crankshaft journal surface finnish

grumpyvette

Administrator
Staff member
I must point out the links hold a great deal of useful info


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federal mogal said:
When refinished, the surface of a crankshaft will develop microscopic peaks which are “tipped” in the direction that the sparks spray during grinding (see the illustration above). If these peaks point toward the oil film area when the engine is running, lubrication is interrupted, and the bearing will show premature wear. It is important that the crankshaft be ground and final polished so that these peaks are tipped opposite the direction that the crank rotates when it is installed in the engine, this is referred to as the “favorable” direction. We recommend grinding the crank in the “favorable” direction, followed by a multi-step polishing process using progressively finer paper. The first polishing operation uses 280 grit paper with the shaft rotating in the reverse direction – this helps to “knock off” some of the raised material left over from grinding. The second polishing process uses 320 grit paper, and the crank should be rotating in the “favorable” direction. A third step polish with a very fine (400 grit) paper is optional, but should again be done in the “favorable” direction. If the thrust surface was contacted during the resizing operation it must also be polished.

HERES WHAT A MACHINE SHOP CRANK POLISHING BELT LOOKS LIKE

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care must be taken to ensure the journal does not get polished unevenly, tapered or egg shaped
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journal surface must be polished so micro burrs face away from the direction of rotation on bearing surface for max durability on bearing surface, burrs far to small too see or feel still induce wear
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http://www.enginebuildermag.com/Article ... nough.aspx

http://garage.grumpysperformance.com/index.php?threads/precision-measuring-tools.1390/#post-68861

http://www.engineparts.com/it_crankgrind.asp

http://engineparts.com/publications/CL77-3-402.pdf


http://www.enginebuildermag.com/Article ... ished.aspx

http://garage.grumpysperformance.com/index.php?threads/can-i-get-it-polished.9214/

http://www.hoigto.com/tech/crankgrind.htm

http://www.circletrack.com/techarticles ... index.html

http://mahleclevite.com/it_crankgrind.asp

http://garage.grumpysperformance.com/index.php?threads/measuring-crank-bearing-journals.5478/

http://www.enginebuildermag.com/Article ... nough.aspx

Crank And Cam Polishing: Are You Smooth Enough?


It is more important than ever for engine builders to be as perfect, or near perfect, as possible when it comes to surface finish requirements.

By Brendan Baker

Brendan Baker

Manufacturers are designing today's engines with tighter tolerances and less room for error. They make more power, live longer, produce less noise, vibration and friction, burn less fuel and produce lower emissions. So in light of all this, it is more important than ever for engine builders to be as perfect, or near perfect, as possible when it comes to surface finish requirements.

Crankshaft and camshaft finishes are no exception. In today's engines, rotating assemblies ride on a thin wedge of oil only .00005? thick in some cases. And to help reduce friction as much as possible, oil itself is much thinner today as well, so it is especially important to achieve the proper surface finish on all components in order to avoid problems down the road.

Engine builders big and small have the same need to produce a smooth surface on cranks and cams, but their respective budgets and business volumes may dictate what equipment is used to get the job done properly. Large production engine remanufacturers (PERs) can justify purchasing a micropolisher and an automated surface finish gauge while small custom engine rebuilders (CERs), by-and-large, feel they can't afford such luxuries. This is not to say that it's out of the realm of possibility for CERs to afford a micropolishing machine or a surface finish profilometer; but higher volume is generally what justifies making such a purchase.

Belt Polishing
Belt polishing is the traditional method, used by engine builders for many years, to polish crankshafts as well as camshafts. In the past, belt polishing worked well to easily produce surface finishes that were extremely close to OEM levels. Today, however, vehicle manufacturers have automated industrial polishing machines that cost many thousands of dollars and produce very smooth and consistent results, results that are increasingly more difficult for engine builders to reproduce with manual equipment. Difficult, perhaps, but not impossible.

"We achieve an extremely fine finish on the grind, before we polish." says Bob Heidbreder, Northampton Crankshaft in Cuyahoga Falls, OH. "Our method is to first dress the grinding wheel very fine and then use polishing belts for the final finish. We do all different styles of crankshafts this way and we've never had a failure or a comeback because of the finish."

Customarily with late-model crankshafts, says Heidbreder, rebuilders mic the journals and go through a two-step polishing process. "If the crankshaft is salvageable and you don't have to grind it, you polish it with a #400 grit aluminum oxide polishing belt. And then, if you need, you would have the option to micropolish the crank to achieve a finer Ra finish. Of course, this depends on the crankshaft you're working on but most want it as smooth as possible."

Aftermarket distributors recognize the demand for near-OE quality finishes and offer rebuilders a number of products. "We have a brand new belt that we introduced about a year ago," says Chris Jensen, Goodson Shop Supplies, Winona, MN. "It's a GSW micropolishing belt and it is the best thing we've found for final crank polishing. You put it on with a little crank polishing rouge and turn it approximately 10 rotations."

According to Jensen, these belts have even worked well enough for a number of NASCAR racing teams to give them a try. These teams now use them for final polishing the cranks in their race engines.

"What is so nice about this belt is that there's a serration on it so it can polish the large radius on these high performance cranks," says Jensen. "High performance cranks have a large radius for strength while many production engines have virtually no radius."

A number of other distributors offer micropolishing belts as well as portable belt polishers. Tom DeBlasis, K-Line Industries, Holland, MI, says portability brings polishing capability to the price range and user abilities of almost every rebuilder.

"We offer a couple of portable crankshaft polishers: one electric and one pneumatic, which can be used on either a crank grinding machine after you move the head away or on a rotating polishing stand," says DeBlasis. "Most guys start out with a #320 grit belt, then go to a #400 grit and then progress to a very fine micro-polishing belt for a few revolutions, depending on the application."

Micropolishing
"A long time ago stones were used to polish cranks and cams but that technology has gone by the wayside now," says Ken Barton, QPAC, Lansing, MI. "Today, micropolishing is technically the most advanced way to achieve OEM-level surface finish on cranks and cams."

Barton says many engine builders believe in some polishing "myths," which can impact a shop's profitability. "Some rebuilders believe if you use very fine grit belts you will not remove any material. That's kind of a misconception. You always take off some metal when you polish," says Barton.

"When you are done grinding the piece with the grinder it looks like a mountain range," Barton continues. "If you put a micrometer on the piece you're going to measure from the highest peak to the highest peak. So when we take those peaks down and you remeasure it, you're going to get a different measurement. It might be just a small amount of material, but some material is being removed."

Typically that amount may be as small as .0002? according to Barton. One of the challenges is convincing shops that such a small amount actually makes a big difference. "Of course, you have to take material off to achieve the correct surface finish. With micropolishing you're taking off the peaks and getting down towards the valleys, and the more peaks you remove the more surface area you have," he says.

Another misconception that some users have is they think they can put a polishing belt on and go crazy with it. "Some people think making the part shiny is enough, but it probably has taper, crown and who knows what else," says Barton. "If it is within a couple of tenths the error is still there. That's why we back our machines up with a rigid shoe behind the tape. So now you have a rigid setup that won't taper or go out of round. That way we keep things round and flat. When you finish you index the tape approximately one inch and go on to the next journal."

Micropolishing machines use a polishing tape instead of a belt like a belt polisher does. The tape comes in approximately 150-ft. rolls, and when used on a micropolishing machine operators index each roll after each use. When a crank or cam is polished on one of these machines, it uses about one inch of tape per journal, so fresh abrasive is used for every polishing job, but because it's such a small amount, many users say they've saved money. "One PER reported a 50-75 percent savings in the cost of tape vs. belts," Barton says.

The type of abrasive tape you should use for grinding cranks and cams varies, according to Barton. "From our position it all depends on what is on the machine. Is it a steel, cast iron or nodular iron part? Is it a hardened steel or is it a forged part? Then once you get through the material, what kind of surface are you grinding? We've had surfaces as high as 45 Ra and as low as 15 Ra. All that determines what abrasive you use on the polishing tape. It could be 9 micron or it could be 50 micron (note: 20 micron is roughly equal to #600 grit). It just depends on how aggressive you need to be."

Barton understands that not every shop will purchase his equipment. "For shops to really see a payoff they need to rebuild about 15 cranks or engines a day. Some specialty and high performance shops that sell engines and cranks for much higher prices than average can also make their ROI."

Measuring the Finish
According to some of our experts there are still engine builders who refuse to use any measuring devices other than a fingernail to measure surface finish. Yet increasingly, it is more important than ever to know exactly what surface finish you have. Without a measuring system, however, you can't know exactly what you have when you finish polishing.

While you don't have to measure every piece, measuring Ra during a spot check is a good step toward safeguarding against problems you may not even know exist. There is hope that this trend is going to change as the industry becomes more aware of the need to measure surface finishes.

K-Line's DeBlasis says that a lot of engine builders are at least starting to look at portable profilometers. "Profilometers are not inexpensive and I think that's why some smaller shops are just looking right now, but we're starting to sell more of them," says DeBlasis. A portable profilometer can cost up to $2,000, according to DeBlasis.

There are three characteristics to understand about surface finish: profile, waviness and roughness. According to John Wilt who works with the American Society of Mechanical Engineers Board of Standards on surface finish specifications, a sand dune is a good example of all three. "The overall shape of the dune would be equivalent to the profile, and as you moved in closer to see the windswept ridges, that would be waviness, and the grains of sand would represent roughness," Wilt says. "Every crank and camshaft has these three features as well, and technically they all fall under the heading of 'surface finish.'"

However, just because they all represent an element of surface finish, it's important to approach each component separately.

"Profile is the size of something," says Wilt. "It could be the diameter. It could be the shape as far as being round or tapered or hourglass shaped. Profile can also be a length measurement. So the journals and the lobes all have to be the right shape and in the right location."

The second phase of surface finish is waviness or lobing. "If you go along the axis of a crankshaft journal or camshaft journal it would be considered waviness. If you go around the part it's now called lobing. And a very frequent slang term used for this is called chatter. If waviness is out of spec, that's where problems with noise and vibration come into play.

"Basically, if you go back to the sand dune analogy, if you were to walk a little closer to the dune and saw all the windswept ridges, there's something separate that caused that to happen than the profile itself," says Wilt.

The third phase of surface finish is roughness. "If you walk up and grab a handful of sand from the dune, you'll recognize that the grains represent roughness. Surface finish is kind of like grains of sand paper that over time creates wear. The valleys in the grains of sand actually are a major contributor to holding lubricant," says Wilt.

Wilt's company, Adcole, manufactures surface finish equipment designed to measure all three categories independently. It can measure a crankshaft or a camshaft, giving the manufacturer or rebuilder the ability to look at the shape, size and location first (profile) to ensure that it's correct. "If the profile is not right the rest doesn't matter. If it doesn't assemble or go together it's not going to work anyway," Wilt notes.

Roughness Average?
When you read the term "Ra" does it mean anything to you? "If you ask anybody in the industry, we've all seen a surface finish symbol: a checkmark with a number, or the roughness symbol," says Wilt. "The problem is that most people assume they know what it means."

While shops can often easily get an Ra reading using a handheld device, they may not really understand what it means. One reading may be different than another. According to Wilt most people think Ra is the peaks and the valleys - but it is only an area measurement.

Wilt continues. "If you say you have an Ra of one micro-inch it's hard for people to understand just what that means. It's like if I say I have one acre of land for sale for $5,000: would you want to buy it? What do you know about my acre of land? You know only one thing - it's an acre and it's got so many square feet. If you know the area is rectangular then you can describe it by its spacing and its peaks, but you really don't know anything about the area. Saying a surface has such an Ra is exactly the same thing."

"When you have your Ra you don't really know what that looks like," Wilt explains. "It could look like a saw tooth or it could look like a square wave, where it goes up and goes across a little and then it goes down or it could be much spikier or wavier. The net result is you have different capabilities for bearing load. In many cases, simply looking at the roughness average is not necessarily the best way to look at that surface. You want to make sure there's enough bearing area and you also want to make sure you don't gouge the bearing surface. So ultimately the part that's contacting the connecting rod bearing, for example, must have enough surface to bear the load. It's not just two contact points that will be crushed as soon as the engine rotates."

Favorable and Unfavorable Direction
One way to achieve the proper surface finish on cranks and cam journals is to grind them in the opposite direction they normally rotate in the engine. Most automotive cranks typically rotate clockwise, but some industrial and marine engines rotate counter clockwise. So you have to know which way the engine rotates before you mount it in the polishing stand.

Polishing the crank or cam in the opposite direction it was ground will also break off more of these ferrite burrs, leaving a cleaner smoother finish as well. Ferrite burrs, if not removed, can cause problems later on because they can wipe away the oil film and cause a bearing failure.

Not all engine builders agree this is necessary, but it should remove the sharp edges of the ferrite burrs leftover from the grinding process and leave what is called a "favorable" finish.

One rebuilder we spoke with says he grinds one direction and polishes the other to get as smooth of surface as possible. However, other engine builders we interviewed say they have not noticed any difference, no matter which way they polished the part.

The main goal in polishing any crankshaft or camshaft is to achieve the smoothest, flattest surface as possible. When cranks and cams are properly lubricated they turn and rotate very smoothly, which does two things: minimizes wear and more importantly it minimizes heat and fatigue. You want valleys for lubrication but also plateau peaks, not sharp peaks, to handle load.

So it is important to remember when you're polishing that although it looks like art when you are finished, it really comes down to a science.

http://www.epi-eng.com/piston_engine_technology/piston_motion_basics.htm

Crankshaft Inspection

  1. Perform a visual inspection of the crankshaft
    • Check for cracks and signs of overheating
  2. Measure Crankshaft
    • Measure journal diameter
    • Measure for out of round
    • Measure taper
    • Measure run out
    • Tools needed: Dial gauge
    • Tools needed: Dial bore gauge
  3. Mark crankshaft for identification
  4. Measure after grinding
    • These measurements are needed for correcting bearin torlerances
    • Use a rifle brush through oil passages to clean out debris after grinding
    • Tools needed: Rifle brush
    • Tools needed: Dial bore gauge
    • Tools needed: Dial gauge
  5. Change pilot bearing

    If no engine balanceing is required go to engine block inspection Task A09
  6. If engine balancing is require; supply the machine block with required parts
http://garage.grumpysperformance.com/index.php?threads/precision-measuring-tools.1390/#post-68861
 
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grumpyvette

Administrator
Staff member
http://www.enginebuildermag.com/Article ... ished.aspx
Crankshaft Polishing: Make Sure The Journals On The Crankshaft Are Properly Polished


By Larry Carley

Larry Carley

Today's high output, close tolerance engines are more dependent than ever on quality remanufacturing procedures, durable parts and precise machining. One of the best ways to assure long bearing life in today's engines is to make sure the journals on the crankshaft are properly polished.

The oil film between the journals on the crankshaft and the loaded portion of the main and rod bearings is only about .00005" thick when the engine is running. If the journals are too rough or have burrs, particles or other debris that sticks up above the surface, it can abrade the bearings and increase bearing wear and the risk of bearing seizure.

Cast iron cranks typically contain about 4% carbon. The carbon forms little nodules of graphite surrounded by a relatively soft form of iron called "ferrite." When the crankshaft journals are ground and polished, the ferrite around the graphite nodules forms little burrs or jagged flaps that protrude above the surface. The height of these burrs can be as much as .00035", which is more than enough to cut across the oil film and dig into the bearings.

Ferrite burrs create a sawtooth-like finish on the surface that is directional, usually facing away from the direction the journal was ground or polished. If the sharp edges face away from the direction that the crankshaft normally rotates, it is said to be a "favorable" orientation because the burrs are less likely to dig into the bearings. On the other hand, if the sharp edges are towards the same direction of rotation, it is an "unfavorable" orientation and is much more likely to cause problems.

The trick, of course, is figuring out which way is which - that is, which way to grind the crank and which way to polish it to achieve the proper orientation of the ferrite burrs.

The ultimate goal when polishing crankshaft journals is to achieve a relatively flat and smooth surface finish (an average roughness of 10 microinches or less) with plenty of bearing surface to support the oil film. But it is also important to orient the remaining ferrite burrs in a favorable direction so they will have less of an abrasive effect on the bearings.

With forged steel cranks, there are no graphite nodules or ferrite to worry about, so it isn't necessary to grind the crank in one direction then polish it in the opposite direction. Even so, for best results, the recommendation is to polish a steel crank in the same direction it rotates.

Polishing techniques
One way to achieve an optimum surface finish on the journals of a cast iron crank is to grind the crankshaft journals in the opposite direction it normally rotates in the engine, then polish it in the same direction it rotates in the engine. This will leave a favorable finish with the sharp edges of the ferrite burrs facing backward. Polishing the crank in the opposite direction it was ground will also break off more of the ferrite burrs leaving a cleaner, smoother finish.

According to Steve Bleggi, sales manager for Abrasive Accessories, Inc., Frisco, TX, a polishing belt with #320 or #400 grit abrasive is typically used depending on the surface finish requirements of the application. The most popular sizes are a 1" x 64" and 1" x 72" belt size.

Ian Bagnall, sales manager at RMC Rogers Machine Co., Bay City, MI, says most automotive crankshafts usually rotate clockwise in an engine. Some marine and industrial engines rotate counterclockwise, so the first thing you have to determine is which way the crank normally rotates before chucking it up in a grinder or polishing stand.

"Most crankshaft grinders and polishing stands rotate the crankshaft toward the operator (clockwise if viewed from the left end of the machine, counterclockwise if viewed from the right end)," says Bagnall.

"If the crank is mounted with the nose to the right, the crank will spin in a counterclockwise direction in the machine - which is opposite its normal direction of rotation in the engine. If the crankshaft is mounted in the grinder or polishing stand with the nose to the left, on the other hand, it will turn in the same direction it rotates in the engine."

Which way should the crank be mounted to achieve an unfavorable orientation when grinding and a favorable orientation when polishing? Bagnall says the grinding wheel on most crank grinders also rotates counterclockwise so the sparks and debris are thrown down as the journals are refinished.

This will leave ferrite burrs that are oriented in an unfavorable direction on the journals if a crank that normally rotates clockwise in an engine is mounted with the nose to the right. If the crank is mounted with the nose to the left, the grinding operation will leave the ferrite burrs with a favorable orientation and reduce the effectiveness of the polishing step.

To produce the best finish, the crankshaft must be turned around after it's been ground so the nose is to the left for polishing. This is necessary because the abrasive surface of the polishing belt that rides on the crank journal moves away from the operator and throws the dust and debris backward and out of the way.

However, if the crankshaft is mounted with the nose to the right and turns counterclockwise in the equipment, the belt will be polishing in the same direction the crank was ground. This will reduce the effectiveness of the polishing step and leave an unfavorable orientation on the remaining ferrite burrs. Turning the crank around so the nose is to the left for polishing will remove more of the burrs and leave a favorable orientation which is the best surface finish for the bearings.

Not everyone agrees with this recommendation. Some say they have achieved good results regardless of which way the crank is mounted, ground and polished. Some rebuilders say they've ground and polished crankshafts in both directions with no bad results. If the finish on the shaft is smooth enough, the rotation in which it is ground shouldn't matter. Even so, a microscopic examination of the surface finish will usually show the best finish on a cast iron crankshaft is achieved with an unfavorable grind and favorable polish.

Why not just mount the crank in the grinder with the nose to the left for both grinding and polishing? This approach saves time because you don't have to reposition the crank after grinding - but it leaves a favorable orientation of the ferrite burrs which will reduce the effectiveness of the polishing operation.

An alternative method is to use a two-step polishing procedure. Though all the bearing manufacturers do not agree on polishing procedures, Ron Thompson, a bearing engineer at Federal-Mogul Corp., Detroit, MI, says an improper crankshaft finish can be especially hard on bearings. When using belt-polishing equipment, he recommends polishing the journals in the unfavorable direction (opposite the direction of rotation) with a #280 grit belt, then finishing the journals in the favorable direction (same direction as rotation) with a #320 grit belt.

Polishing with tape
Another way to polish the crank journals after grinding is with equipment that uses microfinishing tape rather than an abrasive belt. This type of equipment works differently than a belt polisher. Instead of rubbing a rotating abrasive belt against a rotating journal, the abrasive tape remains stationary and is clamped against the journal as the crank turns.

The tape makes contact at four points, which the suppliers of this type of equipment say produces a more even and consistent surface polish - though the appearance may be somewhat duller than what many people are used to seeing. The tape is then advanced about an inch for the next journal, and so on until all the journals have been polished. A lubricant is also used with the tape to help wash away debris.

Compared to belt polishing, which may remove .0002" to .0005" or more inches of metal from a journal depending on the belt grit, length of polish and pressure exerted by the operator, tape polishing removes almost no metal. The abrasive on the tape is very fine. A 15 micron tape abrasive is similar to a #600 belt grit. Polishing a cast iron crank with a ground finish of 12 RA for 15 seconds with the 15 micron tape, for example, can improve the finish to 7 RA or better.

One of the advantages claimed for tape polishing is that it reduces the risk of operator error. The pressure exerted by the tape on the crank is fixed and does not depend on how hard the operator is pushing down on a handle.

Another purported benefit is more consistent results. The cutting action of a polishing belt changes as it wears. A new belt cuts more aggressively than a used belt. Tape polishes the same way every time.

Tape also costs less over the long haul than belts. A roll of tape costs about $33 and typically does about 200 cranks if the tape is advanced about half an inch per journal. But the initial investment in tape polishing equipment is much higher than traditional belt polishing equipment.

Hand belt polishers typically cost $500 to $700 depending if the polisher is air or electric powered. Belt polishing stands typically sell for $2,000 up to $2,600 depending on the size of cranks the stand can accommodate.

Tape polishing equipment, by comparison, can cost from $13,000 for an aftermarket polisher up to $50,000 or more for an OEM type of unit. So a high volume of cranks is usually needed to justify the investment in a tape polisher.

Ken Barton of QPAC Corp., Lansing, MI, says his company supplies the original equipment vehicle manufacturers with tape crank polishers. "The OEMs use tape to polish virtually all crankshafts today," said Barton. "They typically grind a new crank to 25 to 30 RA, then polish it to finish specifications which may be 8 to 12 RA for an automotive crankshaft or 5 to 7 RA for a diesel crankshaft."

Barton says the best results with tape polishing are achieved when the crank is ground the same way it turns in the engine, then polished the same way. He says this gives longer tape life as well as a favorable finish.

Mark Jeltema, product specialist and provider of tech support at K-Line Industries, Holland, MI (K-line has an exclusive agreement with QPAC to supply the tape polishing technology to the aftermarket), also says the best results are achieved when the crank is turned in the same direction it normally rotates in the engine when it is polished with tape.

"Our machine is reversible, and the nose normally mounts to the left," explained Jeltema. "This will leave a favorable finish with a journal finish in the 3 to 6 micron range with a 30 second polish using 15 micron tape."

Polishing experiences
Tom Bagley at Grooms Engines, Parts, Machining, Inc., Nashville, TN, says his company recently acquired a used OEM tape polishing machine. "It's an oscillating type of machine that uses 30 micron tape," he explained. "We run the tape for 10 seconds on each journal with the crank turning first one way, then the other way, the last one being in the favorable direction. This gives us very consistent results in the 6 to 9 RA range."

Bagley says he uses the tape polishing equipment on both cast iron and steel cranks. "Compared to a belt polisher, it takes a little longer to clean up the journals with the tape machine, but we've very happy with the results," he said.

Bagley said he previously used a two-step polishing procedure with a belt polisher. "The tape machine is a one-step process, but we still put a different surface finish on journals that run against oil seals," he said. "We use a 40 micron tape that leaves a 14 to 18 RA finish. We feel this helps hold the oil better for a good seal than a highly polished surface."

Steve Schmidt at Jasper Engine and Transmission Exchange, Jasper, IN, says his crankshaft department has recently switched from belt polishing to tape polishing. "Tape can't do 80 crankshafts a day and is slower than belts, but we're consistently achieving journal finishes in the 7 to 8 RA range with a 3M 30 micron tape," Schmidt said."

Polishing pitfalls
One mistake that's sometimes made when belt polishing a crankshaft is overpolishing the journals. Whether the operator is trying to achieve a bright, chrome-like appearance or is trying to clean up excessive roughness left by the grinding operation, excessive polishing can create a "halo effect" around the oil holes. The depressions created will reduce the bearing area and strength of the oil film which may lead to premature bearing failure.

The amount of pressure that's exerted against the journal by a polishing belt will also affect the cutting action of the belt and the amount of material removed from the journal. A very light pressure is all that's needed, and for no more than a few seconds. Excessive pressure can change the geometry of the journal leading to clearance problems and increased oil leakage.

If a nitrited crankshaft has been ground to more than .010" undersize, the crank will have to be renitrited after grinding, then straightened prior to polishing.

Don't forget to polish the fillet radii and the seal surfaces as well as the journal bearing surfaces. Some crosshatch on rod and main journals is desirable, but seal and thrust surfaces should have a straight line polish.
 
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Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
http://www.autoserviceprofessional.com/article/92691/inspecting-crankshafts

Inspecting crankshafts
Posted on October 17, 2012 by Mike Mavrigian -

After the used crankshaft has been cleaned, the very first check should be for cracks. Here a crank is passed through a magnetic field on a particle inspection station.
M-p48a-1.jpg



Regardless of which crank you choose to use during a customer’s engine rebuild or fresh build (used OE or new aftermarket), take the time to inspect the crankshaft. In the case of a new crankshaft, check for dimensions and runout. With a previously used crankshaft, you’ll also need to check for flaws (cracks). Inspecting the crankshaft before installation verifies its condition and allows you to avoid problems and/or comebacks.

First and foremost, especially when dealing with a used crankshaft, clean the crank thoroughly, preferably in a jet wash or hot tank. Once clean, always inspect the crankshaft for flaws/cracks. This is best done on a magnetic particle inspection station (commonly known as a magnaflux machine (even though “Magnaflux” is actually a brand name, other equipment makers, such as DCM, for example, make magnetic particle inspection equipment). The crank is mounted horizontally on the inspection bench and passes through a large diameter circular magnet and inspected with an ultraviolet (“black”) light. Any cracks are easily found, visible as whitish lines.

If a crank is cracked, don’t even debate the issue — sell it as scrap metal and buy a new one. By crack-checking first, you’ll avoid wasting time by performing further dimensional inspection.

Next, check crankshaft runout. With the crankshaft mounted level on a pair of level V-blocks (resting on the front and rear main journals), set up a dial indicator at the center main journal, placing the indicator probe slightly offset to avoid hitting the journal’s oil feed hole. Preload the indicator by about 0.050-inch and then zero the dial.
Once a magnetic field has been obtained, an ultraviolet light is used to inspect for cracks.
M-p48b-1.jpg


Slowly rotate the crankshaft while observing the gauge. Record your reading. For example, the maximum OE-spec for allowable runout may be listed as 0.000118-inch. If the indicator gauge doesn’t read in the hundredths of a thousandth of an inch, you’ll be hard-pressed to actually determine that tiny number. Generally speaking, if the crank shows less than 0.001-inch runout, it’s probably fine. If the crank shows more than 0.001-inch runout, it needs to be either straightened or replaced. Crank straightening is a precision task that should only be handled by a skilled specialist. Not all cranks can be successfully straightened, by the way.

Using a micrometer, measure the main journal diameter (at the center of the journal) of each of the main journals. Record your measurement and compare this to the specifications for that particular engine.

Published specs will include a tolerance range (max/min), usually of about 0.001-inch (for example, 2.558- to 2.559-inch). Bear in mind that, if using a reconditioned crankshaft, the main journals may have been re-ground to a smaller diameter in order to maintain serviceability (for example, the mains may have been ground –0.010-inch undersized).
Also measure each main journal for taper (measure the journal area at two locations, toward the front of the journal and toward the rear of the journal). Maximum allowable journal taper is generally about 0.0004-inch.

Also, be sure to measure each main journal at several radial locations to check for journal out-of-round. Maximum allowable out-of-round is usually around 0.000118-inch or so (check the make/model engine specs).

Next, measure each rod journal diameter at several radial locations on each rod journal. The tolerance range (min/max) will generally be around 0.0008-inch or so (for example, rod journal diameter might be listed at 2.0991 – 2.0999-inch).


If the crank passes the crack-check, next inspect for runout. Here a crank rests on a stand that allows rotation. A dial indicator is set up at the center main journal and the crank is slowly rotated to inspect for runout.
M-p50-1.jpg



Measure each rod journal for taper (at each end of the journal surface). Maximum allowable rod journal taper is generally around 0.0002-inch. Also measure rod journal width (base of fillet to base of fillet — in other words, the front of the journal and the rear of the journal relative to crank length), and compare this to the listed spec. If journal width is too tight, you’ll have insufficient connecting rod sideplay.

If any beyond-tolerance areas are found in terms of journal diameter, taper, width or out of round, this can be corrected by re-grinding on a dedicated crankshaft grinding machine. In order to correct journals, you’ll end up moving to an undersize (smaller diameter than original), in which case you can easily purchase a set of undersized-I.D. main and/or rod bearings (bearing pairs with a smaller I.D. and thicker walls).




Whether the crank is new OE, reconditioned, used, or new aftermarket, measure each main and rod journal diameter with a micrometer and compare your readings with specifications.
M-p52a-1.jpg



Just remember that bearing size needs to be uniform — if one main journal must be re-ground to then accept an undersize main bearing, then all of the main journals should be ground to that same size. The same holds true for rod bearings. If even only one rod journal needs to be undersized, then all rod journals need to be ground to the same diameter. Always check with your bearing supplier to first find out what undersize bearings are available (-0.0005-inch, –0.005-inch, –0.010-inch, –0.020-inch, etc.). This will determine the diameter of the re-grind.


In order to check crank endplay, you’ll need to temporarily install the crank to the block. Install upper main bearings dry (block saddle and the rear of the bearing must be dry). Once the bearing has been installed, then apply a lubricant to the exposed bearing surface using oil or assembly lube.
M-p52b-1.jpg


3




If a used crank checks out OK and you intend to re-use it (with no need to re-grind), each journal can be polished on a crankshaft belt polisher, using 400 grit, stepped up to 600 grit. Small surface scratches can also usually be eliminated by polishing. NOTE: Different equipment makers may specify different grit-grade abrasives for polishing. The journals should not be “mirror” polished, since microscopic scratches are needed to provide oil cling.


In order to check crank endplay, you’ll need to temporarily install the crank to the block. Install upper main bearings dry (block saddle and the rear of the bearing must be dry). Once the bearing has been installed, then apply a lubricant to the exposed bearing surface using oil or assembly lube.
M-p52c.jpg


Preference on assembly lube?

50% marvel mystery oil
marvel.jpg

and 50% crane moly lube, or the paste moly, the mix of moly paste and M.M.O. is generally applied liberally with the paint brush, in multiple applications to surfaces like cam gears, timing chains, lifters, rockers, and cam lobes, to provide an extra layer of lubrication protection on initial engine start up.
crn-99004.jpg

what Ive used for decades
but this works

permassembly.jpg

I have used J&B WELD EPOXY on a large magnet
https://www.zoro.com/value-brand-ring-magnet-98-lb-pull-10e797/i/G4187224/
Z_o3v-kcpEx-.JPG


on the base of an aluminum 1/2 cup measuring cup I purchased at a yard sale for 25 cents to mix up the mixture, the magnet allows me to stick the cup to the block oil pan rail or engine stand where its handy too get at, and I simply brush on the mix with a 1" paint brush, with synthetic bristles that won,t shed
molypaste.jpg

OH! slide it off the block don,t try to just pull it off , its going to be much less messy that way trust me!
when your done , wipe it clean and stick it inside the lid of your tool box , after placing it in a ziploc bag to prevent it from picking up trash while in storage

1aae7f99-2e0a-4629-aae6-e646e15a8533_1000.jpg


shopping

Buying a replacement crankshaft

Your customers have several choices when purchasing a crankshaft, including a new OE crank, a reconditioned OE crank or an aftermarket crank. OE crankshafts are available in the original stroke dimension, while aftermarket performance cranks are offered in a range of strokes from the OE spec through increments of longer strokes. Quality aftermarket crankshaft makers include Scat, eagle, Lunati, Crower, Ohio Crankshaft and others.




Depending on the application/design, some journal oil holes may feature an extended chamfer to promote oil transfer.
M-p54-1.jpg



General tips

1. If the journal surfaces are damaged (scratched, scored, gouged, burnt), further inspection is required. If the scratches are light enough, the journals may be saved simply by re-polishing with 400 grit, followed by 600 grit abrasive paper. This should be done on a dedicated crankshaft polishing stand, where the crank rotates at a slow speed while an arm-mounted abrasive belt is lowered onto the journal. If the surface damage cannot be eliminated by polishing, the journals mayneed to be re-ground with an abrasive stone wheel on a crankshaft grinder.


The installed bearings do not actually provide a uniform round inner diameter surface. The bearing shells feature a slight taper (thinner near the parting line and thicker at top and bottom). This promotes a “squeeze” ramp for the engine oil, allowing the oil to provide the needed support film to support the journal.
M-p56a-1.jpg


2. If a crankshaft’s mains, rods or both are re-ground to an undersize, the crankshaft MUST be labeled to easily identify any undersizing by stamping or etching the undersize on the forward face of the front counterweight. For example, if the main journals are fine but the rod journals are re-ground to, say, 0.010-inch undersize, the stamping or etching should say “ROD 010,” OR “R –10”, etc., to clearly identify the rod journals as having been undersized by 0.010-inch. A negative symbol (-) preceding the number makes it clear that the re-grind factor of 0.010-inch has been removed.




Main (and rod) bearings feature a slight bit of extension when installed (where the bearings ends protrude slightly beyond the parting line). This provides the proper bearing “crush” to achieve bearing retention and the proper inside diameter profile for correct oil clearance and lubrication delivery.
M-p56b-1.jpg



3. Also make sure to inspect all fillets (the shoulder area where the journal surface blends into the counterweight or throw area). A journal should never be ground to create a sharp corner, since this can lead to an eventual stress riser, which can result in crank failure.



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Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
4. Inspect all threaded holes (the center hole in the front snout and the flywheel holes in the rear flange). Make sure that the threads are clean and are not damaged. A chaser tap (as opposed to a cutting tap) can clean these threads without cutting and removing too much thread material.

5. Inspect all main and rod journal oil feed holes to make sure that they’re drilled through, and that they’re not plugged up with debris.


750px-crank_snout_-_keyway_dimensions-jpg.196499

http://garage.grumpysperformance.co...-bearing-trashed-in-1-2-hour.5141/#post-71921

http://garage.grumpysperformance.com/index.php?threads/thrust-bearing-wear.619/#post-37676

http://garage.grumpysperformance.co...ed-holes-in-bearings-shells.10750/#post-64733

http://garage.grumpysperformance.com/index.php?threads/precision-measuring-tools.1390/#post-52466


http://www.bracketracer.com/engine/mains/mains.htm


Measuring Main Bearing Clearance

Pat Mancuso - patman@bracketracer.com - www.bracketracer.com

Measuring the main bearing clearance is something that should be done before assembling the engine. This is commonly done with plastigage, but that method is not very accurate. Here's how to measure the clearance with an outside micrometer and a dial bore gauge

The obvious answer is to measure the main journal on the crank, and measure the diameter of the main bearing, and subtract. Not so fast. The problem with that method is that you're using two tools to do the different measurements, and there is no way to know what the error was in measuring each.

The textbook measurement for the main journals on a SBC 400 is 2.6500". Let's say you measure 2.6500 on the journal with the outside micrometer, and 2.6525 with a dial bore gauge on the bearing. That sounds like you've got 0.0025 clearance, right? Wrong. Let's say the error rating on the micrometer and on the bore gauge was +/- 0.0005. That means the journal could have really been 2.6505 to 2.6495, and the bearing could have been 2.6530 to 2.6520. Taking the worst pairs, 2.6505 - 2.6520 = .0015 and 2.6495 - 2.6530 = .0035, so your clearance is really somewhere between 0.0015 and 0.0035 once you figure in the potential error. That's a pretty wide range for the tolerances you need to check.

How do you get around that without spending a fortune on more accurate measuring tools? Easy, always use the same tool to measure with and you eliminate half of the error uncertainty. The basic idea is to measure the journal with the outside micrometer, then zero the bore gauge inside the micrometer, then use the bore gauge to measure the bearing. The actual measurement doesn't matter, you end up seeing only the difference in the measurements, which is the clearance measurement you wanted to check in the first place.

Going through the steps, measure the crankshaft journal with the micrometer, zero the bore gauge against the micrometer, and then measure and get .0025 (+/- .0005 error) which means your actual measurement is between .0020 and .0030. Much better than the previous method.

When using this method, you must be sure to measure each journal and re-zero the bore gauge before measuring the corresponding bearing. Measure the #1 journal, zero the bore gauge, use the bore gauge to measure the #1 bearing, measure the #2 journal, zero the bore gauge, use the bore gauge to measure the #2 bearing, etc.

To get an accurate measurement of the bearing diameter, make sure the bearings are installed properly in their correct location, and the bolts are tightened to the final assembly torque specifications.

Using the outside micrometer on the main bearing journal:

mainbearingmeasure01.jpg


Zeroing the dial gauge against the micrometer measurement:

mainbearingmeasure02.jpg


mainbearingmeasure03.jpg


Reading the clearance at the bearing:

mainbearingmeasure04.jpg


Showing .0030 difference:

mainbearingmeasure05.jpg

cle-ms909h.jpg

http://garage.grumpysperformance.co...market-4340-connecting-rods.13321/#post-72048

http://garage.grumpysperformance.co...haft-journal-surface-finnish.2728/#post-72042

http://garage.grumpysperformance.co...kit-1986-chevy-caprice-classic-with-305.3860/

http://garage.grumpysperformance.com/index.php?threads/types-of-crankshaft-steel.204/

http://garage.grumpysperformance.com/index.php?threads/rod-bolts-rpm-vs-stress.341/#post-68856

http://www.scatcrankshafts.com/rotating-assemblies/rotating-assembly-product-search/

http://www.scatcrankshafts.com/rotating-assemblies/rotating-assembly-product-search/

http://garage.grumpysperformance.com/index.php?threads/precision-measuring-tools.1390/#post-68850
dcp3.jpg

It looks like they use eagle crank and rods, which are a common name brand,
I've seen several guys buy and install those cranks and have friends that have used eagle rotating assembly's without issues,
but having compared them side by side with SCAT, ID PREFER SCAT,
the machine work quality and consistency was noticeably more precise on the two 383 sbc cranks,
I compared on the SCAT crank



As a journal rotates, an oil film “wedge” is created, which centers the journal within the bearing I.D. during engine operation. This oil film provides the support for the crankshaft, so that the journals do not actually contact the bearing surfaces as the crankshaft rotates.
M-p56c-1.jpg





6. As far as crank oil holes are concerned, simply deburr the holes to break off any sharp edges. It was commonplace for years for builders to radius-sweep the holes, but you get too much bleed-off doing that, so it’s better to simply deburr the holes, removing as little material as possible.

A note about undersize grinding

If a crankshaft (rod and/or main journals) is to be re-ground to an undersize, this is done on a dedicated crankshaft grinder, using specific-width abrasive stone wheels. When main journals are ground, the crankshaft is mounted and rotated “straight” with zero runout. When rod journals are ground, since they are offset from the crank centerline, the crank is adjusted on the machine to run at an offset, with the rod journals positioned at zero. Cooling fluid is applied during grinding to cool and clean the journal surfaces.

As far as crank service life is concerned, if a crank’s rod or main journals need to be re-ground, say – 0.020-inch, you’ll lose the initial surface hardness. While some builders (or customers) may assume that the crank is no longer usable simply because the surface hardness has been lost, in reality this isn’t a problem. Simply send the crank out for nitriding after the corrective grinding has been accomplished.

General clearance recommendations

Start with 0.0010-inch of clearance per inch of journal diameter. For example: 2.100-inch journal diameter X 0.0010 = 0.0021-inch clearance. For high performance applications, add 0.0005-inch. If, for example, initial clearance is determined to be 0.0021-inch, add 0.0005-inch for a final clearance of 0.0026-inch. From this point, tighten clearance as your experience dictates in specific applications.

NOTE: Use of a dial bore gauge is always the recommended method of measuring oil clearance. Instead of measuring journal diameter and then measuring installed bearing diameter, zero the bore gauge at the actual journal diameter. When you measure bearing diameter, you’ll obtain a direct clearance reading without the need to perform math procedures, avoiding potential math mistakes.

If clearance modification is needed, do not increase or decrease clearance by modifying housing size outside of tolerance limits. An undersize housing will over-crush the bearing; and an oversize housing will reduce crush and bearing retention.




Once the main caps have been installed, follow the specs for torque value (or torque-plus-angle for OE fasteners). Tighten all primary (vertical) main cap fasteners first, in stages and in proper sequence, then tighten main cap side bolts if applicable.
M-p58a-1.jpg



Today’s leading bearing manufacturers utilize finite element analysis computer modeling to examine the elastic deflections of all bearing-related areas. EHL, or Elasto-Hydrodynamic Lubrication, allows engineers to more accurately determine the effects of dynamic forces in relation to forces and oil clearances. This understanding of loads, metal deflection and effects on clearance has allowed a more precise view of what the bearings are subjected to, and furthers engineers’ ability to develop bearings that will function properly in high-stress dynamic racing applications.


If you really want to get nit-picky with regard to bearings, pay attention to not only suggested clearance, but also take into account the bearing surface are from an anticipated load standpoint, as well as bearing speed, based on journal circumference.




Once the main cap fasteners have been tightened to specification, the crank may be rotated. Check for free rotation. If a bind exists, re-check bearings clearances, main bore alignment and/or crank runout. If the crank rotates freely, then set up a dial indicator to check for crank endplay. Using a flat-blade screwdriver (prying between a counterweight and main cap), carefully move the crankshaft fully rearward. Adjust the dial indicator with about 0.050-in. of preload, then zero the indicator gauge. Using the screwdriver, pry the crankshaft fully forward and note the amount of movement on the indicator. Perform this step several times to verify your results. Compare the measured endplay/thrust movement with the OE specifications.
M-p58b-1-1.jpg



In higher end engines, where you plan to run smaller journals sizes, you really need to pay attention to the load carrying capabilities.

In order to provide adequate oil delivery, some high-end race engine builders sometimes drill extra oil holes in the bearings and partial-radius grooves in the housing or saddlearea of the mains to create multiple oil supply points. This is especially important in engines that use smaller bearings and will experience higher loads (don’t try this at home).

As far as bearing clearances are concerned, for street engines that see higher loads, some builders tend to run somewhere around 0.003-inch for mains and around 0.0025-inch for rods. For engines that will see lots of heat for extended periods, such as endurance engines or marine engines, tighter bearing clearances are the norm, to compensate for the fact that clearances will loosen under hot conditions.




If the crankshaft features a reluctor wheel (also called a tone wheel) for crankshaft timing position, it is possible that you’ll need to either replace a damaged wheel or install a wheel to a new crankshaft. This must be done with a dedicated locating tool in order to achieve the correct clock position of the tone wheel. Shown here is Goodson’s reluctor wheel positioning and installing tool.
M-p60a-1.jpg



In a high-speed, high-load engine application, experienced builders tend to run a fairly high crush (where bearing shells mate together), while maintaining this within an acceptable range. Considering bearing load and journal and housing deflection, you want to make sure that the bearing is securely held in place. Where you have oil films that are in the tenths of thousands clearance, the bearing gets very hot. If you don’t have adequate crush, you won’t get enough heat transfer. Avoid taking housings to their maximum size, to avoid inadequate heat transfer.




Crankshaft main and rod journals are machined to size (diameter and width) on a dedicated crankshaft grinding machine. Abrasive stone wheels rotate against the rotating crank while lubricated by the machine’s coolant supply. The main journals are ground with the crank set-up to rotate at its main centerline. Connecting rod journals are ground with the crankshaft offset-positioned to rotate on the rod journal centerline.
M-p60b-1.jpg



When a builder opts for smaller journal diameter crankshafts (to reduce mass) they sometimes modify the crankshaft journal oil holes in order to drive more oil to the rods. As you shrink the rod journal diameter, the load goes up. In order to get extra oil to the rod bearings, they create a slight teardrop groove to the crank main oil holes. The leading edge (attack side) of the oil hole is slightly grooved. As the crankshaft rotates, this slight teardrop-shaped cavity fills with oil and is then force-pumped into the oil hole, increasing boost pressure. This can cure problems with rod bearings that were otherwise seeing too much load. This can be done with a grinder, but is best performed on a CNC machine. However, you need to pay strict attention to the dimensions of the teardrop groove, in terms of width, length and depth. Generally speaking, this teardrop groove is usually around 0.300-inch to 0.400-inch in length. If the groove is too aggressive, you could start starving the mains for oil. The specific profile of this groove controls the amount of oil pressurizing into the rod.

Again, this is nothing for the weekend builder to mess with, and is certainly not necessary for street applications. ●

HERES A FEW OF THE OILS I TRUST, coat flat tappet lifters and cam lobes with crane moly assembly paste lube
crn-99004-1_w.jpg

I usually use 6-7 quarts of oil and 1 quart of marvel mystery oil added in my oil pan 11 quart oil pan and oil cooler system capacity
10W30 Valvoline VR1 Conventional Racing Oil
10W30 Valvoline NSL
10w30 Castrol GTX conventional,
10w30 mobile 1
10w30 KENDAL racing oil

zddp.jpg

and heres a good break in additive for flat tappet lifter cams
http://www.summitracing.com/parts/CRO-86092
http://www.acdelco.com/auto-parts/v...engine-oil-supplement-assembly-lubricant.html
eos.png

How A Stroker Crankshaft Affects Piston Speed and Inertia.
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WisecoLogo.png
SPONSORED BY
Wiseco


001wiseco-piston-speed-k1-stroker-crankshaft.jpg


An intense look at mean piston speed, inertia, and controlling the massive, destructive forces at work inside your engine.

Engine builders have long calculated the mean piston speed of their engines to help identify a possible power loss and risky RPM limits. This math exercise has been especially important when increasing total displacement with a stroker crankshaft, because the mean piston speed will increase when compared to the standard stroke running at the same RPM.

But what if there was another engine dynamic that could give builders a better insight into the durability of the reciprocating assembly?

“Rather than focus on mean piston speed, look at the effect of inertia force on the piston,” suggests Dave Fussner, head of research and development at Wiseco Pistons.

Let’s first review the definition of mean piston speed, also called the average piston speed. It’s the effective distance a piston travels in a given unit of time, and it’s usually expressed in feet per minute (fpm) for comparison purposes. The standard mathematical equation is rather basic:

Mean Piston Speed (fpm)=(Stroke x 2 x RPM)/12

There’s a simpler formula, but more on the math later. A piston’s velocity constantly changes as it moves from top dead center (TDC) to bottom dead center (BDC) and back to TDC during one revolution of the crankshaft. At TDC and BDC, the speed is 0 fpm, and at some point during both the downstroke and upstroke it will accelerate to a maximum velocity before decelerating and returning to 0 fpm.

002wiseco-piston-speed-k1-stroker-crankshaft_2.jpg

As the piston races from bottom dead center to top dead center, for a brief moment, it comes to a complete stop. This places tremendous stress on the wrist pins. Shown, these Trend pins are offered in various wall thicknesses depending on the application.
002wiseco-piston-speed-k1-stroker-crankshaft-300x200.jpg
The mean piston speed takes the total distance the piston travels during one complete crankshaft revolution and multiplies that by the engine RPM. Piston speed obviously increases as the RPM increase, and piston speed also increases as the stroke increases. Let’s look at a quick example.

A big-block Chevy with a 4.000-inch-stroke crankshaft running at 6,500 rpm has mean piston speed of 4,333 fpm. Let’s review the formula again used to calculate this result. Multiply the stroke times 2 and then multiply that figure by the RPM. That will give you the total number inches the piston traveled in one minute. In this case, the formula is 4 (stroke) x 2 x 6,500 (RPM), which equals 52,000 inches. To read this in feet per minute, divide by 12. Here’s the complete formula:

(4 x 2 x 6,500)/12=4,333 fpm

You can simplify the formula with a little math trick. Divide the numerator and denominator in this equation by 2, and you’ll get the same answer. In other words, multiply the stroke by the RPM, then divide by 6.

(4 x 6,500)/6=4,333 fpm

With this simpler formula, we’ll calculate the mean piston speed with the stroke increased to 4.500 inch.

(4.5 x 6,500)/6=4,875 fpm

As you can see, the mean piston speed increased nearly 13 percent even though the RPM didn’t change.

003wiseco-piston-speed-k1-stroker-crankshaft.jpg

Reducing piston weight plays a huge role in creating a rotating assembly that can sustain high rpm. The seemingly insignificant gram weight of a piston is magnified exponentially with rpm.
Again, this is the average speed of the piston over the entire stroke. To calculate the maximum speed a piston reaches during the stroke requires a bit more calculus as well as the connecting rod length and the rod angularity respective to crankshaft position. There are online calculators that will compute the exact piston speed at any given crankshaft rotation, but here’s a basic formula that engine builders have often used that doesn’t require rod length:

Maximum Piston Speed (fpm)=((Stroke x ?)/12)x RPM

Let’s calculate the maximum piston speed for our stroker BBC:

((4.5 x 3.1416)/12)x 6,500=7,658 fpm

By converting feet per minute to miles per hour (1 fpm = 0.011364 mph), this piston goes from 0 to 87 mph in about two inches, then and back to zero within the remaining space of a 4.5-inch deep cylinder. Now consider that a BBC piston weighs about 1.3 pounds, and you can get an idea of the tremendous forces placed on the crankshaft, connecting rod and wrist pin—which is why Fussner suggests looking at the inertia force.

“Inertia is the property of matter that causes it to resist any change in its motion,” explains Fussner. “This principle of physics is especially important in the design of pistons for high-performance applications.”

004wiseco-piston-speed-k1-stroker-crankshaft.jpg

When the connecting rod is lengthened, it provides a softer transition as the piston changes direction. The longer connecting rod also reduces the compression height of the piston and can help pull weight out of the rotating assembly.
The force of inertia is a function of mass times acceleration, and the magnitude of these forces increases as the square of the engine speed. In other words, if you double the engine speed from 3,000 to 6,000 rpm, the forces acting on the piston don’t double—they quadruple.

“Once started on its way up the cylinder, the piston with its related components attempt to keep going,” reminds Fussner. “Its motion is arrested and immediately reversed only by the action of the connecting rod and the momentum of the crankshaft.”

Due to rod angularity—which is affected by connecting rod length and engine stroke—the piston doesn’t reach its maximum upward or downward velocity until
about 76 degrees before and after TDC with the exact positions depending on the rod-length-to-stroke ratio,” says Fussner.

005wiseco-piston-speed-k1-stroker-crankshaft.jpg

Stroker cranks such as this forged LS7 piece from K1 Technologies, are a great way to add displacement. However, when the stroke is lengthened the piston must accelerate faster each revolution to cover the larger swept area of the cylinder wall.
“This means the piston has about 152 degrees of crank rotation to get from maximum speed down to zero and back to maximum speed during the upper half of the stroke. And then about 208 degrees to go through the same sequence during the lower half of the stroke. The upward inertia force is therefore greater than the downward inertia force.”

If you don’t consider the connecting rod, there’s a formula for calculating the primary inertia force:

0.0000142 x Piston Weight (lb) x RPM2 x Stroke (in) = Inertia Force

The piston weight includes the rings, pin and retainers. Let’s look at a simple example of a single-cylinder engine with a 3.000-inch stroke (same as a 283ci and 302ci Chevy small-block) and a 1.000-pound (453.5 grams) piston assembly running at 6,000 rpm:

0.0000142 x 1 x 6,000 x 6,000 x 3 = 1,534 lbs

With some additional math using the rod length and stroke, a correction factor can be obtained to improve the accuracy of the inertia force results.

Crank Radius÷Rod Lenth

“Because of the effect of the connecting rod, the force required to stop and restart the piston is at maximum at TDC,” says Fussner. “The effect of the connecting rod is to increase the primary force at TDC and decrease the primary force at BDC by this R/L factor.”

For this example, the radius is half the crankshaft stroke (1.5 inch) divided by a rod length of 6.000 inches for a factor of .25 or 383 pounds (1,534 x 0.25 = 383). This factor is added to the original inertia force for the upward stroke and subtracted on the downward movement.

006wiseco-piston-speed-k1-stroker-crankshaft.jpg

Both the crank on the left and right are at the same point in their respective rotations. However, the piston on the left will have to travel much faster to reach top dead center at the same time as the piston on the right.
“So, the actual upward force at TDC becomes 1,917 pounds and the actual downward force at BDC becomes 1,151 pounds,” says Fussner. “These forces vary in direct proportion to the weight of the piston assembly and the stroke to rod length and they also vary in proportion to the square of the engine speed. Therefore, these figures can be taken as basic ones for easily estimating the forces generated in any other size engine.”

007wiseco-piston-speed-k1-stroker-crankshaft.jpg

As the piston reaches top dead center on the exhaust stroke, there is no cushion of compression to help slow it down. Instead, the connecting rod takes the full brunt of the force which pulls on its beam and tries to separate its cap. Quality connecting rods are paramount to a high-horsepower, high-rpm engine.
“We know a common measure used for many years to suggest the structural integrity danger zone of a piston in a running engine is mean piston speed,” sums up Fussner. “As the skydive instructor told his student, it’s not the speed of the fall that hurts, it’s the sudden stop. And so it is with pistons. So rather than focus only on the mean piston speed, let’s decide to also consider the effect of inertia force on the piston, and what we can do to reduce that force. And if that is not possible, make sure the components are strong enough to endure the task we have set forth.”

This article was sponsored by Wiseco. For more information, please visit our website at wiseco.com
 
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