The Stovebolt Site Forums The Shop Area Engine Shop Plastigage - what for?

I recently watched a mechanic assemble a short block. Main caps in, crank in, a bit of Plastigage and then torque down the main caps. Removed the caps and measured the length of Plastigage against a scale that came with the product. Then reassembled the main caps.
What is the purpose of Plastigage and what do the measurements mean? What can you do if the measurements are wrong?

he was measuring the width. Plastigage is a precision metered clay rope that comes in different measurment ranges. By laying it across the crank journal and torquing down the caps, it flattens out. The amount that it flattenes out, tells the clearance between the bearing shell and the crank journal. Pretty straightforward if you think about it. The closer the clearance, the more it flattens out and the wider it is. comparing the width to the reference numbers on the package, tells the clearance.

if the measurments are wrong you take the crank back and get another one. crank grinders make mistakes.

i was doing a 400 pontiac had to get 3 cranks to get the right clearances.

dont they make different thickness bearings to compensate ?

leonard

they make bearings for crank grind jobs .0100 under for instance
but what if the crank is ground just a little too much or for what ever reason the bearing was made wrong or just packaged wrong
the point of the 5 dollar plasti gauge is to make sure every thing and every one has done what their supposed too. if the plastiguage shows me a vary small difference i mic the crank and borrow a dial bore gauge and make sure but if its off much the plasti gauge will tell you

In the case of the 216/235/261 Chevy, you would shim the main cap to get the correct clearance.

If you were a professional and you skipped this step, you could send the completed engine out and have it fail. You would be on the hook for the warranty. I have seen mechanics reject a crankshaft that has been ground wrong.

I must be a pedantic bastard,
I plastigauge every journal.
Had a couple of cranks tighten down under assembly and found different clearances across the number of journals!
Then start the arguments with the machining company. :mad:

All part of the fun..........Doc.

I understand all this but when he compares the width with the reference gauge on the package how does that relate to the clearance between the bearing shell and the journal. If a crank is turned .020 and all is correct with the bearing, would the gauge reference show .020? Or does any measurement help as long as its not too tight.

The Shop manual also gives a torque range. If the plastigauge is not quite wide enough (too much clearance between the bearing and the crank), torquing the bolts to the maximum in the range may put the plastigauge to the proper width. And vise versa, if it is too wide (not enough clearance), then going to the min torque in the band might do the job.

Bill,

The Plastigauge system doesn't care if the crank is ground .010 or .020 or .0999998 under or over or anything. What you are trying to determine with the Plastigauge is 'clearance' and this is what you are verifying. A crank journal would/should have the same clearance parameters whether the thing is factory stock or has been ground twenty under.

What would be a typical clearance for a Stovebolt main bearing, .0015 to .002 or .003...something like that I think. This is how the little string would mash and compare with the gauge on the package.

Stuart

olblu,

Each engine has specs that tell how much clearance you need, within a certain tolerance, between the crank and the bearing. When you get a crank ground, then the new bearing inserts have to be a little thicker to compensate for the lost metal on the resized crank. Same concept as using bigger pistons and rings when you have the cylinders bored out. The plastigage won't tell you what grind your crank is (.010, .020, etc.). All it tells you is if you have the proper clearance between the insert and the crank. If the plastigage compresses further than the spec sheet says it should, then you've got to use shims under the journals to get the proper clearance.

In addition, the plastigage also tells you if the fit is consistant across the width of the insert. When you tighten the journals down, you want an even fit across the entire bearing. So, if one end of the plastigage compresses more than the other, you use shims on the tighter end of the journal and keep plastigaging until it's the same or very close all the way across. It's all just part of balancing the engine and setting it all up for even wear and the longest life possible between rebuilds. It can be a tedious process, and I've had times where I did the same bearing 3 or 4 times to get a fit I was happy with, but it's well worth it.

Hope this is all clear as mud :-)

/Bobby

If the crank is ground 0.020, then matching 0.020 undersize bearings are used. The main bearing clearance for a 235 should be from 0.0005 to 0.003, according to the service manual. That's 5 ten-thousandths minimum clearance.

regards,
Leon

Personally I wouldn’t go less than .001”. Plastigauge will never lie to you and reflects actual clearance and not a calculated one. Not everyone has the ability to do precision measuring accurately including machinists. Would you trust these guys?

Clear as mud but I get it. Thanks all. Amazing that a simple gizmo like that can be such a benefit in this day of hi-tech measuring devices.

Okay, let's back up a little. I know this might be basic for some, but I'm still going to put this out. The purpose for a clearance between the crank and the main bearings, or the rod bearings and the crank, is because the crank "floats" in the middle of a cushion of oil. This is what prevents metal to metal contact. When the clearance between the crank and the bearings is what they call on the "tight" side, or less clearance, oil pressure as indicated on the gauge will be higher. If the clearance wears down until it is on the "loose" side, or is outside of specifications, then the oil pressure will be lower. When the clearance gets so loose that the oil can't maintain the floating effect of the crank, then oil pressure becomes very low and you get a knock, which is the sound you hear when the crank is making metal to metal contact with the bearings. If you keep driving in this condition, eventually, the bearings will seize up and the connecting rod(s) will break, and that's what is called "throwing a rod". If you're lucky, it will just exit through the oil pan, but usually, you end up with a hole in the side of the block.

Plastigauge is what is used to determine the exact clearance between the crank and the bearings, so that you know ahead of time what that clearance will be. As long as the clearance is within specs, then the crank will safely ride in that cushion of oil.

Plastigauge measures the clearance, not the undersize. Suppose the clearance range is from .001" to .0025". The amount the Plastigauge flattens shows pretty much what the clearance is. The green Plastigauge has a clearance range from .0005"to .002", I believe. Red is for slightly wider clearances, and blue is for BIG gaps!

On my race engines, I don't trust Plastigauge. I torque up the rod and main bearings with the crankshaft removed, and measure the actual inside diameter, down into the .0001" range, then measure the crank journal and subtract the difference to get the clearance. Then I polish each crank journal individually to get an exact clearance on all the bearings.
Jerry

My father made the mistake on a Perkin's Diesel, several years ago and didn't plastigage the crank before he installed it. He sourced the crank from a company he's been using for years, so he trusted it. Well anyhow, you guys can figure out the end outcome. It was a very costly mistake. Upon inspection, it seemed as though the machine shop may have changed stones in the middle of the project and never reset their machine. Bottom line, don't trust anyone! Always check!

I have no doubt YOU possess the skills to use measuring tools correctly but in supervising a bunch of machinists who constantly swear thier tools are used and calibrated properly CMM inspection continually proves otherwise. For the average person plastigauge eliminates human error and is readily available to anyone at low cost.

well if supervise machinist and are not one i see problems from the start. your cmm is measuring things in a way that the guys in the shop are not thinking. im not saying that the cmm is wrong and believe me i know more about a cmm than i ever wanted to.the shop guys think in xyz the cmm is showing them tuvwxyz. i use plasigauge
and its good enough for most engines it is not the same as a dial bore gauge and a mic. just my 2

The "average person" isn't building an engine that's producing three times its designed horsepower and is expected to last an average of 1,000 full-throttle laps between freshen-ups. They're also not getting paid somewhere in the $100.00 per hour range to build engines. My customers expect miracles, and I do my best to perform them.
Jerry

I agree. If you're checking with an inside caliper or some other precision measuring device, you have to make sure it has been calibrated recently, with traceability to the National Bureau of Standards, and measured in a minimum of 3 locations in each cap and connecting rod. Then I hope you're also checking it with plastigauge and comparing.

I don't really know why you don't trust plastigauge. I use it on my race engines whenever I need to build or rebuild one. I have never had a failure of any of my built engines thus far (knock on wood), although I have experienced a failure in a crate motor.

Plastigauge is OK for a quick check on a production engine. I haven't yet found a plastic ribbon I can read to the 4th. decimal place, or even close. My dial bore gauges read out to the 4th. digit, ditto on my mikes, and I check them repeatedly with standards during the job. I don't have any argument with people who accept more tolerance than I do- - - -they are better able to determine what their work is worth, or what's acceptable to them. 50 years in the trade has made me a little bit picky.
Jerry

That's the difference right there Jerry.
I always use plastigauge on my re-builds because they are just that.........rebuilds.
I do performance engines, but nothing in your category of work.
My next door neighbour is there for that!
Builds race and drag engines all the time, has one of the quickest naturally aspirated engines' in the country and has his own engine dyno business.

He would go to the 4th decimal, but for what we are dealing with, plastigauge is a very handy tool.....when used right!
There is alot of mis-understanding on how it is used and exactly what people think they are measuring (as we have seen here).

I always build my street engines to relatively tight tolerances but always open them out a bit more for a higher revving, high volume oil pump driven, drag only engine with greater diametre galleries and modified oil systems.

But when talking about run of the mill daily driven engines, as long as the clearances are set right to specification, and the plastigauge is used correctly.....you normally don't run into any problems.

Just a matter of having the right knowledge to use the tool at hand..............Doc.

Plastigauge is everybody’s friend and is simple, cheap, and accurate enough for nearly all automotive work. Speaking of standards does anyone else out there have a United States Navy Level 1 sub safe vendor designation or have any idea what it is or takes to get one? The more people claim to have precision measuring skills the more they worry me.

That's the thing Curt,
the reason most of us use Plastigauge.
I'm no machinist and only measure to the best of my ability and tools allow, which is where this stuff comes in handy.

Like I said previously though, as long as people understand the principal of what they are measuring, it is a handy thing to have available.

If people profess to getting anywhere near Jerry's standard of work on an old street driven StoveBolt........they need to get a different hobby!