Rods to close


Active Member
Ok Grumpy
I got my L31 block back from cam bearing installation . I put the cam in, LT4 Hotcam. The rods dont touch the cam, but the large paper clip wont pass between the cam and rod at one certain point in the rotation. This is an eagle rotating assembly externally ballanced. Can I just grind some off each rod? If I can do I need to grind both sides of the rod to keep it ballanced ? Or just the side thats to close? Lol we are still making progress. :roll:


Staff member
IF your installing a longer stroke crank in a SBC, the first step is test fitting and carefully marking the connecting rods and block after carefully checking which rods need clearance, in most cases only two or four cylinders will require cam/rod clearance work, and the amount that is normally required too be removed is small enough that the difference in weight is so minimal it won,t have any significance in the engine balance, so grind the rod only in the potential area of cam to rod interference, only on one side, just at that one location, try hard to allow a .060 rod/cam clearance and no more, and do it in a shallow rounded depression so you don,t create stress risers, with any and all the grind mark lines only parallel to the rods long axis and then polish, the grind marks until the areas smooth with 500 grit sand paper, then clean carefully to remove metallic dust
on many blocks all the block rails will need minor clearance work for the rods to clear the oil pan rails, and don,t forget to verify the counter weight to lower cylinder clearance,


why you need to verify the cam to rod bolt clearance




if you wonder why I suggest using SCAT (H) beam style cap screw connecting rods vs stock or most (I) beam designs this picture should show the increased cam to connecting rod clearance


keep in mind the problem is easy to totally avoid simply by selecting rods designed for stroker engines,EXAMPLE
SCAT and LUNATI both make 7/16" rod bolt rods with far stronger than stock forgings, that have cap screw rods designed for much greater clearance

theres a great deal of useful info in the links so read and take advantage of it ... roker.html ... index.html






both these pictures show a rod that was probably over or excessively ground & clearanced for most engines, careful rechecking as you rough grind,the rod, helps, take off as little material as you can to get the required clearance.


don,t forget to verify these areas also






Active Member
Well I got the rods cleared .060 past the cam last nite. I am going to double check all of them this morning. I had hell with # 2 and 6. I did notice one spot on the pan rail that I did not get cleared enough the first time so I will have to be careful and get it done this morning. I will rewash block when I am satisfied with all clearances.


Staff member
as you do more engines youll get experience and the process becomes easier, almost second nature as they say.
an AIR DIE GRINDER and a carbide burr and cut off wheels make the process fast and easier than electric drills



keep in mind cap screw rods (bolts screw into the upper rod from cap)
generally require less block clearance work, have much more cam-to-rod clearance and are usually stronger than O.E.M. rods
notice how the rod bolts come close to the cam bearings and cam lobes,as the pistons reach top dead canter in the bores, this clearance must be individually checked and should be no less than about .060 (generally you cam use a LARGE plastic tie-wrap


placed between the cam lobe and connecting rod bolts or connecting rod shoulder areas to check clearances as the soft tie-wrap will not damage the cam lobe while you verify clearances)










"I don't see why a 6" rod would require any more clearance than a 5.7" rod. Block and cam lobe clearances are a function of the rod's big end shape and bolt type & height, not rod length. Grumpy?"

first step.
before you start panicking and potentially wasting money and time.

worrying about the connecting rods potentially hitting the cam lobes or the block,
It would be, smart too assemble a single piston and rod assembly without rings,
but ideally with some old bearings on the crank and connecting rod and install the cam, in the block
( indexed with a simple,dot to dot timing on the timing gear sets should be ok at this point)
move that connecting rod and piston to all 8 locations and very carefully verify clearances (remember the rod clearance bevel faces the crank counter weight and the piston valve clearances face the outer block)through the full 720 degree rotational cycle, remember the cam spins at 1/2 the crank speed so the cam lobe comes close to the rod every other rotation,
and actually verify you DO, have or DON,T have a potential clearance problem
theres zero sense in running around pulling your hair out and screaming until,
theres actually a PROVEN ISSUE too SOLVE (THERE MAY NOT BE!)
now if you find theres an issue to be solved you proceed using facts
and while your checking the cam lobe to connecting rod clearance check the connecting rod to block clearance ....yes the same minimum .060-.080 clearance is suggested

generally its a minor easily done clearance job





don,t forget to verify the cam to connecting rod clearances
a cams VALVE LIFT is determined by the DISTANCE the lifter moves as the cam rotates under the lifter base as it moves from the cam lobe base circle
(the closest the lifter comes to the cams center line)
up to the cam lobes ramp to the lobes peak,
(the furthest the lifter up off or from the cams center line)

your forgetting the ROD ANGLE between the crank throw and piston pin, the shorter 5.7" rod is at a degree or two more angle, in relation to the bore center line than the longer 6" rod so the rod bolts ends are slightly closer to the rotational center with the shorter rods, when your concerned with clearance issues where a few thousandths of an inch can cause problems, the rod angle, and rod outer surface,Contour DOES MATTER , and cap screw rod designs do tend to present a much more compact, design allowing more clearance in the out arc the rods surface edges describe as they rotate.

don,t forget to carefully check the piston skirt to crank counter weight clearance, it should be a MINIMUM of .080 thousands
heres some pictures taken of an engine assembly that use a crank designed for a MINIMUM of a 6.25" connecting rod that was used with a 6.135" connecting rod



you can clearly see where the piston pin boss was being hit bye the counter weights, even though the builder checked one piston and found it had .025 clearance during assembly

the result was a trashed engine with lots of damage

piston2.jpg Length.htm
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Staff member
I get asked all the time if 6" or 5.7" rods are the best selection for the 383 sbc builds, the truth is both have some advantages and disadvantages, as an example I selected two very similar forged KB pistons to compare with similar compression ratios that might be used in a kick butt racing combo, with 12.5:1 cpr for a weekend warrior style 383 sbc
so lets look at the differences as they will have similar traits to other pistons

heres a link to KB pistons, Ive used dozens of these sets with good results



advantages & disadvantages of the 5.7" rods and pistons, in a 383 sbc


less clearance for the counter weights , to pass under the skirt, higher thrust loads on cylinder walls
higher reciprocating weight
481 grams piston / 132 grams pin
rings fully supported in oil ring groove, as the piston pin is lower
slightly longer and potentially more stable bore skirt contact area

advantages & disadvantages of the 6" rods and pistons, in a 383 sbc
lower reciprocating weight ... s&P_id=391
more clearance for the counter weights , to pass under the skirt, lower thrust loads on cylinder walls


418 grams piston / 132 grams pin
THATS 63 grams lighter than the 5.7" piston
rings not fully supported in oil ring groove, as the pin by-sects the lower oil ring groove
slightly shorter and potentially less stable bore skirt contact area

related info

Tech Tip - 2005

Rod Lengths/Ratios: Much ado about almost nothing.

Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".

To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!

Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).

To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.

In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!

Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.

Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype.


Staff member
while your checking clearances, remember the valve train, ring end gap and piston to valve clearances, etc. need to be carefully checked
minimum safe clearance?
that depends on the quench distance deck height and cam lift duration and lsa , how the cams degreed in and your rocker ratio
about .100 on the exhaust and about .080 on the intake valve to piston will normally be about MINIMUM required clearance and at lifts over about .480 and duration over about 220 degs @ .050 lift you better start checking carefully, and remember the valves come closest to the piston NEAR 10-20 degrees before and after TDC, not AT TDC

reading thru these threads should help









interesting,(strange actually....)
every machinist I know has a few oz of non-hardening modeling clay to verify oil pump pick-up to oil pan floor and valve to piston, and piston dome to combustion chamber clearances.
but a bit of WD 40 sprayed on modeling clay strips ( about 1" wide and 1/4" thick and 3" long ),keeps them from sticking. and a razor can cut the valve indentation at its deepest point, cutting it cross wise, a dial caliper ... toview=sku

can measure, that thickness and youve got the correct info in about 3 minutes.



once EITHER the duration exceeds about 225 @ .050 or the lift exceeds about .480 in a 383 SBC youll probably have clearance issues with stock connecting rods,and the cam lobes, aftermarket (H) or SOME (I) style stroker rods with 7/16" capscrew bolts are stronger and profiled to clear and use of both a small base billet roller cam and stroker profile rods will usually prevent that


Staff member

BTW if your running a flat tappet cam INSIST ON A P55 core, to have it ground on as they are far more durable than the cheaper cores unless you ask for and pat for the better quality core you get the standard core every time from the big manufacturers, its fully adequate in most cases its just not the best available.
its advisable to order the better core if your looking for max durability and it depends on the cost if its worth it to most guys


cast cores generally look cast like the lower cam while steel billet cores tend to appear to be machined and frequently have that copper coated finish

A small base circle cam is where the cam's core (the round metal part of cam that the lobes and bearing journals are made onto) is a smaller DIAM.

You do this, not for greater lift but for rod bolt/crank to cam clearance on stroker engines.. When you add a longer stroke crank into a block that wasn't meant to hold it..

Like a Chevy 383 stroker (3.75" stroke crank into a block made for a 3.48" stroke)

The rod bolts will hit the cam, as the piston/rod moves up the bore because the crank now pushes rods/pistons .27" (over a 1/4 inch) higher up

By using a cam ground on a smaller core shaft, the clearance prob is gone..

You can also grind the shoulders down on the
tops of the rod bolts, or use aftermarket rods that are already made stroker clearanced and run a std cam
rods that use bolts with nuts like pictured below will be weakened if excessively clearance ground

stroker profile rods offer more clearance to cam lobes, and yes the stroker clearanced profile rods are available in both (h) and (I ) beam designs


generally its a minor easily done clearance job

"Small Base Circle Cams

When building your stroked-crank small-block Chevrolet V-8, one of the main factors in choosing your camshaft will be to decide if a small base circle cam is needed. There are certainly a number of factors that enter into sorting this dilemma out: What is the stroke of the engine? Are the connecting rods steel or aluminum? Have the rods been profiled by the manufacturer for camshaft clearance? What does the rod supplier say about camshaft clearance?

When you have the answers to these questions, you can then order the camshaft. It's always best to use as large a base circle camshaft as possible, as it should be stronger (larger barrel between the lobes), and less likely to wear (larger minimum radius of curvature of the lobes). This is of minimal consequence when using a roller camshaft (either hydraulic or mechanical), as we make special steel cam cores for small base circle grinds. A less pleasant situation occurs if you want to use a cast hydraulic or mechanical flat-faced lifter cam in your engine. The primary issue is wear. When a conventional flat-faced follower lobe is ground smaller, the lobe's minimum radius of curvature (which is usually at, or near, the nose of the lobe) decreases, which then causes the lobe to become more "pointed." This will drastically increase the probability of premature lobe wear, and can also lead to core breakage due to the smaller barrel that will be required. Therefore, we will produce small base circle cast cams on special order, but they will have absolutely NO GUARANTEE regarding wear or breakage. A better choice is to use a full sized cam, and additionally clearance the rods (usually only three of them will have an interference condition), or go to a small base circle roller cam and components.

The term "small base circle" is also a bit misleading, as the rod normally interferes with the lobe where it is well into its lift, not at the base circle. Shorter duration cams should have less possibility of rod interference than long duration cams. The lobe separation angle ("centerline") will also influence this clearance. Higher lift cams will already have smaller base circles, as the lobes can't be higher than the journals. As a general rule, try to maintain .035" minimum clearance between the rods and lobes to be safe. It's best to always check this clearance whenever changing brands of rods, when increasing the crankshaft stroke, or even when advancing or retarding your cam if you've been on the close side of the tolerances before.


(size limited by the 1.868" diam. cam bearing journal).

most stock cams have a base circle of at least 1.12 inches, a small base cam can be made on a steel core down to about .900 or a bit smaller giving you an extra .100 clearance or more

(.900) ... 1&x=34&y=8
(you need to specifically order a small base circle with crower) ... index.html ... 383_cu.htm

failure to keep the pistons from hitting the valves, bending valves, ,over reveing the valve train and having un-controlled valve movement, or having chunks of piston,that detonation can break loose, being compressed against the heads,can result in the cracked cylinders, and bent rods like the pictures below show




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