The Stovebolt Site Forums The Shop Area Engine Shop 1954 235 Factory Shim Question

Hello,

I am in the process of rebuilding a 1954 235 and had a question about the main bearing shims. This engine has insert bearing for the rods.

The engine had 0.005" shims under all of the mains from the factory.

Were these shims installed before the bores were machined? So that the bores are perfectly round with the 0.005" shims installed? Or were the mains bored and then the 5 thou shims installed so the bores are oblong/not a perfect circle from the start?

Thanks.

The simple way to tell would be to torgue the bearing caps up with the crankshaft removed and measure the bores with an inside micrometer or a dial bore gauge. The purpose of the shims is to adjust the oil clearance on a slightly worn bearing by removing a thin layer of shim during an in-the-frame rebuild, same as adjusting rod bearing clearance on Babbit rods.
Jerry

Engine is at the machine shop right now and they are going to assemble it so unfortunately I will not have access to it.

All it takes is a phone call. If they're not willing to do some measuring, find a real machinist.
Jerry

I have their answer but I would like a second opinion if anyone else has dealt with these.

I have wondered the same thing about GM retaining the shims on the blocks which were changes to use insert bearings.
My guess differs from Jerry's theory however.
GM is well known for resisting change in how they design parts. I am guessing that rather than changing the entire manufacturing process of the block, they only altered it to accept insert bearings and used the shims to make the block work.
Where I differ from Jerry's description, is that once the insert bearings lose enough material to impact oil pressure, there won't be enough bearing material left to be useful for further service. Unlike insert bearings, babbit bearings have a significantly thicker layer of material, which makes it possible to remove shims to take up the slack from normal wear.
I would like to see what the differences in the dimensions between the shimmed/insert blocks, and the later non-shimmed/insert blocks. That would tell the us something.
Carl

Assuming that the shims were added before machining and the bearings are a perfect circle with the 0.005" shims.

Is it a bad idea to polish a crank that is 0.001" undersized and remove a shim during an engine rebuild? Or is it frowned upon to remove a shim to fit the undersized crank?

Should the crankshaft be ground to the next undersized bearing instead or are both ways acceptable for these engines?

Do the shims overlap the ends of the bearing shells, or do the bearings butt together and the shims go only between the block and main bearing cap? If the shims overlap the ends of the bearing, removing a layer of shim will alter the oil clearance, just like on a Babbit rod. If they're only there to make the hole round and the bearing doesn't contact the shim, it would be necessary to also file the end of the bearing a little to alter the oil clearance by removing shims.
Jerry