The Stovebolt Site Forums The Shop Area Engine Shop Bearing Clearance, plastigage & Shims

I’m just about to start reassembling my ’53 235 powerglide engine. The block was bored 0.030” over and the crank is now 0.010” under on the mains and rods, the block and main caps were not line bored.

The machine shop told me to start with 0.004” shims under all of the main bearing caps. Then use plastigage and adjust the shims until I get between 0.003” and 0.001” clearance in all of the main bearings. I understand putting the plastigage in, tightening the bearing cap bolts to the torque specs, then taking it back apart to read the width that the plastigage has been squeezed to.

My questions are

Do I do all 4 mains at the same time?
Do I make sure all surfaces; both sides of the bearing inserts, the crank journals and the bearing caps are all wiped clean of oil
Where do I place the plastigage? Directly across under the center of the cap at12 o’clock, or maybe two strips at 10 and 2 o’clock

One thing that really has me confused. I bought a shim kit at the local NAPA in the kit there was 1 pair of shims for each bearing cap, but each of the 4 pairs was a different thickness from 0.0065” to 0.0085”.

I took shims out when I disassembled the engine. These were about 0.004” to 0.005”. I cleaned and saved them and marked each for where it came from, maybe I should start with these.

Since I have not done this since high school in the early 60’s I sure could use some advice from someone who has more recent experience

Thanks

Kip

I start out with no shims, and snug the bearing caps one at a time to be sure the crankshaft can be locked, or at least made hard to turn by tightening the bolts. Add shims until the crank turns freely, then use the Plastigage to do a final check. Use one piece of Green PG at the top of the crankshaft journal, torque the cap, and remove it without turning the crankshaft. Yes, the bearing and crank needs to be dry. I like to shoot for a bearing clearance of .0015", with .0025" as an absolute maximum. Check and adjust the bearings one at a time. The insert-type bearings don't wear as quickly as the Babbit types do, but getting the clearance down to minimum will make for a longer-lasting engine.
Jerry

Since we’re on the subject of Plastigauge, something that has always bugged me about using Plastigauge vs. mechanical measurement tools. With the Plastigauge laying on top of the crank journal and the caps tightened down the clearance at the bottom is closed up, which opens up the apparent clearance at the top of the bearing surface. http://www.pbase.com/dennygraham/image/122580617/large
Since the nominal oil clearance is symmetrical it would seem to me that you get a false reading using a thickness measurement media only at one point on the bearing. Now if you had a thread of material every 90° around the bearing then you would get an accurate read. This isn’t discussed on the Sealed Power site and I’ve never seen it brought up in any discussion about the use of Plastigauge. http://www.plastigaugeusa.com/how.html
Any thought or input on this?
Now mind you my
Denny Graham
Sandwich, IL

The new mains for my 216 were pinching the crankshaft near on the sides near were the two halves fit together, despite plastigauge placed on top indicating .003 clearance. However, if one did try using plastigauge on the sides, it would probably smear due to the sliding action of the bearing shell against the crank at that location when tightening.

Denny, nothing beats torquing up the bearing caps with nothing inside, and doing a 3-point measurement of the bore. That's how we do the race engines. Plastigauge is simply the best, most cost-efficient method of estimating oil clearance. back in the dark ages, we used a piece of shim stock between the bearing and the crank. If it locked things up, we assumed that thickness of shim was the clearance. If the next size thinner shim would let the crank turn, the clearance was pretty well established.
Jerry

Well Jerry that's just about exactly the way I’ve done it in the past and the way I’m checking this one. That is, using the 3" mic, bore gauge and indicator to check the crank clearance.
But my question really was why you never see a detailed explanation of just how they design the measurement or calculate the calibration. There is a formula on the Plastigauge site but I haven’t tried to dissect it, I never was much of a math wizard.
I have to assume that Sealed Power has calibrated Plastigauge taking into consideration that all of the clearance will be read off of one side of the bearing surface. In other words in the example I posted the nominal .002” oil clearance when every thing is torqued down will in reality be .004” where the Plastigauge strip is placed. And the paper gauge strip is calibrated to give you a reading half that amount. That is the only thing that seems to make any sense.

And Jon, I didn't actually mean that one would place plastigauge at 90°, I just meant theoreticly that would be the only way to find the true symetrical oil clearance.
Denny Graham
Sandwich, IL

Kips41,
those shims that they are selling are pretty useless. I ran into the same thing and made my own. You should have multiple shims of .002" each. I would be starting with three .002s on each side. I think bolting it down with no shims to start wil make sure that the shells are seated properly.

I'd be a little careful bolting the caps down without shims. That Babbitt is very thin and it doesn't take much to distort it or the shell. I think I'd sneak up on it from the other direction like the service manual suggests, starting out high and removing shims. If everything is clean and there aren’t any burrs the shells should snap in place right on size.
I've heard from quite a few sources that the shim packs available today aren't the best offering and that they ended up making their own shims.
I have shim stock that I've gotten from McMaster Carr and to make a really fine adjustment I'm planning on making my own shim stacks with .001” and .0005” shims.
Laminated Brass shim stock is available in .002” per layer, which is probably what the vendors are selling in their shim stacks.
Denny Graham
Sandwich, IL

When I used PG to check main bearing clearance in the vehicle, I'd put a small bottle jack under the crankshaft, and top the clearance out so all the slack would be on the bottom side. As long as there's a pretty good metal-to-metal contact with a minimum of oil film between the parts, it's very accurate, compared to an actual measurement of different diameters. For instance, if the plastic string compresses to indicate there's a .002" clearance, that's actually a .001" film of oil all the way around the shaft while the engine is running.
Jerry

Denny,
the shims I got from LMC were .006" thick! I can't remember who made them but it was a prominent name. I think someone read in the shop manual about starting out with .006" of shims and decided that is the thickness they should be.

Sounds like a Bummer Fred, I'll bet they didn't even give you a kiss when they gave it to ya!!
I think I remember Pete saying the same thing about the store bought shims that he bought being useless.
Just another case of, you're better off making it yourself.
Denny G

I did make my own got the shim stock from Mc Master carr and just cut out ,I got a punch for the holes.I ordered a set from Egges and from the filling station than made my own anyway. I used .002 and .004 brass stock.Easyer than I thought.

Pete

Thankx guys, for all the suggestions.

I should be starting next week and will post progress here

Here is an up date. Today I installed the crankshaft.

The block and crank have been back from the local machine shop for two weeks now. I knew they would be stored in my unheated Michigan garage for a while so I sprayed all the internal and machined surfaces with WD40 and put a plastic bag over everything. I started by cleaning the oil off of the crank main journals, block half-rounds and the main bearing caps. Next I placed the block halves of each bearing insert in place making sure they were the ones with the oil hole and the locating bump on the back side. Then I set the crank into the block on the new bearings being careful not to move or damage the bearings (Not all that easy, the thing is very heavy).

After assembling the other half of each bearing into its respective cap I set each cap in place and tightened then one at a time to insure that each would lock the crank from turning. Next all the caps were removed. Then starting with the rear cap I inserted a home-made 0.003’’ shim on each side, placed a piece of 0.001’’ to 0.003’’green plastigage across the top of the bearing and tightened the bearing cap bolts in 10 ft-lb increments to 100 ft-lb. After carefully removing the cap I checked the crushed plastigage with the scale on the package and found the bearing clearance to be 0.0015’’. I was lucky again on the #3 main, it was also 0.0015’’ with 0.003’’ shims on the first try. The #2 main took 3 tries and ended up at 0.0015’’ with a pair of 0.002’’ shims on each side. The front main took two tries and ended up with 0.002’’ shims on each side for 0.0015’’ clearance.

I then removed all the caps and the crank and installed the rear main seal. One more cleaning of the crank journals and bearings then I coated both with STP (recommended by the machine shop as a great assembly lube). Finally I reassembled everything and torqued all the bolts to 105 ft-lb. Now came the critical moment when I attempted to rotate the crankshaft. It took 40 ft-lb to start it the first time and around 15 ft-lb to keep it turning.

The whole process, including making the shims, took about 6 hours. I’m happy.