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General Tech Plasti-Gauge on Rod Bearings

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KVH

KVH

Obi Wan
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My TR was rebuilt twice over the past 20 years. When I replaced the rod bearings 14 years ago, they were sized 1000 under and the crank looked great, so I just used the same. I remember the plasti-gauge being inconclusive back then, seeming too tight, but I ignored it and the car ran great.

Well, I want to replace the bearings again since I'm inside the engine doing all sorts of mischief (new cam, etc.), and I just plasti-gauged the rod bearings again with new 1000 under bearings. The result showed about .0020 (or maybe even .003, like .0017 less) clearance, and factory spec, if I'm reading it correctly, is .0028. In other words it's still too tight. Is plasti-gauge that exact, even from a package 14 years old?

I'm inclined to ignore this as I did 14 years ago and drive scenic roads with a big foolish grin on my face.

Any serious caution out there?


(thoughts for all hurricane victims and those displaced; they have some issues worthy of real consideration)
 
I don't know the clearance specs, but I always use Plastigage to confirm any work I've had done and shoot for the tighter range of the spec, never outside it.
Rut
 
You can get a decent digital caliper for very little money today that will confirm or contradict your plasitgauge. You also get the advantage of seeing the shells and journals independently.
Tom
 
KVH, you clearance is fine, most would love to be that good.

Wayne
 
While plastigage if fine for verifying freshly completed machine work, the problem with using plastigage on old work or higher mileage is that usually the problem with journals, if damaged, is that they are 'out of round'.
Plastigage is only checking a few of the 360 degrees.
 
KVH said:
My TR was rebuilt twice over the past 20 years. When I replaced the rod bearings 14 years ago, they were sized 1000 under and the crank looked great, so I just used the same. I remember the plasti-gauge being inconclusive back then, seeming too tight, but I ignored it and the car ran great.

Well, I want to replace the bearings again since I'm inside the engine doing all sorts of mischief (new cam, etc.), and I just plasti-gauged the rod bearings again with new 1000 under bearings. The result showed about .0020 (or maybe even .003, like .0017 less) clearance, and factory spec, if I'm reading it correctly, is .0028. In other words it's still too tight. Is plasti-gauge that exact, even from a package 14 years old?

I'm inclined to ignore this as I did 14 years ago and drive scenic roads with a big foolish grin on my face.

Any serious caution out there?


(thoughts for all hurricane victims and those displaced; they have some issues worthy of real consideration)
Click to expand...

I'm so sorry. I miss-typed. I meant to say: "I just plasti-gauged the rod bearings again with new 1000 under bearings. The result showed about .0020 clearance (or maybe even .0003 less than that, like .0017), and factory spec, if I'm reading it correctly, is .0028. In other words it's still too tight." I could be .0008 too tight, maybe even .0010 too tight.
 
KVH, if you are not typing these clearances wrong, you have way too much. I think you are reading the plasti-gauge wrong.

I think your new bearing shells are .010 over and you say the results is about .0020 clearance or maybe even .0003, that is a lot of difference. I think all is OK, you are just interpolating it incorrectly.

Look on the paper part of the plasti gauge and count the zero's, a .0001-0003 in very small, just room enough for a oil slick.

Wayne
 
Good points on checking journals for roundness( a quick had polish with some 600-1000 paper quickly shows any wear pattern) with an outside mic which is fairly easy to get accurate repeatable readings.I use Starret inside snap gauges for cylinder id's but repeatability,for me anyway, is hard to get better than .0005 1/2 thou.Fine for cylinder work but the plastigage is pretty much fool proof repeatable for closer work like bearing journals if you don't have an expensive bore mic in the correct range.
Tom
 
KVH said:
I'm so sorry. I miss-typed. I meant to say: "I just plasti-gauged the rod bearings again with new 1000 under bearings. The result showed about .0020 clearance (or maybe even .0003 less than that, like .0017), and factory spec, if I'm reading it correctly, is .0028. In other words it's still too tight." I could be .0008 too tight, maybe even .0010 too tight.
Click to expand...

I'm causing confusion. It's my fault for trying to give too much information. What I actually said in my corrected post was that my reading showed about .0020 clearance, or maybe ".0003 less than that," meaning as tight as .0017. At .0020 I'm .0008 too tight, and at .0017 I'm near .0010 too tight. Again, sorry. I'm just being too picky about the details perhaps. My local shop says that if my plasti-gauge is years old, and if I know I've been running on .010 undersize rod bearings, just put it back together, drive and have fun. I think that's my plan.
 
There is a technique to plastigage. You must ensure that you use no oil and pull the rod hard into the crankshaft before tightening the cap. If you get twist on the rod as you torque it, it will overly compress the gage and show an erroneous tight clearance. I agree. If you were running .010" bearings before, the crank has not grown. Unless the bearings are mis-boxed, you are golden.

As a final check, make sure the crank will turn after every rod is torqued. After all the bearings are torqued you should still be able to turn the crank without too much effort. If it acts locked...then take it apart and see what's too tight. Even then, it is normally caused by a warped bearing rather than a tight clearance.
 
I am not a machinist, but I knew this guy that was and he would help me all the time. Anyways, he would always have me pull the rods out and bring them to him and then he would true them up or round them out --the cost was not much. This is probably what Poolboy was getting at.
steve
 
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