Staff member
youll generally find that
4000 FPM (FEET PER MINUTE)of piston speed with stock parts
4500fpm with VERY good quality forged parts is the reasonable limit on the lower end ROTATING ASSEMBLY stress,
if you don,t think selecting high quality components and correctly assembling them is important, heres a visual reminder of the results of component failure under high stress.

but the valve control issues tend to become the limiting factor before the lower assembly causes problems,assuming you've used good quality parts and balanced the rotating assembly,and your avoiding lubrication and detonation issues,as a general rule you'll have sufficient strength in a 2 bolt block to run at about 4000 ft per minute in piston speed,over a long life expectancy, exceed that stress level by much and your increase stress will eventually cause durability issues
its commonly the valve train,control issues, the lubrication and cooling systems failures or detonation in the cylinders that are common factors in engine failures
keep in mind stress on the rods and main caps goes up rapidly with increased rpms
the aftermarket forged 4340 steel connecting rods with the 7/16" ARP cap screw connecting rod bolts, has about a 20% larger cross sectional area , and in many cases a 100% or greater strength advantage more than the factory connecting rods with 3/8" rod bolts, its generally not needed till you exceed about 4200fpm in piston speed, but it seldom is a bad idea to use 4340 forged connecting rods with the 7/16" ARP rod bolts, as they are not that expensive, and your engine durability depends on good rods



a good set of SCAT FORGED 4340 forged connecting rods costs less than $400 and they are 150%-200% stronger than MOST OEM chevy SBC rods
it will cost you almost that much to replace the bolts with ARP wave lock bolts, balance and polish and resize stock rods and you have far weaker rods when your done
keep in mind most posted info is meant to give good guide lines and related info,theres no way of pin pointing the the exact rpm that your particular components will fail, but experience has shown that having a decent cushion in the component strength is a very good idea.
rarely will I get specific,s to one particular build unless it states that, now your combo, your building may have forged rods and 3/8" ARP rod bolts that are significantly stronger than stock OEM 3/8" rod bolts, and forged rods that are much stronger than the OEM powdered metal connecting rods,and it may do fine up into the 6500rpm range for very brief and fairly infrequent use,and Id think that anything under about 6300rpm for only brief periods should be safe with a decent forged rod with ARP rod bolts in a 383 combo, but it certainly won,t allow you to spin the engine at near 6500 rpm for prolonged duration, if only because the valve train most guys use in a street build 383 is unlikely to work well under those conditions, especially with any hydraulic lifter valve train..
a 7/16" rod bolt is about 20% larger in cross sectional area than a 3/8" rod bolt and most 7/16" rod bolt rods are significantly thicker in cross section making them much stiffer and less likely to fail under loads


ARP posted these pictures of failed bolts






rcf6.jpg ... ewall.html


lets do a bit of math with a high rpm 383 combo, it might help here

lets take this connecting rod (645 grams)
this piston (527 grams)
and just temporarily ignore the rings,and bearing weight

thats about 18210 grains at 4500 fpm in piston speed thats 75 ft per second
6588 inertial pounds the piston weight per piston at just over 7000rpm, and your looking to reverse its direction of travel , at over 116 times PER SECOND at 7000 rpm, effectively doubling even that load of the stress on the exhaust stroke ,if you don,t think thats absolutely amazing that its potentially possible to do without instantly self destructing you have zero grasp on the potential levels of stress, then we add the fact that theres potentially 600 psi of pressure on the power stroke over a piston or about 7700 pounds resisting the piston on the power stroke but not on the next exhaust stroke and it mind boggling it holds together for even a second or two if we throw in the rings and bearing weights

4 bolt blocks using all forged rotating assembly's can usually be safely operated for brief periods at up to 4500 ft per minute in piston speeds, but the stress levels are cumulative and the higher the average rpm rage the lower the life expectancy
hitting the engines red line doesn,t mean the engines going to sustain damage, but it generally induces significant stress, stress that WILL eventually cause DAMAGE, it might happen instantly or require hundreds of repetitions BUT it will eventually happen if its exceeded regularly, because STRESS IS CUMULATIVE.

the 4000fpm rule is a general V8 rule with STOCK components,and it takes into account several factors like lower end stress and likely valve train components, if you upgrade to ALL FORGED & balanced components, and aftermarket valve train, you can usually go up to 4500fpm in piston speed, keep in mind 4000fpm=48,000 inches per minute and you need to use twice the stroke per revolution in calculations ... speed.html


generally hydraulic lifters max out at about 6500rpm or lower
and stock rockers and valve trains rarely control valves well even with solid lifters above 7000-7500 rpm ... ILEID=9290 ... re=related

a reasonable limit on cast pistons usually falls near 4000 feet per minute in piston speeds
so your stroke is a factor not just rpms

a balanced set of quality forged pistons can probably handle 4500fpm, or a bit more

keep in mind that's max PEAK engine rpms, that should only rarely be reached ,your engine will NEVER stay together if subjected to those rpms consistently for more that brief moments before shifting, hold any engine at redline for more than a few seconds and bad and expensive things are likely to happen
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many guys don,t realize that the rod bolt material and cross sectional area are critical to durability , especially in a high rpm range combo,while the rods themselves occasionally fail, its much more likely that the rod bolts lost their clamping strength, stretched a bit first and that was a major contributing factor in the bearing failure or the rod failure engines red-line is the rpm limit where the stress is approaching the strength limitations of the weaker components in your engine, in most cases that's the rod bolts, rods, or a valve train control issues, on most engines.
Id also point out that valve springs tend to loose tension and load rate as they age and stress on valve train parts is cumulative , as is wear on the bearings, and stress on rods and pistons, etc.,so the rpm range you could safely maintain when the engine was new will be reduced as the engine ages.
on most American V8 engines that's reached at about 4000 fpm (feet per minute) in piston speed, with high quality forged components and a solid lifter valve train, that's at about 4500fpm,now Id point out Im not about to suggest that at 4100 fpm a stock engine self destructs,because theres a wide variation in components, and different engine designs but stress is cumulative, and those are reasonable limits as a guide,for a engine designed for long term street use durability now your engine may exceed that limit hundreds of times or only a few times before problems occur, but exceed the limits and eventually youll over stress rotating assembly ,components until they fail.
What I like is, building cost effective,easily duplicate-able engines, from off the shelf components if what I need is available, if not I have zero problem fabricating some components or modifying whats available if required,engines that use mostly built from well matched correctly fitted components. I like having total predictable control on a valve train components and long term durability, with low maintenance and in most cases large displacement high compression engine combos
I prefer to keep the piston speeds under 4300feet per minute , and Id prefer to keep the compression as high as I can , simply because its generally going to make for a more responsive combo with higher torque in the useable rpm range.
now in most cases thats engines of 400 cubic inches or larger displacement, and when I can 11:1 or higher compression, so 7000rpm is about where Im comfortable limiting valve train speeds, and I have zero issues building bigger displacement combos that might never see 6500rpm.
given the choice Id gladly give up that last extra 2%-5% of potential horse power in exchange for an added 20%-to-50% greater engine life expectancy, which is in many cases a choice you ARE forced too make

your fpm is found by multiplying the engine rpms x the stroke x 2
on a 383 sbc the stroke is 3.75", so 4000fpm on a 383 is reached at 4000rpm x 12"(inches)= 48,000 inches per minute of piston travel
so 48,000/7.5" of stroke =6400rpm, (7.5" is twice the engines 3.75" stroke) exceeding that is a reasonably sure way to over stress the engine or (get into valve control issues ( which is a different can of worms)

this is a good place to point out that ARP makes much stronger rod bolts and main cap studs,and that the better aftermarket forged 4340 forged connecting rods are far stronger than most OEM connecting rods and that a 7/16" arp rod bolt is on average 200%-250% stronger than a stock 3/8" OEM rod bolt.
balanced components tend to put significantly lower stress on the engine components


obviously valve float and improper lash clearance can cause problems but in some cases, lash caps can reduce wear


in most cases when you see valve tip damage like this its the result of valve float or a weak valve spring , in many cases youll need to swap to a higher spring load rate and new springs to prevent or reduce this damage
interesting info from ARP

540 RAT posted this info
First let’s look at some typical strength values of various bolts, to get a general feel for how they compare.

Grade 2 hardware store general purpose bolt:
Yield strength = 55,000 psi
Tensile strength = 74,000 psi
Cost = a few cents each
Grade 5 hardware store general purpose bolt:
Yield strength = 85,000 psi
Tensile strength = 120,000 psi
Cost = a few cents each
Grade 8 hardware store general purpose bolt:
Yield strength = 120,000 psi
Tensile strength = 150,000 psi
Cost = a few cents each
ARP 8740 chrome moly “connecting rod” bolt:
Yield strength = 180,000 psi
Tensile strength = 200,000 psi
Cost = $120.00 per set of 16 at Summit Racing Equipment, or about $8.00 each.
ARP 2000 “connecting rod” bolt:
Yield strength = 180,000 psi
Tensile strength = 220,000 psi
ARP 2000 rod bolt material has twice the fatigue life of 8740 chrome moly rod bolt material.
Cost = $200.00 per set of 16 at Summit Racing Equipment, or about $13.00 each.
ARP L19 “connecting rod” bolt:
Yield strength = 200,000 psi
Tensile strength = 260,000 psi
ARP L19 rod bolt material is subject to hydrogen embrittlement, and stress corrosion. It also cannot be exposed to any moisture, including sweat and/or condensation.
Cost = $200.00 per set of 16 at Summit Racing Equipment, or about $13.00 each.
ARP Custom Age 625+ “connecting rod” bolt:
Yield strength = 235,000 psi
Tensile strength = 260,000 psi
ARP Custom Age 625+ rod bolt material has nearly 3 ½ times the fatigue life of the ARP 3.5 rod bolt material.
Cost = $600.00 per set of 16 at Summit Racing Equipment, or about $38.00 "EACH".
ARP 3.5 “connecting rod” bolt:
Yield strength = 220,000 psi
Tensile strength = 260,000 psi
Cost = $855.00 per set of 16 at Summit Racing Equipment, or about $53.00 "EACH"!!!

So, as you can see above, hardware store general purpose bolts are considerably weaker than “purpose built” connecting rod bolts. And we won’t even bother getting into the differences in fatigue life. Suffice it to say, we CANNOT use general purpose hardware store bolts in our connecting rods
A connecting rod bolt’s maximum tension loads are determined by the mass of the parts involved, the rod length, the stroke length, and the max rpm. That’s it. It has absolutely nothing what so ever to do with the amount of HP being made. The max tension loads on the rod bolts will never change, no matter if you add Nitrous, a Turbo, or a Blower to an engine, as long as the short block and redline don’t change. That max tension loading occurs at TDC on the exhaust stroke as the mass involved is brought to a dead stop, and has its direction reversed. In order to change the max tension loading on the rod bolts, you’d have to change the short block configuration and/or the redline. And vacuum pulling on the rod bolts when chopping the throttle at high rpm, is not a concern. Because those affects don't even begin to build until well past TDC, which of course is "AFTER" the mass of the parts involved has already been brought to a stop, and their direction reversed.

The rod’s big end “clamp-up preload” provided by stretching/torquing the rod bolts, must always be HIGHER than the “cyclic tension load” applied to the bolts at TDC exhaust, in order to prevent rod bolt failure. And the larger the difference between the preload and the cyclic load, the better. Precision detailed "Strength Analysis" calculations can be performed using sound Engineering principles, to determine the “Margin of Safety” (MOS) between the “cyclic tension loading” and the “clamp-up preload”, to make sure you have a sufficient MOS for the engine to be reliable. I’ll spare you all the involved and complicated math, and just show you the results.

Before we go on, first a comment on “cap screw” rod bolt sizes. Your rod bolts are NOT the size you think they are. If you run 3/8” rod bolts, only the threads are 3/8”. But, the part of the bolt that matters regarding the stretch, is the shank. And the main length of the shank is only 5/16”, not the 3/8” you might have thought. And if you run 7/16” rod bolts, the threads are 7/16”, but main length of the shank is only 3/8”. So, where you are most concerned, the bolts are one size SMALLER than you thought.

And if that isn’t enough detail, you must also consider, in addition to the main section of the shank, the other diameters involved which come from the radius transition between the threads and the shank, the radius transition between the shank and the shoulder right under the bolt head flange, and that shoulder itself right under the bolt head flange. The bolts stretch the whole length between the threads and the bolt head flange. And all those individual sections contribute to the total stretch by different amounts.

So, the rod bolt “Strength Analysis” must take into account all those various diameters, as well as the length of each of those diameters. Because the stretch has to be calculated for each individual section of the shank between the threads and the bolt head flange. If this is not done correctly, the “Strength Analysis” results will simply end up being wrong and worthless. But, for the results shown below, all those details were carefully worked out for those portions of the “Strength Analysis”. So, the answers below are all accurate.

Rod bolt "Strength Analysis" performed on known real world Street Hotrods, Street/Strip cars and Sportsman Drag cars, being operated at their typical maximum rpm, indicates the following:

• An engine with a max rpm rod bolt MOS of around 125% or higher, results in the engine being as safe and reliable as a stock grocery getter, or in other words essentially bullet proof. This is our design target when planning a new build. Having a MOS higher than this can’t hurt of course, but in terms of strength requirements, there is really no added value for doing that. However, a higher MOS can help with rod bolt fatigue life, if that is critical for a particular application. More on fatigue life later.

• If you are a little more aggressive, and run a maximum rpm rod bolt MOS between 100% and 125% only “on occasion”, which limits the number of cycles at this higher stress level, you will still generally be able to keep the engine together.

• But, if you were to run a typical maximum rpm rod bolt MOS under 100%, your rod bolts will be expected to fail prematurely.

As mentioned above in the definition of Yield Strength, we CANNOT stretch our rod bolts beyond the yield point. Because once the yield point is passed, it is considered a “failed” condition for a bolt, and the bolt must be discarded. So, a typical conservative Engineering approach in most general applications is to use a preload clamp-up of about 75% of yield. That way you have a good range between the installed preload and the yield point, in case the bolts get stressed even more during operational use. However, typical engine connecting rod bolt preload clamp-up in most reliable engines, can vary from a low of about 60% of yield to a high of about 90% of yield, with 75% of yield, the sweet spot you might say, right in the middle.

Since rod bolt stretch specs have generally become the standard in High Performance engine builds, the stretch called for is more often around 90% of the yield point. Stretching to this higher percentage of yield, is used to maximize preload clamp-up, in an effort to try and help minimize rod big end distortion at high rpm, which can cause additional undesirable rod bolt bending that would add to the bolt stress.

So, this high level of stretch is a good idea from that standpoint, but at the same time, you are left with a smaller range between the installed preload clamp-up and the yield point. But, this common 90% of yield has worked out quite well in the real world for Hotrods, Street/Strip cars, and Sportsman Drag cars. Even though there is less range between the installed preload clamp-up and the yield point, the yield point in properly selected rod bolts is not typically reached in actual operation, so all is good.

You may also have noticed that through all this discussion of rod bolt strength, there has been no mention at all of rod bolt tensile strength. That’s because we CANNOT go beyond the yield strength which is reached well “BELOW” the tensile strength. So, what good is tensile strength then? For a large number of steels, there is a direct correlation between tensile strength and fatigue life. Normally, as tensile strength increases, the fatigue life increases. So, while tensile strength does not come into play during rod bolt "Strength Analysis", it is a factor in rod bolt fatigue life.

Rod bolt fatigue life is important to Road Racers because of the number of cycles they see. And rod bolt fatigue life is absolutely critical for Endurance Racers like NASCAR. And NASCAR teams do an incredible job managing the fatigue life of their rod bolts. But, for our Hotrods, Street/Strip cars and Sportsman Drag cars, rod bolt fatigue life isn’t typically a big concern, if the motors are built with the correct rod bolts in the first place. That is because these bolts won’t typically see enough cycles in their lifetime to cause a failure due to fatigue. But, with that said, it is still a good idea to keep fatigue life in the back of your mind, when it comes to choosing your rod bolts. It can be a tie breaker, in the event that multiple rod bolts are being considered for a certain build. More on that below.


Even though there are various companies that offer rod bolts, below we will compare 5 different rod bolts offered by Industry leader ARP.

So, let’s take a look at a typical 540ci BBC motor, running steel rods with 7/16 “cap screw” rod bolts, and uses 7,500 rpm as its typical maximum, which results in a cyclic tension load on each rod bolt that = 7,280 lbs or about 3.6 tons:

• For general reference, let’s first take a look at rods installed the old school traditional way, here using ARP 2000 rod bolts that are torqued to about 75 ft lbs with original ARP moly lube.
Bolt stretch is about .005”, which = 76% of yield strength
Clamp-up preload on each rod bolt = 16,531 lbs or about 8.3 tons
Margin of Safety (MOS) for this setup = 127%, which meets our MOS design target for being safe, reliable and essentially bullet proof.

Now, for the rest of the rod bolts we’ll be looking at, we’ll preload them to the more common higher percentage of yield strength, which is typical of the stretch called for these days.

• Using ARP 8740 chrome moly rod bolts (this has the same yield strength as ARP 2000)
Bolt stretch = .006” which = 90% of yield strength
Clamp-up preload on each rod bolt = 19,686 lbs or about 9.8 tons
Margin of Safety (MOS) = 170%

• Using ARP 2000 rod bolts (this has the same yield strength as 8740 chrome moly)
Bolt stretch = .006” which = 90% of yield strength
Clamp-up preload on each rod bolt = 19,686 lbs or about 9.8 tons
Margin of Safety (MOS) = 170%

• Using ARP L19 rod bolts
Bolt stretch = .0066” which = 90% of yield strength
Clamp-up preload on each rod bolt = 21,655 lbs or about 10.8 tons
Margin of Safety (MOS) = 197%

• Using ARP Custom Age 625+ rod bolts
Bolt stretch = .0078” which = 90% of yield strength
Clamp-up preload on each rod bolt = 25,445 lbs or about 12.7 tons
Margin of Safety (MOS) = 250%

• Using ARP 3.5 rod bolts
Bolt stretch = .0073” which = 90% of yield strength
Clamp-up preload on each rod bolt = 23,821 lbs or about 11.9 tons
Margin of Safety (MOS) = 227%

As you can see above in all 6 examples, whether torqued the traditional way to a lower stretch value, or stretched to the more recently called for higher percentage of yield value, all these rod bolts are above the minimum 125% MOS target for safety and reliability. Therefore, all these configurations would operate without issue, just like a stock grocery getter. So, if a builder chooses any of these bolts or stretch values between the 127% and the 250% "Margins of Safety" above, he could NOT go wrong, no matter how much HP the motor makes. Remember that HP has NOTHING to do with the max tension loads on rod bolts.

Since most Hotrods, Street/Strip cars, and Sportsman Drag cars, with their lower number of cycles, can live almost indefinitely with some of the more affordable mainstream rod bolts above, it’s rather hard to make a case for using the much more expensive and higher strength 625+ or 3.5 bolts, even if they do have higher fatigue life values.


So then, all we REALLY NEED, from a conservative Engineering standpoint, is to at least reach the 125% MOS target for safety and reliability, no matter how much HP is being made. And anything above that 125% is fine, but not necessary.


But, things aren’t always wine and roses, because some engines will NOT stay together and live like the well built configurations above. I've done "failed" rod bolt "Strength Analysis" on two smaller very high revving engines, after the fact, to take a look at why they failed. One blew-up catastrophically when a rod bolt broke, costing its owner 20 grand. And the other engine was found to have rod bolts stretched beyond the yield point, during a teardown for other reasons. So, its fuse had been lit, but fortunately it was caught just in the nick of time before they let go, saving its owner a ton of money and agony.

In both cases, the rod bolt "Strength Analysis" revealed that they had been built wrong, and that they were well BELOW 100% MOS, which predicts premature rod bolt failure. One had only a 67% MOS and the other had only an 86% MOS. If rod bolt "Strength Analysis" had been performed before these engines were built, during the planning stages, then all that grief and cost could have been avoided. They have since been rebuilt much stronger, with MOS values now well ABOVE that 125% safe target. And they have now been raced for some time without issue.



• ARP 8740 chrome moly rod bolt - a strong affordable rod bolt, but it has only a moderate fatigue life, which makes the ARP 2000 rod bolt which is in the same general price range, a much better choice since it has twice the fatigue life.

• ARP 2000 rod bolt - considering how good its strength and fatigue life are, this rod bolt is an excellent choice for most Hotrods, Street/Strip cars, and Sportsman Drag cars.

• ARP L19 rod bolt - the strength and fatigue life increases this bolt provides over the ARP 2000 are not significant enough to overcome the concerns the L19 has with hydrogen embrittlement, stress corrosion, and the fact that it CANNOT be exposed to any moisture, including sweat and/or condensation. Don’t forget that every engine forms condensation inside, at every cold start-up. Plus, oil rises to the top of, and floats on water because of density differences, which can leave portions of the rod bolts exposed to water even after the engine is built. Therefore, it is best to avoid the L19 rod bolt altogether, especially since the ARP 2000 rod bolt already provides way more than enough strength and fatigue life than is typically required by most Hotrods, Street/Strip cars, and Sportsman Drag cars. So, there simply is no good reason to select the ARP L19 rod bolt. If you are currently running L19 bolts, I’d suggest you consider replacing them with different bolts the next time you have the motor apart.

• ARP Custom Age 625+ rod bolt - a very pricey bolt, but with its excellent strength and its impressive fatigue life, this bolt is one of the very best rod bolts on the market.

• ARP 3.5 rod bolt - this bolt has excellent strength, but its staggering cost is 43% HIGHER than the 625+ bolt, yet the 625+ bolt is superior to the 3.5 bolt in virtually every way. So, there is no good reason to select the 3.5 bolt either.



Of the 5 rod bolts above:

• The ARP 2000 rod bolt is an excellent value, considering how good its strength and fatigue life are. And it should be considered the rod bolt of choice for most Hotrods, Street/Strip cars, and Sportsman Drag cars, no matter how much HP they make. And this is why you most often see quality aftermarket rods come with these bolts.

• ARP Custom Age 625+ rod bolt has a price that is not for the faint of wallet, but it should be considered the rod bolt of choice for very high revving engines, road race engines, and endurance engines, which require the utmost in rod bolt strength and/or fatigue life.

540 RAT"

I know most guys only learn the flaws of using an O.E.M. block,
after having one catastrophically fail under loads
" you can PAY NOW for good parts or PAY MORE AGAIN LATER ,
keep in mind valve train control and rod bolts failures,
and keeping the cooling and lube systems fully functional,
and selecting components that will exceed the stress limitations,
and verifying clearances ARE key factors to maintaining long term durability


Other Stresses & related info





It must be realized that the direct reciprocating load is not the only source of stresses in bolts. A secondary effect arises because of the flexibility of the journal end of the connecting rod. The reciprocating load causes bending deformation of the bolted joint (yes, even steel deforms under load). This deformation causes bending stresses in the bolt as well as in the rod itself. These bending stresses fluctuate from zero to their maximum level during each revolution of the crankshaft.

Fastener Load

The first step in the process of designing a connecting rod bolt is to determine the load that it must carry. This is accomplished by calculating the dynamic force caused by the oscillating piston and connecting rod. This force is determined from the classical concept that force equals mass times acceleration. The mass includes the mass of the piston plus a portion of the mass of the rod. This mass undergoes oscillating motion as the crankshaft rotates. The resulting acceleration, which is at its maximum value when the piston is at top dead center and bottom dead center, is proportional to the stroke and the square of the engine speed. The oscillating force is sometimes called the reciprocating weight. Its numerical value is proportional to:
It is seen that the design load, the reciprocating weight, depends on the square of the RPM speed. This means that if the speed is doubled, for example, the design load is increased by a factor of 4. This relationship is shown graphically below for one particular rod and piston

"are all rod stretch gauges created equal "

obviously no more than all girls are equally good looking
but most of the gauges are functional, some just have more features or more precise calibrations, some are adjustable in length ,over a wider range, some have digital read outs, ETC.

sum-900015.jpg ... index.html

a reasonable limit on cast pistons usually falls near 4000 feet per minute in piston speeds
so your stroke is a factor not just rpms

a balanced set of quality forged pistons can probably handle 4500fpm, or a bit more

rod bolts can fail for a couple dozen plus reasons
OVER tightening
UNDER tightening
lack of bearing lubrication
lack of rod to block clearance
piston rings locking in the bore when hot
failure to measure stretch or use a torque wrench
detonation damaged pistons
valve train failures
over revving the engine
lack of quench clearance
valve to piston contact
broken valve springs
lack of cam to rod clearance
lack of rod bearing to crank edge clearance
etc. ETC.ETC.

very few are directly related to the rod bolt strength limitations under designed operational conditions, itself failing, most are operator or engine assembly induced problems yet the

but the results similar in most cases




and don,t skip the sub linked info
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on many American V8 engines with FORGED and BALANCED COMPONENTS, its not the lower end strength, lack of oil pressure or the rotating assembly thats likely to be the CAUSE of a failure,but it is the other often over looked factors, Id bet serious money that most guys that find a bent connecting rod, blame the lower end rotating assembly, never thinking further into the cause,when in many cases thats the result not the cause of the failure.
detonation can destroy a piston or bearings in minutes, lack of oil flow over the rings,valve springs, or pistons will cause them to overheat rapidly, and valve control issues are far more likely than lower end problems
lack of cool pressurizer oil flowing over valve springs, will cause them to rapidly over heat and loose temper reducing their load carrying capacity, and rockers,lack of cool pressurizer oil flowing over lifters and cam lobes will cause very rapid over heating, lack of oil flow can cause rapid wear issues and when a cam or lifter starts to wear metallic crap can rapidly ruin bearing surfaces, and destroy clearances reducing oil control and reducing pressure.
your oil filter WON,t generally catch all the metallic crud before it can cause problems.
the cause of an engine failure can be tuning issues, lack of correct clearances, lack of lubrication or cooling , the result is frequently a piston trying to compress a non-compressible object like a bent valve that didn,t fully seat due to valve float or valve control issues or a chunk of busted ring land that fractured from over heating due to detonation , lack of a film of cooling oil, bad fuel/air ratio,or incorrect ignition advance curve.
One other factory to think about ,is that ARP main studs are stronger than factory main cap bolts, and and if a main cap starts moving under high stress at the upper rpm limits the oil pressure tends to drop as the clearances tend to increase.

Calculating Maximum Safe RPM

Max. Safe RPM = Mean Piston Speed (ft/min) x 6
Divided by Stroke in Inches

Example for a budget aftermarket forged crank in a 4-inch stroke small-block Chevy:
4,800 x 6 = 7,200 rpm

keep in mind valve train control problems can occur before piston speed is at max limits

Maximum Mean Piston Speeds for Above Formula:
Factory cast-iron cranks 3,750 ft/min
Aftermarket cast-steel cranks 4,500 ft/min
Factory forged cranks 4,600 ft/min
Budget aftermarket forged cranks 4,800 ft/min

Typical race aftermarket cranks 5,500 ft/min
High-dollar custom endurance race cranks 6,000 ft/min
ProStock/Mountain Motors 7,500 ft/min
Formula One 7,500+ ft/min [/size]
Written by David Reher

Recently I've had several conversations with racers who wanted to build engines with long crankshaft strokes and small cylinder bores. When I questioned them about their preference for long-stroke/small-bore engines, the common answer was that this combination makes more torque. Unfortunately that assertion doesn’t match up with my experience in building drag racing engines.

My subject is racing engines, not street motors, so I’m not concerned with torque at 2,000 rpm. In my view, if you are building an engine for maximum output at a specific displacement, such as a Comp eliminator motor, then the bores should be as big as possible and the stroke as short as possible. If you’re building an engine that’s not restricted in size, such as a heads-up Super eliminator or Quick 16 motor, then big bores are an absolute performance bargain.

I know that there are drag racers who are successful with small-bore/long-stroke engines. And I know that countless magazine articles have been written about “torque monster” motors. But before readers fire off angry e-mails to National DRAGSTER about Reher’s rantings on the back page, allow me to explain my observations on the bore vs. stroke debate.

In mechanical terms, the definition of torque is the force acting on an object that causes that object to rotate. In an internal combustion engine, the pressure produced by expanding gases acts through the pistons and connecting rods to push against the crankshaft, producing torque. The mechanical leverage is greatest at the point when the connecting rod is perpendicular to its respective crank throw; depending on the geometry of the crank, piston and rod, this typically occurs when the piston is about 80 degrees after top dead center (ATDC).

So if torque is what accelerates a race car, why don’t we use engines with 2-inch diameter cylinder bores and 6-inch long crankshaft strokes? Obviously there are other factors involved.

The first consideration is that the cylinder pressure produced by the expanding gases reaches its peak shortly after combustion begins, when the volume above the piston is still relatively small and the lever arm created by the piston, rod and crank pin is an acute angle of less than 90 degrees. Peak cylinder pressure occurs at approximately 30 degrees ATDC, and drops dramatically by the time that the rod has its maximum leverage against the crank arm. Consequently the mechanical torque advantage of a long stroke is significantly diminished by the reduced force that’s pushing against the piston when the leverage of a long crankshaft stroke is greatest.

An engine produces peak torque at the rpm where it is most efficient. Efficiency is the result of many factors, including airflow, combustion, and parasitic losses such as friction and windage. Comparing two engines with the same displacement, a long-stroke/small-bore combination is simply less efficient than a short-stroke/big-bore combination on several counts.

Big bores promote better breathing. If you compare cylinder head airflow on a small-bore test fixture and on a large-bore fixture, the bigger bore will almost invariably improve airflow due to less valve shrouding. If the goal is maximum performance, the larger bore diameter allows the installation of larger valves, which further improve power.

A short crankshaft stroke reduces parasitic losses. Ring drag is the major source of internal friction. With a shorter stroke, the pistons don’t travel as far with every revolution. The crankshaft assembly also rotates in a smaller arc so the windage is reduced. In a wet-sump engine, a shorter stroke also cuts down on oil pressure problems caused by windage and oil aeration.

The big-block Chevrolet V-8 is an example of an engine that responds positively to increases in bore diameter. The GM engineers who designed the big-block knew that its splayed valves needed room to breath; that’s why the factory machined notches in the tops of the cylinder bores on high-performance blocks. When Chevy went Can-Am racing back in the ’60s, special blocks were produced with 4.440-inch bores instead of the standard 4.250-inch diameter cylinders. There’s been a steady progression in bore diameters ever since. We’re now using 4.700-inch bores in NHRA Pro Stock, and even bigger bores in unrestricted engines.

Racers are no longer limited to production castings and the relatively small cylinder bore diameters that they dictated. Today’s aftermarket blocks are manufactured with better materials and thicker cylinder walls that make big-bore engines affordable and reliable. A sportsman drag racer can enjoy the benefits of big cylinder bores at no extra cost: a set of pistons for 4.500-inch, 4.600-inch or 4.625-inch cylinders cost virtually the same. For my money, the bigger bore is a bargain. The customer not only gets more cubic inches for the same price, but also gets better performance because the larger bores improve airflow. A big-bore engine delivers more bang for the buck.

Big bores aren’t just for big-blocks. Many aftermarket Chevy small-block V-8s now have siamesed cylinder walls that will easily accommodate 4.185-inch cylinder bores. There’s simply no reason to build a 383-cubic-inch small-block with a 4-inch bore block when you can have a 406 or 412-cubic-inch small-block for about the same money.

There are much more cost-effective ways to tailor an engine’s torque curve than to use a long stroke crank and small bore block. The intake manifold, cylinder head runner volume, and camshaft timing all have a much more significant impact on the torque curve than the stroke – and are much easier and less expensive to change.
Written by David Reher

Looking back at the 2004 season, I can attribute much of the performance improvement in Pro Stock to faster engine speeds. It’s difficult to believe that 500cid Pro Stock engines now routinely turn 10,000 rpm, but the truth is plain to see on the data recorders and on the time slips.

The trend toward higher and higher engine speeds was also evident in NASCAR stock car racing until the rulemakers applied the brakes with new restrictions on rearend and transmission gear ratios. Now the growing interest in fast bracket racing, Top Sportsman, and Top Comp eliminators is bringing this same high-rpm technology to sportsman drag racers.

Why does turning an engine higher make a race car run faster? This is my final column of the year, so I’ll offer my ideas and hope that they give racers something to think about over the winter break.

The simple explanation is that raising rpm effectively increases an engine’s displacement. This might seem nonsensical because the volume displaced by the pistons doesn’t change, but consider the effects of filling and emptying the cylinders faster in real time. An internal combustion engine is an air pump, and if we turn that pump faster, we can theoretically burn more fuel in a given amount of time and consequently produce more power. For example, an eight-cylinder engine running at 6,000 rpm fires its cylinders 24,000 times in one minute (assuming perfect combustion). Increase the engine’s speed to 8,000 rpm and it will fire 32,000 times per minute, a 33 percent increase. The volume of air and fuel that moves through the engine is now equivalent to an engine with a much larger displacement. There are also 8,000 additional power pulses per minute transmitted to the crankshaft that can be harnessed to turn the wheels and accelerate the car.

Raising engine speed is analogous to supercharging or turbocharging a motor; the goal is to increase the volume of air and fuel that moves through the engine. The airflow is increased with a forced induction system by pressurizing the intake system; in a naturally aspirated engine, the airflow is increased by raising rpm. If done correctly, both approaches will increase power.

A higher revving engine also permits the use of a numerically higher gear ratio to multiply the engine’s torque all the way down the drag strip. Let’s say an engine that produces 1,000 horsepower at 7,000 rpm is paired with a 4.56:1 rearend gear ratio. If this engine is then modified to produce 1,000 horsepower at 8,000 rpm, it can now pull a 4.88:1 or 5:14:1 rearend gear without running out of rpm before reaching the finish line. The numerically higher gear ratio gives the engine a mechanical advantage by multiplying its torque by a greater number to accelerate the car faster – in effect, it has a longer lever to move the mass.

I learned this lesson many years ago when I started drag racing. I raced my little 302cid Camaro against 426 Hemis and 440cid Max Wedge Mopars. The big-inch engines had thunderous low-end power, but my high-revving 302 with a 4.88:1 rear gear would just kill them because they were all done at 5,800 rpm. My small-block had much less torque and horsepower, but I could multiply the power it had with a steeper gear ratio. The same principle applies to racing a Pro Stock or a Top Sportsman dragster. By turning more rpm, we can use a greater gear ratio to produce more mechanical advantage to accelerate the car.

There are limits to engine speed, of course. Higher rpm increases parasitic losses from friction and windage. The stability of the valvetrain also restricts engine rpm. However, with the technology developed in NASCAR and in Pro Stock, racers are learning how to build engines that operate reliably at high rpm. Research and development on valve materials, springs, rocker arms, and pushrods are now being applied to serious sportsman drag racing engines. In fact, I wish that I had some of the parts that we now install in our high-horsepower sportsman engines for our Pro Stock program a few years ago!

While increasing rpm is generally a good thing for a racing engine, it also puts more responsibility on the owner. A high-rpm combination requires more vigilance and more maintenance than a low-rpm motor. It’s important to check the valve lash frequently and to look for early warning signs such as weak or broken valve springs. Neglecting these parts in a high-rpm racing engine can produce some very expensive problems.

Raising an engine’s operating range also requires complementary changes in the drivetrain and chassis. A high-rpm sportsman engine really needs a high-stall torque converter to realize its potential. With an automatic transmission, the engine speed should ideally drop 1,000 to 1,300 rpm after a gear change. For example, if the converter stalls at 6,700 rpm, the engine should be shifted at around 8,000 rpm. Shifting this engine at 7,000 rpm would simply put the engine back on the converter, causing the converter to operate inefficiently and wasting horsepower by heating the transmission fluid.

I’m excited about the emerging trend toward fast sportsman drag racing. I enjoy working with customers who want to go fast because it gives me an opportunity to deliver the benefits of our Pro Stock R&D to other racers. Not every racer wants or needs a high-rpm engine, but if the goal is to have a fast car, raising the redline is a proven approach.

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"HEY GRUMPY, I got a GREAT deal on some used imported small block connecting rods, from a local machine shop,Ive never heard of the company, but they say they are forged rods with 3/8" rod bolts" ... tions.html
link too bore vs stroke info on hundreds of engines
Id recommend either of these,5.7" sbc connecting rods , yes they are more money, but Ive used them both in several builds,each, with both scat 9000 cast steel and scat 4340 forged steel cranks and they have quality rod bolts, and don,t have a wide variation in weight either, guys are using them even on nitrous engines that regularly spin 6600rpm with zero problems
the $170-$200 or so you save will be looked at as a really bad idea if the engine comes apart at high rpms and rods and rod bolts are highly stressed components, those cheaper import rods are NOT the same quality, and while they look like a bargain , they won,t continue to look like a bargain if one comes apart and the only thing you can salvage is the water pump and intake and valve covers like one guy I know had left when he tried spinning stock rebuilt chevy rods at 6700rpm
most chevy small block factory rods are VASTLY inferior in strength to many of the mid range and better aftermarket rods available.
a 7/16" cap screw type ARP rod bolt is EASILY 200%-300% stronger than a stock 3/8" factory rod bolt and frankly, the cost & TIME to correctly modify and prep stock rods is a total waste, its almost always cheaper to buy decent aftermarket rods.
just for discussion sake here.....
lets discuss why you factor in piston speeds

Ill point out that the probable strength limitations,
in the 355 block and rotating assembly
(assuming good forged components and ARP main studs are used)
yes as pointed out earlier, both engines below would be pushing well past the limitations imposed by a STOCK, O.E.M. production block
but assuming that the O.E.M block will hold up at least for a hundred 1/4 mile passes,before it comes violently apart at peak rpm, that stress limitation,
will be imposed by both common valve spring limitations and piston speed on the rotating assembly and block,
on a 355 chevy with its 3.48" stroke , (assuming a 4200 fpm max reasonable piston speed)
has a peak rpm near 6700-7200 engine rpm, selecting a cam that peaks below about 6700 rpm-7000 rpm,)
leaves the potential advantage of that shorter stroke the 355 sbc on the table.
this is one reason that the 383 with its longer 3.75" stroke is currently the most popular combo with a 4.30 bore block.
if your going to build a 355, then you may as well take full advantage of the engines potential strong points.
a similar longer stroke 383 would be limited to about 6300 rpm-6700 rpm but the trade off is about 40 ft lbs more torque over most of the potential power band
the formula for hp is
torque x rpm/5252= hp
if both engines make about 1.2 hp per cubic inch
the 383 will peak about 400 rpm lower than the 355 if both are built to maximize the engines potential strong points
just for giggles and to point out
why you maximize a RACE ENGINE potential RPM LIMITS

lets assume the 355 power peak is at about 6800 rpm and,
the similar 383 would be peaking nearer 6400 rpm
if you have 1.2 x 355=426 ft lbs x 6800 rpm /5252=551 hp
if you have 1.2 x 383=460 ft lbs x 6400 rpm /5252=560 hp
theres an old saying in racing,

The connecting rods are a vital link between the pistons and crankshaft. They connect the reciprocal motion of the pistons to the rotational motion of the crank. The weight of the rods is important because it affects the reciprocating forces inside the engine. Lighter is usually better because less weight means faster throttle response and acceleration. But strength is even more important. Connecting rods have to be stout enough to handle all the horsepower the engine can make, and be strong enough to withstand the tension forces that try to pull the rod apart when the piston hits top dead center on the exhaust stroke. If a rod is going to break, more often than not it will fail at TDC on the exhaust stroke than at any other point in its travels. Consequently, piston weight and maximum engine rpm are more important factors to consider than how much power the engine will make when selecting a set of rods.

Basically, you want a set of rods that are as light as possible, but are also capable of handling all the forces the engine can generate (rpm and horsepower).

If you are building an engine for a sprint car that is constantly on and off the throttle, an ultra light crankshaft with the lightest possible rods and pistons will deliver the kind of performance that works best in this application. But if you are building a large displacement, relatively low rpm, high load drag motor, truck pulling motor or a marine engine, you need the reliability of a heavier crank and the strongest possible rods.

The best advice when selecting a particular set of rods is to talk to your parts suppliers and ask them what they would recommend. Every rod supplier we interviewed for this article said rod selection depends on a number of things. First and foremost is the application. In other words, what kind of engine are you building and how will it be used? The rods that work best in an all-out drag motor probably wouldn’t be the best choice for a street performance engine. Nor would rods designed for a circle track sprint car be the best choice for a NASCAR engine or a marine endurance engine.

If you choose a set of rods based strictly on a catalog or Web site description, or you choose a set based solely on length, weight or price, you may not be making the best choice. That’s why a few minutes spent on the phone with your rod supplier can be so valuable. They may recommend a particular type of rod you hadn’t considered, or they may have some new product offerings that have not yet been added to their catalog or Web site. Catalogs get out of date very quickly, and many Web site are not updated as frequently as they should be.

The engineers who design connecting rods know how to analyze the forces that act on rods. Years ago, the design process involves a lot of trial-and-error testing. An engineer would design a rod configuration, test it until it broke, then try to beef up the areas of the rod he felt were weak. Today, most of the development work is done with computers and sophisticated software. Engineers nowadays use finite element analysis (FEA) to analyze the compression and tension forces on a rod. The software creates 3-D images with color coding that indicates the areas of highest and lowest stress. This allows the engineer to visualize what’s actually happening to a rod at various loads and speeds. He can then tweak the design on his computer screen to add metal where extra strength is needed, and to remove metal from lightly loaded areas where it isn’t needed. By repeating the FEA process over and over with each design change, he can optimize the rod to deliver the best possible combination of weight, strength and reliability — in theory anyway. It still takes real world testing to validate the design. But with today’s design and analysis software, most of the work is done before a prototype part is manufactured.

One rod supplier said using FEA on their current rods allowed them to increase strength 12 to 15 percent with less than a 2 percent increase in overall rod weight.

Computer controlled numeric (CNC) machining also allows manufacturers to machine billets and forgings in ways that were previously too difficult, too time-consuming or too expensive. This allows manufacturers to offer a wider variety of rods in terms of rod length and beam construction. It also allows them to produce custom made-to-order rods very quickly. In fact, some rod suppliers say the majority of the rods they sell today are custom order rods rather than standard dimension rods from off the shelf stock.

Rods essentially come in two basic types: I-Beam and H-Beam. Some rod suppliers only make I-Beams, others only make H-Beams, and some offer both types. I-Beam rods are the most common and are used for most stock connecting rods as well as performance rods. I-Beam rods have a large flat area that is perpendicular (90 degrees) to the side beams. The side beams of the rod are parallel to the holes in the ends for the piston pin and crank journal, and provide a good combination of light weight, and tensile and compressive strength. I-Beam rods can handle high rpm tension forces well, but the rod may bend and fail if the compressive forces are too great. So to handle higher horsepower loads, the I-Beam can be made thicker, wider and/or machined in special ways to improve strength.

Rod suppliers produce a number of variants on the basic I-Beam design. The center area may be machined to create a scalloped effect between the beams, leaving a rounded area next to both beams that increases strength and rigidity much like the filets on a crankshaft journal. These kind of rods may be marketed as having an “oval-beam”, “radial-beam” or “parabolic beam” design.

H-Beam rods, by comparison, are typically designed for engines that produce a lot of low rpm torque. This type of rod has two large, flat side beams that are perpendicular to the piston pin and crankshaft journal bores. The center area that connects the two sides of the “H” together provides lateral (sideways) stiffness. This type of design can provide higher compressive strength with less weight than a comparable I-Beam, according to the suppliers who make H-Beam rods. That’s why H-Beam connecting rods are often recommended for high torque motors that produce a lot of power at low rpm (under 6,000 rpm). Of course, an I-Beam rod can also be strengthened to handle almost any load but it usually involves increasing the thickness and weight of the rod and/or using a stronger alloy.

Over 60 percent of late model connecting rods are powder metal I-beam rods. Powder metal (PM) rods are made by compressing powered steel in a mold and then heating the mold to a temperature where the powder melts and fuses into a solid part. This method of manufacturing allows parts to be cast to very close tolerances. This reduces the amount of machining needed to finish the rod, which lowers its cost. Powder metal casting also allows the ingredients in the steel alloy to be combined in ways that are impossible with conventional metal casting techniques, and the finished parts have less internal stress as a result of the fusing process. PM rods can also be up to 20 percent lighter than a comparable rod made of forged steel. Only one aftermarket rod supplier (Howards Cams) currently offers performance rods made of powder metal.

The special alloys that are used to make powder metal rods allows the rod caps to be “cracked” (separated) from the rod rather than cut. Score marks are cast into the part along the rod parting line, and the cap is then sheared off in a large press. The cracking process leaves a slightly irregular surface along the parting line between the cap and the rod that is like a jigsaw puzzle and only goes together one way. The result is better cap alignment and a stronger rod when the cap is bolted to the rod.

One of the drawbacks of powder metal rods is that the caps can’t be reground to compensate for bore distortion or stretch. Consequently, if the rod bore is out-of-round or worn, the rod usually has to be replaced unless a replacement bearing with an oversized outside diameter is available.

Stock rods are typically designed for 5,500 to 6,500 rpm, and 300 to 350 horsepower in a V8 engine. In an overhead cam four or six cylinder engine, the rods may be designed to handle up to 7,000 rpm but probably only about 200 to 250 horsepower. As a rule, most stock connecting rods can handle up to 25 to 40 percent more horsepower than an unmodified engine was originally designed to produce. So for a typical budget street performance engine or a Saturday night dirt track racer, the stock rods may work just fine.

Even so, to ensure reliability the rods should always be “Magnafluxed” to check for cracks. Any flashing, burrs, nicks or other defects along the sides of the rods should also be ground off (grind lengthwise, never sideways) to eliminate stress risers that could lead to cracks and rod failure later on. Shot peening is also recommended to improve fatigue resistance. When the shot strikes the surface, it compresses the metal slightly and actually relieves stresses that might lead to cracking and rod failure.

If an engine is being built to turn significantly higher rpms than the stock motor, or to produce significantly more power (more than 40 to 50 percent), the connecting rods will probably have to be upgraded to assure adequate reliability. For a high revving engine, some type of stronger aftermarket I-Beam rods would be a good choice. For a low rpm torquer motor, either H-Beam or heavier I-Beam rods would work well.

Most aftermarket performance rods are made using 4340 billet or forged steel. This is a chrome moly alloy with high tensile and compressive strength. A word of caution, though, is that all “4340” steel alloys are not necessarily the same. Heat treatments can vary, and this will affect the properties of the steel. Some rod manufacturers also tweak the alloy by adding their own proprietary ingredients to improve strength and fatigue resistance. Several rod suppliers said the 4340 steel that some offshore rod manufacturers use falls short of American Society of Metals quality standards, and is not as good a steel as they claim it is.

There is also a debate over the relative merits of “Made-in-USA” forgings versus foreign forgings that are machined in the USA or rods that are forged and finished overseas. Labor costs are far cheaper in China and other Third World countries, so there are cost advantages for suppliers who source their forgings and rods from offshore manufacturers. Patriotic and international balance-of-payment issues aside, a connecting rod that meets metallurgical quality standards, is heat treated properly, and is accurately machined to specifications is the same no matter where it comes from or who made it. The engine won’t know the difference. So as long as the rod supplier stands behind their product with their brand name and reputation, the “foreign versus domestic” rod debate shouldn’t matter.

The mistake you don’t want to make, however, is to use low priced “economy” rods in an engine that really needs a set of top quality performance rods. A growing number of rod suppliers are now offering lower cost performance rods as economical upgrades over stock rods for street engines and other entry level forms or racing. Most of these rods are made overseas (in China, primarily) and typically sell for less than $600 a set for a small block Chevy V8. The companies who sell these rods say their products are ideal for racers who otherwise might not be able to afford better rods for their engine. Consequently, these budget-priced rods allow engine builders to offer their customers more options and more affordable alternatives for upgrading an engine. For big buck racers or really demanding applications, though, these kind of rods would not be the right choice. You would want to use a set of top-of-the-line performance rods that are capable of handling the highest loads.

Over the past couple of years, the price of high quality steel as well as many other metals such as copper and titanium has shot up dramatically for a variety of reasons (China’s exploding economy being one, the ongoing war in Iraq being another, and changes in the steel industry itself being a third reason). Some rod suppliers are now having to add a steel “surcharge” to their current prices to help offset their higher cost of materials (which doesn’t matter where they buy their steel because the higher prices are world-wide and affect everybody). The soaring cost of titanium has almost priced this metal out of the aftermarket. Some rod suppliers have discontinued making rods from titanium. Those who still offer titanium rods say the only people who are buying them today are the high end professional racing teams with deep pockets. One rod supplier said titanium has become “unobtanium” for the average racer.

Connecting rods made of light-weight titanium rods can reduce the reciprocating mass of the engine significantly for faster throttle response and higher rpms, but at a cost of up to $1000 or more per rod, who can really afford them?

Another lightweight material that has long been used for performance connecting rods is aluminum. Many drag racers run aluminum rods because they cost less than titanium and provide a good combination of lightness and strength. Most aluminum rods are fairly stout and typically much thicker than a comparable steel I-Beam rod. The added thickness may require additional crankcase clearance, and it increases windage and drag — which at really high rpm may cost a few extra horsepower to overcome. The rods also require a dowel pin to keep the bearings from spinning because the bores stretch more than a steel rod. Also, the rod itself can stretch and grow in length at high rpm. This means extra clearance must be built into the engine so the pistons won’t smack the heads.

Though aluminum rods are popular for drag racing and other high rpm forms of racing, most of the rod suppliers we spoke with do not recommend aluminum rods for street engines. Why? Because steel rods will hold up much better over the long run than aluminum rods. Aluminum rods are fine for a drag motor that will torn down after 200 runs and freshened up or rebuilt with a set of new or reconditioned rods. But for street applications or engines that have to run at sustained high speeds and loads for long period times, steel rods are usually better.

It’s interesting to note that aluminum rods are only available from a few suppliers (GRP is one), and at least one supplier who used to offer aluminum rods (Manley) has discontinued them.

Another material that is used for many high performance rods is 300M, which is a modified 4340 steel with silicon and vanadium added, plus higher amounts of carbon and molybdenum. The 300M alloy is up to 20 percent stronger than common 4340 alloys, and was originally developed for aircraft landing gear. Now it is used for high end connecting rods.

The strength and fatigue resistance of most metals can also be improved by “cryogenic” processing after the rods have been heat treated. Heat treating causes changes in the grain structure of steel that increases strength and hardness, but it can also leave residual stresses that may lead to fatigue failure later on. By freezing parts down to minus 300 degrees below zero in special equipment that uses liquid nitrogen, the residual stresses are relieved. The super cold temperatures also cause additional changes to occur in the metal that help the parts last longer and run cooler. That’s why cryogenic freezing is used on everything from engine parts to tool steels, aerospace hardware and even gun barrels.

The cryogenic process is a slow one, taking anywhere from 36 to 72 hours depending on the parts being frozen, and it must be carefully controlled to achieve the desired results. Most rod suppliers have their own cryogenic vendors who treat their rods for them. But you can also have ordinary untreated rods (even stock rods) frozen to achieve the same results.

Other factors that affect the selection of rods are rod length and rod ratio. Rod length depends on the stroke of the crankshaft and the deck height of the block. If you are switching to crankshaft with a longer stroke, you are obviously going to need rods that have a shorter overall length. Even so, replacing the pistons with ones that have a higher wrist pin location can allow you to use longer rods.

Racing legend Smokey Yunick used to say that the longer the rods are, the better. His logic was based on the fact that a longer connecting rod for a given stroke allows the piston to dwell longer at TDC before it starts back down on the power stroke. This allows pressure to build longer in the combustion chamber before it starts to shove the piston down. The result is usually a broader, flatter torque curve than the same engine with shorter rods. An engine’s horsepower and torque curves depend on a lot of variables other than rod length alone. But if everything else is equal, many engine builders say longer rods produce a broader torque curve. Others disagree, and say it doesn’t really matter.

Rod suppliers say the only trend they see in rod lengths today is that there is no trend. Engine builders are buying just as many standard length rods as they are longer rods.This brings us to rod ratio, which is the length of a connecting rod (center to center) divided by the stroke of the crankshaft. The range in engines today may be from 1.5 to 2.1, but most performance engine builders are going with ratios in the 1.57 to 1.67 range. Some say that going with a rod ratio over 1.7 makes engine torque too “peaky.” Lower rod ratio numbers are typically associated with lower rpm torque motors (a 383 Chevy street motor with a stroker crank and a rod ratio of 1.52, for example), while higher rod ratio numbers tend to be high revving high horsepower motors (a 302 high revving Chevy with a rod ratio of 1.9).

Another dimension to consider is the pin offset of the rod. On most rods (except Chevy “LS” engines), the pin bores are offset slightly. The change in pin geometry reduces the stress on the piston pin and small end of the rod when the piston reaches TDC and changes direction. It also reduces the rocking motion of the piston as it passes TDC to reduce piston slap and noise.

One new trend in this area is to run rods that do not have bronze bushings in the small end. Several racing teams are running bare rods with specially plated pins to improve durability. Eliminating the bushing, they say, leaves more meat in the small end of the rod for added strength. The only drawbacks are that the fit between the rod and pin has to be much more precise, and the wrist pin has to have a wear-resistant coating to prevent wear and galling. Also, if the pin bore becomes worn or out-of-round, the rod and piston will both have to be machined to accept a slight larger diameter pin.





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every once in awhile I get asked questions,
that just rather obviously show me that the person asking the question,
just does not have a grasp on the basic concepts , well enough too even understand the fact that the question ,
is based on a false concept or lack of an understanding of what the question implied?
(QUOTE) WHY do I need better more expensive aftermarket connecting rods in a 383 SBC engine,
I am building,that I want to spin to, 7500-7800RPM,
when everyone I talk to swears that they used to own a 302 SBC that would spin 8000-rpm,
with the stock connecting rods, they are both SBC engines so why the difference?[/quote]
OK let me point out the basic flaw in the thinking, here!
RPM alone has very little to do with the problem, its STRESS or INERTIAL LOADS that your dealing with and you need to be able to calculate the differences that increase those loads.
ok, a few basics, a 302 has a 3.0" stroke,
a 383 has a 3.75" stroke, that may not sound like a great deal but lets look at that first.
as a crank journal rotates the engines "stroke" is the distance the crank journals centerline is located in rotational distance from the crank shaft center-line
in the case of the 302 with its 3" stroke that indicated that distance is 1.5" of offset,
thus the crank journal at TOP DEAD CENTER TDC is 1.5" up toward the blocks deck surface,
and at 180 degrees in rotation further at BOTTOM DEAD CENTER BDC, its 1.5" down toward the oil pan floor.
with a 302 the crank journal swings in a circle 1.5" out away from the crank shaft center-line
1.5" up above, then 1.5" below= 3" total piston movement

with a 383 the stroke is 3.75" so the journal is spaced 1.875" from the crank center-line
1.875" up above, then 1.875" below= 3.75" total piston movement




now think about that, if we spin a 302 with its 3" stroke at 8000-rpm each full crank shaft revolution the piston travels from TDC, to BDC and back to TDC or TWICE the 3" stroke thats a distance of 6", at 8000-rpm thats 8000 x 6" or 48000 inches if we divide that by 12 to get feet per minute we get 4000-feet per minute

the 3.75" stroke thats a distance of 7.5", at 8000-rpm thats 8000 x 7.5" or 60000 inches if we divide that by 12 to get feet per minute we get 5000-feet per minute

the general consensus of most engine builders is that even the best stock connecting rods are best kept to stress levels of under 4200 feet per minute, if long term durability is important


$58 EACH or $468 a set (still need balancing)
next Id point out that the stock chevy connecting rod uses a 3/8" rod bolt,
rated at no more than 180K per square inch
a 3/8" bolts cross sectional are is .1106 square inches

the 7/16" bolts in the aftermarket rods have about a 35% larger cross section
and are about 15% stronger steel, the difference is easily a 50% or greater strength

sca-25700_w.jpg $358 a set
the better aftermarket connecting rods like SCAT sells use a 7/16" ARP rod bolts rated at 200K psi
a 7/16" bolts cross sectional are is .1506 square inches

If you do the math and if you select common piston weights in each application youll see that to provide nearly equal stress the longer stroke 383 would be limited to 6400rpm vs the 302s 8000 rpm, and the stronger and less expensive scat rods are far stronger



We have all heard the adage “there’s no replacement for displacement.” The more air an engine can displace, the more fuel it can burn. Anytime you can add more fuel, more power is sure to follow. This is the reason that turbos and nitrous oxide are so popular. Both force more oxygen into the combustion chamber, allowing additional fuel to be burned. The same concept holds true for displacement.

There are three ways to increase the displacement of an engine: increase the number of cylinders, increase the bore size of the cylinders, or increase the stroke of the crankshaft. The first choice requires a completely different engine block to achieve, so for this article, that narrows the choice down to bore and stroke. Increasing bore size is easy, and is the most common practice. However, an increase in bore size is typically restricted to less than one-hundred-thousandths of an inch (0.100)due to cylinder wall thickness on a stock block. Aftermarket blocks or the installation of cylinder sleeves does allow for more of a size increase, but again that requires a different block or major machine work.

An engine’s stroke on the other hand, can typically be increased by five-hundred-thousandths (1/2-inch) or more in some stock blocks. The result is a large increase in cubic-inch displacement. Using an aftermarket block could allow for even more of an increase, and of course, there are always limitations and other things to consider. An engine builder must consider all factors when designing a stroker engine for a particular application. Chapters could be written covering all of those factors, but for this article, the focus will be kept on the physical (dimensional) and dynamic (operational) properties associated with selecting connecting rod length for a stroker engine. We spoke with Tom Lieb of Scat Enterprises, Trip Manley of Manley Performance, and Kirk Peters of Lunati so we could get their take on the effect of engine performance in regards to connecting rod length. It should be noted that these concepts are based on differing rod lengths using the same stroke (comparing a 5.7-inch rod to a 6.0-inch rod in a 383 ci Chevy stroker) not necessarily rod length in general (comparing rod length in a 383 ci small-block to rod length in a 632 ci big-block) unless otherwise noted.

Rotating Assembly Height

When you increase the stroke of a crankshaft, each journal will rotate on a larger diameter. Think ofcrankshaft stroke as a circle. The centers of the crankshaft’s main journals represent the center of the circle. The centers of the connecting rod journals represent the outside of the circle. As the crankshaft rotates (circular motion) the rod journal travels in a circle, which has a diameter equal to the stroke.

When a given cylinder is at top dead center (TDC), the rod journal is directly above zero degrees of rotation on the center of the circle. At bottom dead center (BDC), it is 180 degrees directly below the center. Although the big end of the connecting rod (connected to the crankshaft) travels in a circular motion, the small end (connected to the piston) travels in a reciprocating motion (up and down). The connecting rod converts the rotation of the crankshaft into a reciprocating motion of the piston. The total movement of the piston from TDC to BDC is equal to stroke.
Rotating assembly height is equal to half of the stroke, plus the connecting rod length, plus the compression height of the piston. The goal is to achieve a rotating assembly height that will provide the desired deck volume or clearance for a particular application. Deck volume or clearance is determined by finding the difference between rotating assembly height and the deck height. Deck height is measured from the center of the main journal bores to the top of the block’s deck. An engine where rotating assembly height and deck height are equal is considered a zero-deck engine.

There may be multiple combinations of connecting rod length and piston compression height available for a particular stroker engine. A long rod will require a short compression height piston (distance from the center of the wrist pin to the top of the piston crown), and a short rod will require a tall compression height to achieve the same assembly height. Before selecting which combination of rod and piston you will use, there are a few factors to consider.

Engine Balance

Once the desired assembly height has been determined, rod length and piston compression height are selected. A short rod will require a taller compression height piston than a long rod would require and vise-versa. The weight of the components should be considered. A piston with more compression height will also weigh more than a piston with less compression height for the same application. A heavier piston requires the crankshaft to have heavier counter weights to offset the additional reciprocating weight of the piston. This may even require additional weight to be added externally to the harmonic balancer and flywheel. When this is the case, the engine is considered to be externally balanced.

Any additional weight incurred by using a longer connecting rod has less of an effect on counter balance weight because the connecting rod is both reciprocating and rotating. Reciprocating weight requires more weight to offset than rotating weight. The difference in connecting rod weight is split between rotational and reciprocating while differences in piston weight is only applied to reciprocating weight. Using a lighter piston will allow for lighter crankshaft counterweights and may not require any additional weight to be added externally. When this is the case, the rotating assembly is considered to be internally balanced.

Lieb, says that many times, the connecting rod length is determined by whether or not the engine builder is looking for an internally or externally-balanced engine.

Piston Design and Stability

While on the topic of piston compression height, it is worthy to note that more compression height will allow for more room between the top of the piston crown and ring pack. Manley states, “Performance engines today are all about power adders. For the tuner crowd it’s boost, and for the drag racer, it’s big-blocks on nitrous. With a short rod, the piston pin is moved lower on the piston, creating a better ring pack for boost.” In addition, more compression height can increase the thickness of material on the deck of the piston, which provides increased strength for higher cylinder pressures created by power adders.
Stability of the piston should also be considered. A longer connecting rod will keep the piston further up in the cylinder bore when at BDC than a short rod would in the same application. Keep in mind, this comparison is true, only if we are using the same piston and only changing connecting rod length. If a different piston is used, the location of the wrist pin in relation to compression height could be different, thereby changing piston location at BDC.

This is important if the piston skirt comes out of the bore. The further the piston skirt moves out of the bore, the more piston rock becomes an issue. Piston rock ultimately causes a loss of ring seal. The piston skirt contacting the cylinder wall is what limits the rocking motion of the piston.

Rod Angle

As the crankshaft rotates the big end of the connecting rod, the small end is moving up and down. This creates an angle between the cylinder wall and the connecting rod. The severity of the angle is determined by the ratio of rod length to stroke (rod ratio). Rod ratio is determined by dividing the rod length by the stroke.

Common Formulas For Building Stroker Engines

A few formulas you need to know when building a stroker engine:

  • Displacement in cubic inches = Bore x Bore x Stroke x Number of Cylinders x .7854
  • Assembly Height = (Stroke / 2) + Rod Length + Piston compression height
  • Rod Ratio = Rod Length / Stroke
  • Mean Piston Speed (feet per second) = (2 x Stroke x RPM / 60) / 12
A shorter rod will decrease rod ratio, while a longer rod will increase the ratio for the same stroke. As the ratio decreases, the rod angularity, or angle between the connecting rod and cylinder wall, will increase. The maximum achieved angle always occurs at 90 degrees before and after TDC. Increasing rod angularity (decreased rod ratio) increases the amount of thrust acting on the cylinder wall, and the result is increased frictional loss and wear on the piston skirt and cylinder wall in some cases.

All three rod manufacturers that we consulted had a slightly different view when it came to rod ratio.

According to Lieb, “Any angle that does not exceed 20, 21, 22 degrees is a non-event. When you look at a 410 ci Chevy sprint car engine with a 6-inch rod, that angle is pretty severe, and those engines run pretty good.”

A 410 ci small-block’s rod ratio when using a 6-inch rod, will be in the 1.5 to 1.6 ratio range, depending on the bore and stroke combination used to achieve 410 cubic-inches. The maximum rod angle for a 1.5 rod ratio is just under 19.5 degrees, and that falls into Lieb’s non-event category. He adds, “When you get into big-block stuff where you have a 4.750-inch stroke, then you get into some issues.”

Manley pointed out the large range of ratios from 1.87 in the Nissan GTR engine to less than 1.5 in some big-block strokers. “Rod ratio is not as important as other factors,” stated Manley, referring to moving ring location down with a short rod for boosted engines.

Peters suggests using, “As high a ratio as possible,” citing less rod angularity, reduced reciprocating weight due to a shorter compression height piston (remember, although a long rod will weigh more, the difference is not as significant because it is split between rotating and reciprocating mass), and reduced piston rock as benefits.

Rod length and ratio further affect one of the most important aspects of a stroker engine’s performance — piston speed.

Piston Speed

It is common to see formulas and calculators that will determine mean piston speed. This is simply the average speed of the piston for the given stroke at a set RPM. Mean piston speed will always be the same for the given stroke, regardless of connecting rod length. Peak piston speed, on the other hand, is dependent on rod length.

[A change in performance] has nothing to do with rod length, per se, it has to do with the relationship of the piston when the valves open or close. – Tom Lieb, Scat Enterprises, Inc.

Piston speed is zero at TDC and increases as it accelerates toward BDC. The speed peaks at a specific degree after TDC (ATDC), and then decelerates back to zero at BDC. The piston accelerates on its way back toward TDC reaching its maximum speed at the same specific degree before TDC (BTDC). The peak piston speed (at a given RPM) is determined by the actual rod length and stroke, while the degree of rotation at which it occurs is determined by the rod ratio.

A common error that is made regarding peak piston speed is assuming that it occurs at 90 degrees of rotation — which is not true. Peak speed actually occurs somewhere around 70 to 75 degrees BTDC and ATDC (depending on rod ratio) due to the angle of the rod affecting piston speed and location. Peak piston speed is higher with a short rod compared to a long rod (stroke being the same), because the shorter rod creates a greater angle.

As mentioned previously, the rod ratio determines at what degree in rotation peak speed occurs. As rod ratio decreases (shorter rod), the number of degrees before and after TDC at which peak speed occurs also decreases (in other words, peak speed occurs closer to TDC). This also means the piston starts to decelerate sooner in rotation with a shorter rod. Therefore, piston speed is less with a short rod on the lower half of the stroke (across BDC) than (refer to the graph provided by Prestige Motorsports).

The significance of a rod length’s effect on piston speed is ultimately dependent on piston speed in relationship to valve events. “The rod length and stroke of the crankshaft determines piston speed,” Lieb says. “[A change in performance] has nothing to do with rod length, per se, it has to do with the relationship of the piston when the valves open or close.”

In today’s engine building, one would use a shorter rod when the engine builder wants to improve the scavenging effect at lower RPM. – Kirk Peters, Lunati

This applies to both piston position and speed. The greatest difference in piston position will occur at the largest rod angle, or 90 degrees before and after TDC. A short rod will put the top of the piston further down the bore at this point as compared to a long rod on the same stroke (due to the angle of the short rod being greater). The difference in position has the largest effect on exhaust valve opening and intake valve closing. The opening of the intake and closing of the exhaust occur near TDC where piston position only differs by a few thousandths-of-an-inch or less (because the difference in rod angle between a short and long rod at this point in rotation is minimal).

“In today’s engine building, one would use a shorter rod when the engine builder wants to improve the scavenging effect at lower RPM,” Peters stated.

This is true because piston speed has a greater effect than piston position during overlap. Piston speed is near its peak when overlap begins before TDC. A short rod will carry more speed from the peak back to TDC, and again back toward the peak (in other words, there are less degrees of rotation between peaks). Therefore, rod length can significantly affect the scavenge effect due its affect on piston speed. A short rod will increase piston speed during overlap allowing the benefit of scavenging to occur at a lower RPM than a long rod.
The camshaft’s intake lobe opening ramp also follows right along with piston acceleration. A short rod will provide more piston speed on the opening side, but lower speeds on the closing side. The exhaust lobe, on the other hand, is opening and closing on the BDC side of rotation where a short rod provides slower piston speeds. Therefore, a long rod will increase piston speed during the exhaust events.


Stroker engines provide a significant increase in displacement. While an increase in displacement alone will provide for additional power, there are many factors to consider to get the most out of the increased stroke. Connecting rod length is one aspect to consider when designing a stroker engine.

Rod length changes both the physical and dynamic properties of the engine. Factors such as assembly height, engine balance, piston ring location, and cylinder length are physical features that must be considered, while rod angle and piston speed are dynamic characteristics affected by rod length. The dynamic characteristics will change engine performance based on their relationship to camshaft events.

As an engine builder, it is important to take all aspects into consideration, and understand how one component will affect the overall combination. Rod length alone cannot be generalized as providing a certain change to every engine. Rather, any change in engine performance is due to the rod length’s role in changing the dynamic properties of the entire combination.

Chevy V8 bore & stroke chart

Chevy V8 Crankshaft Journal Sizes

Here's a list of Chevy V-8 crankshaft journal sizes. All journal sizes are given in "STANDARD" sizes. Your crankshaft may have been cut down in size previously by a machine shop. Make sure your crank will work in the block you have. Blocks were made for each crank main journal size. If you are putting a "small" or "medium" journal smallblock crank into a "medium" or "large" journal smallblock block you will need crank bearing "spacers" or use special "thick" bearings available from aftermarket suppliers.

Chevy Smallblock V8 Crankshaft Journal Sizes

Gen.I, "Small Journal"

Gen.I, "Medium Journal", includes "Vortec" 305 and 350 thru '98

Gen.I, "Large Journal"

Non-production Gen.I combination, using Gen.I 400 crank in Gen.I 350 block
383...400 crank, Mains cut to 2.45"-Rods-2.10"

Non-production Gen.I combination, using Gen.I 350 crank in Gen.I 400 block
377..."Spacer" or "thick" main bearings with 350 crank-Rods-2.10"

Gen.II, "Medium Journal", includes "L-99" 265, "LT-1" 350, "LT-4" 350

Non-production Gen.II combination, using Gen.II 265 "L-99" crank in Gen.II 350 block

Gen.III, includes '97-2005 "LS-1" Corvette, Firebird, Camaro

Corvette "ZR-1", DOHC, "LT-5"

262 = 3.671" x 3.10" (Gen. I, 5.7" rod)
265 = 3.750" x 3.00" ('55-'57 Gen.I, 5.7" rod)
265 = 3.750" x 3.00" ('94-'96 Gen.II, 4.3 liter V-8 "L99", 5.94" rod)
267 = 3.500" x 3.48" (Gen.I, 5.7" rod)
283 = 3.875" x 3.00" (Gen.I, 5.7" rod)
293 = 3.779" x 3.27" ('99-later, Gen.III, "LR4" 4.8 Liter Vortec, 6.278" rod)
302 = 4.000" x 3.00" (Gen.I, 5.7" rod)
305 = 3.736" x 3.48" (Gen.I, 5.7" rod)
307 = 3.875" x 3.25" (Gen.I, 5.7" rod)
325 = 3.779" x 3.622" ('99-later, Gen.III, "LM7", "LS4 front wheel drive V-8" 5.3 Liter Vortec, 6.098" rod)
327 = 4.000" x 3.25" (Gen.I, 5.7" rod)
345 = 3.893" x 3.622" ('97-later, Gen.III, "LS1", 6.098" rod)
350 = 4.000" x 3.48" (Gen.I, 5.7" rod)
350 = 4.000" x 3.48" ('96-'01, Gen. I, Vortec, 5.7" rod)
350 = 3.900" x 3.66" ('89-'95, "LT5", in "ZR1" Corvette 32-valve DOHC, 5.74" rod)
364 = 4.000" x 3.622" ('99-later, Gen.III, "LS2", "LQ4" 6.0 Liter Vortec, 6.098" rod)
376 = 4.065" x 3.622" (2007-later, Gen. IV, "L92", Cadillac Escalade, GMC Yukon)
383 = 4.000" x 3.80" ('00, "HT 383", Gen.I truck crate motor, 5.7" rod)
400 = 4.125" x 3.75" (Gen.I, 5.565" rod)
427 = 4.125" x 4.00" (2006 Gen.IV, LS7 SBC, titanium rods)

Two common, non-factory smallblock combinations:

377 = 4.155" x 3.48" (5.7" or 6.00" rod)
400 block and a 350 crank with "spacer" main bearings
383 = 4.030" x 3.75" (5.565" or 5.7" or 6.0" rod)
350 block and a 400 crank, main bearing crank journals
cut to 350 size

ALL production big blocks used a 6.135" length rod.

366T = 3.935" x 3.76"
396 = 4.096" x 3.76"
402 = 4.125" x 3.76"
427 = 4.250" x 3.76"
427T = 4.250" x 3.76"
454 = 4.250" x 4.00"
477= 4.5" bore x 3.76" stroke
496 = 4.250" x 4.37" (2001 Vortec 8100, 8.1 liter)
502 = 4.466" x 4.00"
557T= 4.5 bore 4.375" stroke
572T = 4.560" x 4.375" (2003 "ZZ572" crate motors)

T = Tall Deck

ALL production big blocks used a 6.135" length rod.

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