The whole purpose of shimming the main bearings is to establish the right oil clearance. If the clearance is right without shims, no problem. My Federal-Mogul master bearing catalog lists the allowable main bearing clearance on all four bearings in the 1953-55 engine series as .0005" to .0031". I like to set the clearance at about .0015" unless very lightweight oil like 10W-30 will be used exclusively. Then you might go a little tighter. These engines were designed for straight 30 weight oil, so the larger clearances like .002" or more were acceptable with the heavier oils.
If you're going to shim the main bearings, be sure the shims slip all the way over the ends of the bearing shells. If only the bearing caps are shimmed, the bearings will try to rock back and forth in the block and they will eventually spin, ruining the bearing and the crankshaft. The shim stock should be sandwiched tightly between the top and bottom bearing shells so nothing is able to move. Instead of using Plastigage, I like to cut a small sliver of shim stock, maybe 1/4" wide by 1" long, and place it between the lower bearing shell and the crankshaft. If .002" is the desired clearance, use that size shim stock. Torque the cap bolts and try to turn the crankshaft by hand. Don't try full turns, just rock the crank back and forth a little. If it's tight, add a thin piece of shim between the cap and the block and try again. When the crank can be turned with just a slight amount of drag, the shim pack is right on that bearing. Remove the checking shim, torque the cap bolts, and proceed to adjust the next bearing. Go to the next one and do the same thing. Continue until all the bearings are shimmed to the correct oil clearance.
The shim thicknesses listed in the F-M catalog for Chevrolet 6-cylinders appear to be far too thick to be of much use. The shim set #FSM-124S lists shims of .0102" to .0085", and the FSM 105S shim pack has .006" to .0085" shims in it. I'm accustomed to making my own shims from brass stock in the .0015" to .003" thickness range.
Jerry
