2008 Polaris ATV Sportsman 300 400 H.O. Service Manual, Page 71Get this manual
ENGINE (300) Piston Inspection
1Measure piston outside diameter at point mm up from the bottom of the piston at right angle to the direction of the piston pin5Measure connecting rod small end IDReplace crankshaft if out-of-round
Piston Pin 6Measure piston ring to groove clearance by placing the ring in the ring land and measuring with thickness gaugeReplace piston and rings if ring-to-groove clearance exceeds service limitsPiston Ring