olblu,
Each engine has specs that tell how much clearance you need, within a certain tolerance, between the crank and the bearing. When you get a crank ground, then the new bearing inserts have to be a little thicker to compensate for the lost metal on the resized crank. Same concept as using bigger pistons and rings when you have the cylinders bored out. The plastigage won't tell you what grind your crank is (.010, .020, etc.). All it tells you is if you have the proper clearance between the insert and the crank. If the plastigage compresses further than the spec sheet says it should, then you've got to use shims under the journals to get the proper clearance.
In addition, the plastigage also tells you if the fit is consistant across the width of the insert. When you tighten the journals down, you want an even fit across the entire bearing. So, if one end of the plastigage compresses more than the other, you use shims on the tighter end of the journal and keep plastigaging until it's the same or very close all the way across. It's all just part of balancing the engine and setting it all up for even wear and the longest life possible between rebuilds. It can be a tedious process, and I've had times where I did the same bearing 3 or 4 times to get a fit I was happy with, but it's well worth it.
Hope this is all clear as mud :-)
/Bobby
