I have a piston/rod from an old LS block and I recently got a micrometer and have been practicing measuring internals of an engine...anyways... My question is, that I looked in the repair manual and it states that the piston diameter is supposed to be in the range of 3.1882" up to 3.1886". I measured this piston out to be 3.1776". That is too small, and outside of the serviceable range. Is it an actual possibility that the piston rubbed up against the walls so much that it essentially ''shrunk'' the piston?

 

You should measure piston diameter on the skirts, approx 0.5in up.