Standard-diameter rod journals are 2.311"-2.312" so a 2"-3" mike would be appropriate. Common undersizes are .010"-.030" so the minimum diameter you're likely to encounter would be 2.280" or so. There are 4 different size main bearings, ranging from 2.683 at the front to 2.775 at the back. It takes a very special-design mike to measure mains in the block, something that would be difficult to find outside a specialized engine rebuilding shop. A pretty good guesstimate could be done with a modified set of outside calipers and an inside mike, after a LOT of practice. I've been measuring crankshafts since the early 1960's, and I wouldn't be very confident of getting a good reading that way.
Jerry
