i was plastigauging my rod/crank and it was coming up tight. about 15 thou. but when we miked it it came out about 25 thou. thats with good mikes. is this common.

Old plastigage? Once it's past it's shelf life, all bets are off for accuracy.

Experienced with measuring tools? Perhaps you need to develop better feel for the micrometers???

actuallty it is pretty old. didn't know it had a shelf life. i don't have a lot of mic time but the guy looking over my shoulder has been building top fuel drag bikes for about 20 years.

Greg, the .0025 that you got, was that per side or total difference between bearing and journal? If it is the total, that yields .0012 clearance. I order to have .0025 clearance, you would have mic'd out tot .005 - which is quite a bit difference.

What kind of Mic are you using?

I made the almost made the mistake of relying on a set of micro-calipers that cost me $20. Turns out the plastigauge was more accurate than they are!

Greg, the .0025 that you got, was that per side or total difference between bearing and journal? If it is the total, that yields .0012 clearance. I order to have .0025 clearance, you would have mic'd out tot .005 - which is quite a bit difference.
Dooh! brain fart. :D thanks. i had rounded off the numbers. . damn. that plastigauge is on the money now. Ok so now need more clearence.

Dooh! brain fart. :D thanks. i had rounded off the numbers. . damn. that plastigauge is on the money now. Ok so now need more clearence.

Wait, are you using a dial bore guage and a mic? because if so the total clearance should be the difference between teh ID of the housing and the OD of the crank, no need to split things in half. wait a second here, now i'm confused LOL

Clearance is the diff. of the ID & OD as far as I know.

Greg, the .0025 that you got, was that per side or total difference between bearing and journal? If it is the total, that yields .0012 clearance. I order to have .0025 clearance, you would have mic'd out tot .005 - which is quite a bit difference.
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This is incorrect if I am reading it right. :confused:
The total clearance is the difference between the journal & the bearing bore or in this case, 0025 which as you sated is .00125/side but this is not the clearance you are measuring.
Plastigauge reads total clearance (assuming you have no oil on the bearing) as does subtracting the journal from the bore so I am completely confused as to where half of this comes into play except for oil film thickness :confused:

In other words, if the bearing bore is 2" & the journal is 1.9980 you have .002 bearing clearance.

now i am really confused. i used the plastigauge. and came up with .0015 in. that was with both red and green PG at 50lb tq with the journel and bearing wiped dry. i figured that is to tight but before i start playing with stuff i took my moroso dial bore indicator. and measured the tourqued rod with the bearing in it. then i took my starret micrometer and took a measurment of the rod journel on the crank. then i took my bore indicator and put it between the anvils un my locked down starret micrometer. that got me about .0028 in. so my clerance is not computing. either i am screwing up tghe PG measurment or the micrometer. i'll do both all over agai. i don't have much eperiance with micrometers or PG as i am not an experianced engine builder. my buddy also took the mics and went behind me to check my measurments. it was the end of the day so we put it all away to start next weekend. i am doing it at his house so if i have any questions he can help me.

No Mike you were reading it right, I was confused on exactly what he had measured and posted. Personally, I prefer to use a mic and take actual measurements, besides being more accurate, I think it is faster also.

Check your bore gage with the micrometer and see how they compare.

I'm not familiar with the bore gage you have so I don't know how it sets. The ones I use have a micrometer head and a dial indicator, that way you can set the bore gage with the micrometer(or setting ring)and zero the indicator, that way you can read the clearance directly.

by the way these are new SCAT H beam rods and new SCAT pro lightweight crank with speed pro bearings.

Check your bore gage with the micrometer and see how they compare.

I'm not familiar with the bore gage you have so I don't know how it sets. The ones I use have a micrometer head and a dial indicator, that way you can set the bore gage with the micrometer(or setting ring)and zero the indicator, that way you can read the clearance directly.

What Big James said, measure the journal with the mic, lock it on the correct measurement, zero the bore gauge in the mic, measure the bearing bore in the rod, what your bore gauge reads is your clearance.

Throw the plasti-gauge away.

If I were you, I would tend to believe the Plastigage; there's a reason why it was invented. Unless you are well practiced in the use of a micrometer and bore gage, your chances of getting an accurate measurement are pretty slim, especially when measuring an ID and an OD. There are just too many opportunities for errors to add up.

Tom

What Big James said, measure the journal with the mic, lock it on the correct measurement, zero the bore gauge in the mic, measure the bearing bore in the rod, what your bore gauge reads is your clearance.

Throw the plasti-gauge away.

Throw it my way. Why do it the hard way when there's an easier, quicker, more accurate way?

My $0.02

I tend to think Plastigauge works better when measuring clearance in a used set of bearings where the eccentricity has been reduced from wear. I think the issue with P-G in a brand new bearing set is that it will not force the journal completely over to one side on a brand new set of bearings. The few times I have tried P-G on a new install, the results were similiar - on the low side and similiar to what he reported - often close to 1/2 of the total clearance. A lot of the error is probably due to not having the the bearing, rod or cap surfaces absolutely bone dry - any oil film will displace the P-G.

Alot of bore gauges are not made to measure bores that have eccentricity in them do to the housing bores being checked with bearing in the rods or mains as we have seen this before with some of our customers using cheaper bore gauges other than a Sunnen.

We clearance out a lot of rotators for customers that assemble there own engines and most of them check my work with PG and I have been told many times the clearances were right on. So my findings are that PG is not to far off.

by the way these are new SCAT H beam rods and new SCAT pro lightweight crank with speed pro bearings.

We find that on new rods that we have to open the big ends of the rods to the high side and also the pin end of the rods are always tight as well and also check the pin bores of the pistons as we find that is an area that is over looked which in most cases we have to hone out up to .0009 sometimes to get the proper clearance as a wrist pin will go through the bore with .0002 clearance. And if this proceedure is not done we have seen the rod busshing with alot of wear do to the rod bushing doing all the work and the skirts and the cylinder bores will show more wear as well.

Might as well add my 2 cents worth ... I dont see how you can use plastigage on a rod bearing and get any type of good reading. It is almost impossible to keep the rod from moving around some while you are torqueing the bolts, and then again when you are taking the cap back off. PG seems to work pretty good on mains, we use it for a quick check sometimes, but 99% of the time we use the good old Sunnen bore gauge.

We find that on new rods that we have to open the big ends of the rods to the high side and also the pin end of the rods are always tight as well and also check the pin bores of the pistons as we find that is an area that is over looked which in most cases we have to hone out up to .0009 sometimes to get the proper clearance as a wrist pin will go through the bore with .0002 clearance. And if this proceedure is not done we have seen the rod busshing with alot of wear do to the rod bushing doing all the work and the skirts and the cylinder bores will show more wear as well.
I would take issue with the part of the statement here about changing either rod or main hole sizes to the "high" dimension to help achieve bearing clearances. This is downright wrong. The holes, both rods and mains should ALWAYS be held to the "low". This gives better bearing crush which in turn, helps keep the bearings from spinning. Some people out there believe the bearing tabs do this, but it's not the case. All the tabs do is locate the bearing and have no other purpose. The "crush" on the bearings keeps them from going around. If you can't get the clearances where you need them with "small" holes, you have to work with the bearings themselves or the crankshaft. We've ground hundreds (maybe thousands) of cranks over the years to get the correct clearances with ALL holes remaining on the "low". Up until a few weeks ago we could actually make a crankshaft from a piece of stock, this was fairly easy for us to do with the equipment my son had been using. Most shafts we grind for clearances still remain Std./Std. Most engine builders out there will know what I'm saying. Thanks, Gary in N.Y.
P.S. The mains are relatively easy to get "right", assuming you have a decent crank grinder, the rods are a little harder. Plastigage belongs in the "dumpster".

I would take issue with the part of the statement here about changing either rod or main hole sizes to the "high" dimension to help achieve bearing clearances. This is downright wrong. The holes, both rods and mains should ALWAYS be held to the "low". This gives better bearing crush which in turn, helps keep the bearings from spinning. Some people out there believe the bearing tabs do this, but it's not the case. All the tabs do is locate the bearing and have no other purpose. The "crush" on the bearings keeps them from going around. If you can't get the clearances where you need them with "small" holes, you have to work with the bearings themselves or the crankshaft. We've ground hundreds (maybe thousands) of cranks over the years to get the correct clearances with ALL holes remaining on the "low". Up until a few weeks ago we could actually make a crankshaft from a piece of stock, this was fairly easy for us to do with the equipment my son had been using. Most shafts we grind for clearances still remain Std./Std. Most engine builders out there will know what I'm saying. Thanks, Gary in N.Y.
P.S. The mains are relatively easy to get "right", assuming you have a decent crank grinder, the rods are a little harder. Plastigage belongs in the "dumpster".

Even with the rods or the mains to high limiits it is still with in the bearing manufactures specs as we have seen many blocks that have come from the Cup teams and have found them .001 over or more then the high side and same on the rods and there did not seem to be any issues there as your only talking .0005 per bearing shell even own the engines we build we may take them over the hight limits from time to time and have been doing this for 29 years now with zero problems.

I have built 2 SBC blower engines that one of them is on its 9 th year of running and the other has 7 years on it and these are aluminum blocks 1 Dart and 1 Brodix block and the mains on both are on the high limits and have spun any bearings yet.

And if your spinning bearings cause there on the high side which is with in the manufactures spec. I would say that you are doing something wrong.

And I have check with the Sunnen rep and he says he does not know any one on Long Island that is named Gary"s or I believe you go by the name of G&K Automotive and as far as building crankshafts I have been to King's Crankshaft in N.C. and I know what it takes to do that type of work and if you was building cranks from billet material I am sure I would have heard about you by now. I don't know who you are tring to impress here but its not me ( IF YOU WANT BLOW SMOKE UP MY A$$ YOU BETTER BUILD A BIGGER FIRE THEN THAT.)

And your the guy that said you don't need to use a TORQUE PLATE WHEN HONEING BIG BLOCK REMEMBER.

I don't belittle anybody from using plastigauge at least they are taking the time to check there clearances as PLATIGAUGE probably has been around longer then you.

I would take issue with the part of the statement here about changing either rod or main hole sizes to the "high" dimension to help achieve bearing clearances. This is downright wrong. The holes, both rods and mains should ALWAYS be held to the "low". This gives better bearing crush which in turn, helps keep the bearings from spinning. Some people out there believe the bearing tabs do this, but it's not the case. All the tabs do is locate the bearing and have no other purpose. The "crush" on the bearings keeps them from going around. If you can't get the clearances where you need them with "small" holes, you have to work with the bearings themselves or the crankshaft. We've ground hundreds (maybe thousands) of cranks over the years to get the correct clearances with ALL holes remaining on the "low". Up until a few weeks ago we could actually make a crankshaft from a piece of stock, this was fairly easy for us to do with the equipment my son had been using. Most shafts we grind for clearances still remain Std./Std. Most engine builders out there will know what I'm saying. Thanks, Gary in N.Y.
P.S. The mains are relatively easy to get "right", assuming you have a decent crank grinder, the rods are a little harder. Plastigage belongs in the "dumpster".
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You are kidding right,, :(
You are saying you have crank grinders that you can mount a crank that has been ground in one machine, put it in your machine & get within .0001 so you can "regrind" clearance??
All I can say is,,, well,, you really are the best :D

If what you are saying regarding housing bore sizing is true then I guess every bearing manufacturer & engineer I have talked to in the last 30 or so years is "downright wrong" too.
Along with almost every automobile manufacturer,, as I would have to say with few exceptions, every rod & main bore I have measured from Honda to GM to Ford to whoever has been in the mid to high side of spec & in a lot of instances over hi spec when new.
And I do have some pretty good measuring equipment,,,,

I have been both building engines & had the privilege of driving some pretty high end deals for more than a few years & this is circle track to GT1 to Blown Alcohol & in every case that needed clearance adjustment either "select fit bearings" (.001 over / .001 under & std) &/or the housing bore has been honed in some cases to achieve the desired clearance.
These are street engines that see 5000 RPM to race engines that see 10,000 RPM & I guess either we are all either just dumb as a post or just plain lucky or we have been doing something right as we didn't & still don't have a ton of bearing trouble.

Now I agree that if you are grinding or having a crank ground & know the sizes needed the crank can be ground to a specific size but I know of very few crank grinders that are willing to grind every journal different for variances in tolerances,,
I do know of a few that seem to be able to grind them to "different" sizes though,,, :D

And as for throwing the Plastigauge in the dumpster, I encourage any of my customers that are assembling their own engine to use it as it is pretty damn accurate when used properly.
Not saying it is as accurate as a quality bore gauge used properly but it is usually pretty damn close & certainly better than not checking & if you find a problem at least you can go to your machinist or come on a forum like this & question your findings as Greg had the good sense to do.

And Greg, if you are getting a difference of .001 between the gauges & the Plastigauge & it is fairly fresh I would take the time to visit your friendly performance machine shop that has some quality measuring equipment & a few years of experience to see where the discrepancy lies before going further.
Most real performance machine shops I know of will not mind taking the time to let you see their equipment as well as allow you to see how it's done.
I know we don't normally mind,,, we are proud of the fact we have some pretty fair equipment ;)
As was stated a few times, it does take a little practice & or someone helping a couple of times to get a "feel" for bore gauges.
Also, a lot of the gauges with sharp points are real bad at "gouging" the bearing & giving you a false high reading especially with the softer bearings but it can do the same with performance bearings too.
And, as Carl stated, a lot of the less expensive gauges are not real good at measuring anything that isn't round.
BTW, I am assuming you are measuring the clearance at the same position in the bore?? This would be at 90 degrees to the parting line.

I'll also add,
when I resize rods, I usually take them to the high side of spec. By the time they cool back to room temp, they have "shrunk" a couple of ten thousandths back from the very high side toward the middle of spec...

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You are kidding right,, :(
You are saying you have crank grinders that you can mount a crank that has been ground in one machine, put it in your machine & get within .0001 so you can "regrind" clearance??
All I can say is,,, well,, you really are the best :D

If what you are saying regarding housing bore sizing is true then I guess every bearing manufacturer & engineer I have talked to in the last 30 or so years is "downright wrong" too.
Along with almost every automobile manufacturer,, as I would have to say with few exceptions, every rod & main bore I have measured from Honda to GM to Ford to whoever has been in the mid to high side of spec & in a lot of instances over hi spec when new.
And I do have some pretty good measuring equipment,,,,

I have been both building engines & had the privilege of driving some pretty high end deals for more than a few years & this is circle track to GT1 to Blown Alcohol & in every case that needed clearance adjustment either "select fit bearings" (.001 over / .001 under & std) &/or the housing bore has been honed in some cases to achieve the desired clearance.
These are street engines that see 5000 RPM to race engines that see 10,000 RPM & I guess either we are all either just dumb as a post or just plain lucky or we have been doing something right as we didn't & still don't have a ton of bearing trouble.

Now I agree that if you are grinding or having a crank ground & know the sizes needed the crank can be ground to a specific size but I know of very few crank grinders that are willing to grind every journal different for variances in tolerances,,
I do know of a few that seem to be able to grind them to "different" sizes though,,, :D

And as for throwing the Plastigauge in the dumpster, I encourage any of my customers that are assembling their own engine to use it as it is pretty damn accurate when used properly.
Not saying it is as accurate as a quality bore gauge used properly but it is usually pretty damn close & certainly better than not checking & if you find a problem at least you can go to your machinist or come on a forum like this & question your findings as Greg had the good sense to do.

And Greg, if you are getting a difference of .001 between the gauges & the Plastigauge & it is fairly fresh I would take the time to visit your friendly performance machine shop that has some quality measuring equipment & a few years of experience to see where the discrepancy lies before going further.
Most real performance machine shops I know of will not mind taking the time to let you see their equipment as well as allow you to see how it's done.
I know we don't normally mind,,, we are proud of the fact we have some pretty fair equipment ;)
As was stated a few times, it does take a little practice & or someone helping a couple of times to get a "feel" for bore gauges.
Also, a lot of the gauges with sharp points are real bad at "gouging" the bearing & giving you a false high reading especially with the softer bearings but it can do the same with performance bearings too.
And, as Carl stated, a lot of the less expensive gauges are not real good at measuring anything that isn't round.
BTW, I am assuming you are measuring the clearance at the same position in the bore?? This would be at 90 degrees to the parting line. Hi Wolfy, That's good advise about going to the machine shop and observing the proper method of measurement with good equipment. My son, who does high tolerance machine work on very good equipment showed me some good stuff about micrometer calibration recently. I found out, with the use of his companies calibration blocks, that the little thimble on my Starret mic gave readings that were slightly tight. After adjusting the mic with the small spanner wrench and learning the proper procedure every reading comes up the same every time.

Might as well add my 2 cents worth ... I dont see how you can use plastigage on a rod bearing and get any type of good reading. It is almost impossible to keep the rod from moving around some while you are torqueing the bolts, and then again when you are taking the cap back off. PG seems to work pretty good on mains, we use it for a quick check sometimes, but 99% of the time we use the good old Sunnen bore gauge.

I agree with this. I recenly used some Scat cap screw type rods with dowl locators. No way you could separate those without crushing the PG. Not that I could figure out anyways.

well. :O I bought some fresh PG today. my dial bore gauge does have a sharp point on it. so maybe mic the crank, mic the bearing by itself in the middle, then use the bore gauge in the big end of the rod. do the math and compare to fresh PG.

Lots of good info here from all viewpoints but one thing I always do no matter how I measure the clearances!!!

I always turn the engine by hand every time I torque a main or rod, and if it feels tight or wont turn, I STOP, and regroup!!! And, I also grab each rod and see how it moves on the journal as I assemble the engine, if one doesn't feel the same as the others I'll investigate!

Always use your sense of feel to get an idea of how things are fitting. I'm betting the engine builders here will all do that same thing.

I've found some glaring errors in machine work with my simple turn and feel and look method. Years ago I was ready to put the heads on a BBC I'd assembled, after rolling the engine over I decided to spin the crank one more time for good luck, low and behold one piston was not coming all the way to top dead center!!! :confused: As it turned out, the machine shop had somehow put a shorter rod on one piston!!! :clonk: I still don't know how they did it but the point is sometimes the simple checks find the biggest problems!!! :thumbsup:

Lots of good info here from all viewpoints but one thing I always do no matter how I measure the clearances!!!

I always turn the engine by hand every time I torque a main or rod, and if it feels tight or wont turn, I STOP, and regroup!!! And, I also grab each rod and see how it moves on the journal as I assemble the engine, if one doesn't feel the same as the others I'll investigate!

Always use your sense of feel to get an idea of how things are fitting. I'm betting the engine builders here will all do that same thing.

I've found some glaring errors in machine work with my simple turn and feel and look method. Years ago I was ready to put the heads on a BBC I'd assembled, after rolling the engine over I decided to spin the crank one more time for good luck, low and behold one piston was not coming all the way to top dead center!!! :confused: As it turned out, the machine shop had somehow put a shorter rod on one piston!!! :clonk: I still don't know how they did it but the point is sometimes the simple checks find the biggest problems!!! :thumbsup:
They gave you someone's "409" rod by mistake, no doubt! Thanks, Gary in N.Y.

FWIW, we frequently check bearing and rod clearance with PG upon tear down. It does a very good job of showing any "hills and valleys" that may be developing on the journals... and a whole lot easier than getting the same information with a bore gauge.

Plastigage is very accurate when used properly. Only PG-1 (green) should be used to check bearing clearances, unless your total clearance is over .003".
When checking bearing clearances, whether using Plastigage or bore gauge/micrometer, you are checking the TOTAL clearance between the journal and the bearing. This clearance should check the same (or very close) whether using Plastigage or mics, and the clearances can also be checked accurately whether the bearings are new or used.