VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Main bearing adjustment from the past...

So I dug into adjusting the main bearings in the '29 today starting with the rear... which is where I stopped. I'm not the first one in there (no surprise really). There was one .003 shim on the drivers side and five .002 shims on the passenger side. the bearing bottom itself looked ok, no cracks or chunks missing and the journal on the crank looked good.

The 29 repair manual says to take one shim off each side, I've read elsewhere that there can be one more on either side for the right fit but a ratio of 1-5 doesn't seem right.

Just to run a quick little test I removed two of the .002 shims from the high side... one I left out totally, the other I moved to the side with only one shim increasing it to .005 and reducing the other side to .006. I wasn't sure of the torque spec on the main bolts so I took them up to 50ft/lb and the crank still spun. (I should note here that i'm trying the foil go/no go method of adjusting with .002 and .001 shims)

Should I just start from scratch with new shims (the ones in there now are home made and not so great) and keep them equal on each side or will this mess up the bearing that has worn into the crank with so many shims on one side?

If it were ME, I would go with equal on each side. Shim to .0015.

I agree that you should try to equalize the shims.

I set my clearances in my '37 using the method of removing shims and reinstalling the cap until you felt drag. Remember that you might have to loosen the other main caps during this process.

After I reinstalled shims so there was no drag I then used Plastigage to get a rough measurement of clearance. Because the engine was in the car I used a bottle jack to push up on the crank. That way I knew that all of the clearance was between the journal and the cap.

I also checked the clearance at a second position that was 90 degrees rotated from the first position. Yes, it took some extra time but gave me confidence that the clearances were within the specified range and that the journal was not noticeable out-of-round.

I know that many people especially machine shops will tell you that Pastigage is a waste of time. My experience is that it works great for these older, slower speed engines. I agree it is not appropriate my 7000 rpm Miata engine.

Don't understand the negative on plastiguage. Works for over 8k rpm on my engine.

I also have used plastigauge to get an idea of the gap between bearings. It is not accurate enough to do much more. The problem with plastigauge on thick babbitt bearings is that the plastigauge is stiff enough to slightly indent the babbitt. That results in an indication that the gap is wider than actual. Modern engines only have a few thousandths thick babbitt. It does not indent like the thicker babbitt does.

How do you feel drag? With plugs out it still takes some effort to get the crank moving to overcome the pistons in the cylinders... not sure I have confidence enough to try that and then fire it up for fear of burning out a bearing...

Does anybody know a torque value for the mains and rods? The repair manual only gives vague specifications about being tight and loose.

Yes, it can be tricky to feel the difference between friction in the cylinders and bearing drag.

Here are some ideas to consider.

1) Squirt some motor oil into each cylinder and let it sit for a while. Then turn the engine over quite a few revolutions to get that oil on the cylinder walls. This will give you lower and more consistent friction between the rings and cylinder wall.

2) Loosen all the main bearing caps and feel the drag as you turn the crank. This is your baseline or "least drag" condition.

3) Tighten one main bearing cap and turn the crank to determine if there is more resistance. Make sure you keep a little bit of oil on the main bearings. Just squirt some where the bearing and journal meet. If the resistance does not increase then remove a shim from that cap and try again. Keep removing shims one at a time until you feel more drag. Then add one shim back to that cap and confirm the drag is lower. Then loosen that cap and move on to the next one.

4) One other trick is to make sure you feel the drag as the crank is rotating. It always takes a little more "umph" (technical term) to get things moving.

With respect to torque specs I use a reference table with torque values for grade 2 hardware. There was very little higher strength hardware in cars of this era.

Great info Rusty... I'll give it a go when I get a chance to cut my laminated shim stock into the appropriate shapes for front, middle, and rear mains... waiting on a hole puncher.

I also made my own main bearing shims for my '28 engine rebuild last year. I drilled the holes with a stepped, unibit drill bit.

Here is a pointer to that write up, which you might find interesting: '28 four banger shim discussion

Cheers, Dean

A method to drill shim stock without doing unwanted damage is to clamp the stock between 2 pieces of otherwise not needed material prior to cutting out.
Tony

Thanks guys!... Its supposed to rain here tomorrow so i'll be back in the garage working on the bearings.

Rustoholic, nice thread about you building up your engine... I'm going to have to get me one of those lifter adjuster tools, pretty slick!

Us ex a Whitney punch for clean holes.

OK guys i'm getting no place...

I've made my shims no problem, I started with the rear main with .006 worth of shim and it spun freely. Removed .002 from both sides and it still spun freely. got down to .001 and I think there is some drag, but man I can't really tell... no frame of reference here. if I take all the shims out it locks up and won't spin so that's good I guess. This is very frustrating, lol.

I've loosened all the mains and connecting rods so the only tight bearing is the rear main, torqued to 44ft/lb as per the filling stations torque chart for that size bolt.

So if I feel some drag at .001 then I should add an additional .001 to both sides and call it good?

If you are testing with dry bearings yes .001 each side but if testing with oiled bearing you could try putting a .001 on 1 side only. A ,001 side to side difference will have no noticeable effect.
Tony

I use .001" and .002" brass shim stock for this. I lay a 1/4" wide piece across the bearing and snug up the bearing. When it binds on.002" and doesn't on .001" I know it is where I want it.

Are you tightening the cap bolts all the way when testing or just snug?

I always tighten the cap to the normal clamp load when checking clearances. The cap and block will be a slightly different shape and closer together when clamped for normal operation compared to just snug.

That's the way i've always done bearings also... good to know that its consistent with the chevy 6. Thanks!