IMO the issue was not with the old (+.040") or new (+.060") piston O.D. dimensions. I measured these prior to assembly and were within +/- 0.0005" of the advertised diameter before top end assembly, in other words, within the accuracy of my measuring tools. I measured with both a micrometer and dial vernier caliper with the same results.
Because the old piston skirt showed scuff marks 180 degrees apart, I conclude that the cylinder bore before the most recent re-bore was oval, at least locally, and not enough clearance piston wall - to - cylinder existed at operating temperature, but adequate when running just after a cold startup. Hence the piston would bind after warming up to operating temperature and knock / stall.
The re-bore was machined out at a 0.004" clearance gap between the new +.060" piston and cylinder wall. Everything seems OK now.
