The best thing to do is measure the piston with a caliper. Digital calipers cost about $100 at industrial tool companies. A digital caliper is easy to use and gives accurate measurements on the piston diameter and cylinder bore. Measure the widths of the piston (front to back) just above the intake cutaway because this is the widest point of the piston. Check the maximun wear specs in your service manual. Check the piston for detonation marks in the crown, cracks in the shirt, or seizure marks. Look at the underside of the piston crown for a large black spot. The spot is burnt oil deposits that adhered to the piston because the piston crown temperature was too hot. This is an indicator that the carbâ€™s main jet needs to be richer.