There's an item known as a "taper shim" or sometimes called a "feather shim" that was used for correcting excessive bearing clearance errors. It's a piece of shim stock that's twice as thick in the middle as it is on both ends. For instance, if you want to correct for .002" too much clearance, the shim is .002" thick at the center, tapering to .001" thick on both ends. That prevents the bearing shell from being squeezed too tight at the sides, where the correction would be .004" with a shim the same thickness from end to end. It's a "shade tree" fix at best. It's also necessary to file the end of the bearing shell a little to prevent getting too much "crush" on the bearing shells as they're squeezed together when the bolts are torqued.
It's not uncommon to get a slightly wider clearance at the rear main bearing, especially if a rope oil seal is being used. I always check for clearance BEFORE installing a rope seal, and then I'm VERY careful not to sandwich a piece of seal fiber between the main cap and the block.
I print out some folks' technical advice, shred it and apply it to my garden!
Jerry
