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
Last edited by 52Carl; 05/01/2017 2:57 AM.
