Exactly what does Plastigage tell you?

[Jeff Ivers]

I once used plasti-gauge, but will never use it again. Years ago, I took an MGB engine in for boring and align bore, etc. Got the engine back, used plasti-gauge, assembled the engine, drove it 10,000 miles and spun a rod bearing. Took it apart, took the block to a second machine shop, told them the history and asked them to verify the block was good and if so do necessary machining. Got the engine back, drove it 10,000 miles, and spun a rod bearing. Went out and bought micrometers and telescoping gauges, etc and started testing everything myself. The original line bore had wobbled the boring bar in the horizontal plane so the clearance there was like .03 inches instead of .003 on the number one cylinder. Plasti-gauge cannot detect a major machining error like that (only tests in the vertical plane). Interestingly, I took the block to a 3rd machine shop, one known for race engine prep, told them the whole story and asked if the block was still usable or needed to be trashed. They called me up and told me the block was ready to go, charged me for all the machine work. I took the block home, checked everything and found they had not corrected the problem. Took the block back to them and proved it and then they did a different block at no charge. That engine ran for many years and was still going strong when the car was parked.

 

[sberry]

Its not meant to find that. Doesn't mean it's not good.

 

[scissorman]

The measurement is the total clearance (difference between ID of bearing shells and OD of crankshaft).

 

[Jeff Ivers]

Its not meant to find that. Doesn't mean it's not good.

Didn't say it was meant to find that. Didn't say it wasn't good. I said I would not use it because why would I not want to thoroughly check machine shop work after being sold shoddy work?

 

[pop pop]

Using plastigage requires some assumptions that should be verified.
Assume the journal is round - better to verify.
Assume the journal is sitting at 0 clearance on the bottom while measuring the top - verify by only having two supports for a crank while checking, one at a time.
Assume the bore is round - better to verify before using plastigage.

So, if you have a dry journal, sitting on the bottom with 0 clearance, on a crank supported only with one other point/bearing cap, in a round bore, with a round journal, you'll get an accurate reading. Anything less is just that, less accurate. If you drop the pan and check one bearing on the crank with others installed, all you get is the clearance where the plastigage was sitting. You wont' know if it's total, partial or what clearance.

PS: I always use it as a final check at assembly - one bearing at a time.

 

[sberry]

Didn't say it was meant to find that. Didn't say it wasn't good. I said I would not use it because why would I not want to thoroughly check machine shop work after being sold shoddy work?

I don't understand this. It is part of the check and has saved 1000's of problems.

 

[Schurkey]

I say Plastigage is a last-chance confirmation of what should have already been checked (and corrected as needed) by the machinist. It's not meant to "measure" as much as to confirm that there is adequate but not excess clearance. Yes, it assumes that both the journal and the bore is round within acceptable limits. It should never be used as a primary reference for clearance--it's the quick-and-dirty double-check.

Keep in mind the poor bastards who use plastigage on crank main bearings IN THE VEHICLE. The Plastigage is squashed flatter than roadkill. Engine comes out, goes to the machine shop, machinist says crank and bearings are fine. Oooops, the weight of the crankshaft on the main cap was what flattened the Plastigage.

I was always told that Plastigage needed to be "fresh", or it wouldn't crush properly. I don't know if that's true or not any more. I hope not, I've got Plastigage that's fifteen years old sitting in the junk-drawer of my tool box.

 

[sberry]

I doubt fresh has much to do with it. You're is newer than mine.

 

[Ironcrow]

Sorry,
There is no magic here. If you compress a piece of plastigage until it is .002" thick, it will measure .002" on the "ruler" on the package. Nothing magic about it. It is used in many other applications and it measures the actual thickness it is compressed to.

In the case of automotive crankshafts it is measuring the bearing clearance which as everyone else has stated is the difference between the crankshaft OD and the bearing ID. Simple.

What? I thought everyone would understand that I didn't really mean literally "magic".

What kind of plastigage are you talking about with a one to one correspondence .002" (thick? wide?) to .002 inch (clearance)? The paper ruler I've seen has a stripe about 1/8 inch wide to line up with the smashed plastic and it is next to a notation .002 inch clearance.

 

[Jeff Ivers]

I don't understand this. It is part of the check and has saved 1000's of problems.

The OP did not indicate the context in which he was using plasti-gauge. The purpose of my post was to encourage people to not assume that plasti-gauge will insure success in all situations. If one is checking everything with micrometers and other machinist tools, there is no reason to also use plasti-gauge, as a thorough measuring job checks more than Plasti-Gauge is able to check. If I had bought the appropriate tools and checked the original machine work on my engine, I would have saved thousands of dollars. Through my experience, I found out that a lot of machine shops, including highly recommended ones do not adequately check their work. Plasti-gauge has to be used inline with the direction that the joint is tightened, it cannot tell you anything about the joint 90 degrees from that.

 

[A_Pmech]

Plastigage works just fine and always has. It was designed for use on machinery thousands of times more expensive than any 350 Chevy where it still shines.

Where it gets a bad rap is mostly due to user error.

 

[padroo]

Plastigauge was never intended to take the place of specking out precision parts, is is just another tool to help verifyou clearences.

If I were installing a crankshaft and even after checking with a micrometer all clearences measured what they were suppose to and it was tight or would not turn Plastigauge would be a good tool to double check your clearences.

 

[bullnerd]

Plastigage works just fine and always has. It was designed for use on machinery thousands of times more expensive than any 350 Chevy where it still shines.

Where it gets a bad rap is mostly due to user error.

Yup. Plasti is probably better than cheap measuring equipment.

Hand five different people a snap/bore setup and you'll get five different measurements.

Never liked trusting them without master rings to set up.

 

[rayra]

Ok, you crushed the plastigage, right? Then there is no clearance on the other side. Plastigage measures the actual bearing clearance.

Yikes, you guys scare me.

yep. And there's a chart that comes with it that explains it. used to anyway, been 20yrs+ since I last used it.

RTFM.

 

[Falcon67]

Plastigage is telling you to go buy some micrometers and a dial bore gauge. Not trusting my $2500+ rotating assembly to a piece of wax string pirated from a roll of LiveSavers.

 

[VoodooCLD]

When i assembled my 440 engine I was getting about .0015 with a dial bore gauge and micrometer and a bit over .002 with the plastigauge. Lucky for me both of those tolerances were just fine. I'd be more concerned if i had one reading at .0015 and one at .0007


I also use the method of measuring the crank journal with a mic, and then using the micrometer anvils to zero out the dial bore gauge. Measure the bore with the dial bore gauge you get a direct reading of the bearing clearance. Just have to make sure you pay attention to which direction the needle is moving to tell if you your hole is undersized or oversized.

 

[sberry]

I usually rotate the engine as I assemble rods make sure something doesn't suddenly bind.

 

[ms fowler]

Great discussion.....Now--what motor oil is best? No No No--just kidding---
If I thought I stirred up a hornets' nest asking about Plastigage, I KNOW what the motor oil discussion would do.

 

[TractorJeff]

Motor Oil?
OMG!

 

[Zeke]

Ok, you crushed the plastigage, right? Then there is no clearance on the other side. Plastigage measures the actual bearing clearance.

Yikes, you guys scare me.

Every day.

 

[bwringer]

If you want a really fun wild goose chase, the kind where you really get to know the obscure greasy corners of your city and county, go out and try to buy some Plastigauge.

I was working on a motorcycle engine where the factory procedure for choosing the correct bearing shells depended on Plastigauge. So one bright morn, off I went on my quest. I figured "hey, I live in Indianapolis, a major hub of motorsports -- this won't take long".

I never saw so many blank looks. Few had heard of Plastigauge. Fewer yet had heard of the correct stuff (several tried to sell me other random plastic objects). I finally found it in an ancient independent store that still had the big metal rack of catalogs.

Plastigauge works fine as long as you actually follow the directions and understand what you are measuring and what you are not measuring.

 

[Charlie51]

Wouldn't you just take a straight edge and place it across the bearing saddles to see if the line bore job on the engine block were true?

 

[ms fowler]

Wouldn't you just take a straight edge and place it across the bearing saddles to see if the line bore job on the engine block were true?

Only if your eyesight is VERY good.

 

[maxpower_hd]

I have used plasti gauge several times with success. Typically when I have an engine out and just want to check the clearance for wear but not fully disassembled. I used it last year when I replaced my 5.3 in my GMC with a used engine. I just wanted to check to see what the bearings looked like for starters and to see if the clearance was still in spec or if I was going to need to do a rebuild.

If I do a rebuild I bring everything to the machinist I trust and have used several times over the years for cleaning, checking and so forth. I may rebuild the old motor that bent a connecting rod. If I decide to do that then I will go to the machinist.

 

[Falcon67]

Wouldn't you just take a straight edge and place it across the bearing saddles to see if the line bore job on the engine block were true?

I haven't line honed a block in a long while. Put the bearing shells in the end saddles, set the crank, put a dial indicator on the center main and spin the crank. Runout should be .0005 or less if it's good. then you can put all the bearing shells in, lube with 10wt machine oil and rotate the crank to check for any binding. For the race engines I like to be .0025 or better static clearance. Note that when these things are running, it's not like a precision shaft spinning in a lathe - the crank is bouncing and wobbling all over the place in response to the firing pulses, sliding back and forth with the .008~.010 thrust bearing clearance, oil wrapped around the thing like a rope and so on.

 

[BMW Rider]

Wouldn't you just take a straight edge and place it across the bearing saddles to see if the line bore job on the engine block were true?

We did exactly that when I worked as an engine rebuilder. The straight edge we used for that task was a round bar style. Feeler gauges were used to determine if there was any misalignment in the bores just as one would use a straight edge to check a surface for flatness.

As for the original topic on Plastigauge, it is not a replacement for good workmanship and careful measuring; it is just a final verification tool used to ensure all the prior measuring is correct and all the parts are in fact fitting together as expected. A good final check to be sure all is right.

 

[LS6 Tommy]

As for the original topic on Plastigauge, it is not a replacement for good workmanship and careful measuring; it is just a final verification tool used to ensure all the prior measuring is correct and all the parts are in fact fitting together as expected. A good final check to be sure all is right.

Exactly. From Clevite's website:


"Clevite 77 Plastigage was designed to be a final check of total vertical oil clearance during reassembly. It is not meant to be a replacement for properly measuring crankshaft journals, housing bores, or bearing dimensions before engine reassembly with accurate micrometers and gauges. Plastigage is available in four different sizes to check total vertical oil clearance on connecting rods and main bearings. Each package has a measuring scale printed in inches and millimeters. The strips are color-coded for easy size identification and are soluble in oil."

Tommy

 

[KenC]

I say Plastigage is a last-chance confirmation of what should have already been checked (and corrected as needed) by the machinist. It's not meant to "measure" as much as to confirm that there is adequate but not excess clearance. Yes, it assumes that both the journal and the bore is round within acceptable limits. It should never be used as a primary reference for clearance--it's the quick-and-dirty double-check.

Keep in mind the poor bastards who use plastigage on crank main bearings IN THE VEHICLE. The Plastigage is squashed flatter than roadkill. Engine comes out, goes to the machine shop, machinist says crank and bearings are fine. Oooops, the weight of the crankshaft on the main cap was what flattened the Plastigage.
When I was taught to use Plastigage in that type of measurement, the procedure was: Remove all Main Caps, put a paper shim in the bottom bearing shell of all but one, Torque them up, put Plastigage in the other one, install, remove and read clearance. Repeat for each cap. The shims assure that the little plastic thingy isn't bearing any of the rotating assembly weight.

I was always told that Plastigage needed to be "fresh", or it wouldn't crush properly. I don't know if that's true or not any more. I hope not, I've got Plastigage that's fifteen years old sitting in the junk-drawer of my tool box.

Like most work, attention to detail and technique is important. see above

 

[pancho400cid]

PG is handy for checking the clearance - or lack thereof - between the bearing cap and the bearing shell on sleeve bearing electric motors, or between the OD of a roller bearing and the cap. Last time I used it was on a spherical roller bearing with a 360 mm (14.17") OD. The cap had about .004" clearance which was expected.... no need to measure it to the .0001" in that case.

As said, results depend on who's taking the measurements. You have to understand what you are trying to achieve and work with care.

 

[like2wheel]

I once used plasti-gauge, but will never use it again. Years ago, I took an MGB engine in for boring and align bore, etc. Got the engine back, used plasti-gauge, assembled the engine, drove it 10,000 miles and spun a rod bearing. Took it apart, took the block to a second machine shop, told them the history and asked them to verify the block was good and if so do necessary machining. Got the engine back, drove it 10,000 miles, and spun a rod bearing. Went out and bought micrometers and telescoping gauges, etc and started testing everything myself. The original line bore had wobbled the boring bar in the horizontal plane so the clearance there was like .03 inches instead of .003 on the number one cylinder. Plasti-gauge cannot detect a major machining error like that (only tests in the vertical plane). Interestingly, I took the block to a 3rd machine shop, one known for race engine prep, told them the whole story and asked if the block was still usable or needed to be trashed. They called me up and told me the block was ready to go, charged me for all the machine work. I took the block home, checked everything and found they had not corrected the problem. Took the block back to them and proved it and then they did a different block at no charge. That engine ran for many years and was still going strong when the car was parked.

There are a LOT of issues with this story, and I can see nowhere that Plastigauge is to blame....

 

[493 scamp]

I assembled my own 493 stroker from parts bought over a period of several years. My parts were all taken to a machine shop that was highly recommended. It was align bored,bored with honing plates,balanced. Everything was supposed to have been checked per my request. On assembly I used plastigauge on the mains and found the clearances around .0025. Oiled and torqued mains and crank would not turn. My new $790 Ohio Crankshaft main journals were great in the middle but tapered larger near the counterweights. Crank had to be turned to correct. I now use a full width piece of plasigauge instead of a short piece. I also use brake clean to clean off the plastigauge.
One I found .0035 on a stock chevy 350 main and could feel it felt funny when torque wrench was used. I found metal shavings in the bolt holes,cleaned it out with magnet and air,re torqued and showed .0015. I still use it as a final quick check on assembly.

 

[Jeff Ivers]

There are a LOT of issues with this story, and I can see nowhere that Plastigauge is to blame....

Nobody said that plasti-gauge was to blame. THE POINT OF THE STORY IS THAT USING PLASTI-GAUGE WITHOUT UNDERSTANING ITS LIMITATIONS CAN RESULT IN MANY LOST DOLLARS. IT IS A TOOL WITH SPECIFIC PURPOSES - HELPS CHECK CLEARANCES IN THE DIRECTION THAT A JOINT IS TIGHTENED - DOES ABSOLUTELY NOTHING FOR THE THAT JOINT IN ANY OTHER PLANE.

 

[Schurkey]

My new $790 Ohio Crankshaft main journals were great in the middle but tapered larger near the counterweights. Crank had to be turned to correct.

Ohio Crankshaft is infamous for improper machine work (too much polishing) on cheap Chinese cores. The taper you saw is exactly what happens when the journals are polished excessively.

 

[493 scamp]

Exactly, and I knew to tell the machine shop to check it all out. I had other issues as well and the owner ended up firing the guy that did the work. I traveled 3 1/2hrs each way and spent $2400 for pistons and labor. They charged me another $80 to turn crank and did not supply any bearings to go with it. Ohio Crank ignored my emails as well. The rods I sent came back with only one opened from the sealed bags they came in. The owner claimed that they only need to check the weight on one because they are all very close. So did the big ends check out ok... no response.
Machine shop was in Iowa city,Iowa.