2000 Yamaha YFM400FWA(M) Factory Service workshop Manual, Page 152Get this manual
CYLINDER AND PISTON
3.Measure: Ring end gap Out of specification ReplaceNOTE: You cannot measure the end gap on the expander spacer of the oil control ringIf the oil control ring rails show excessive gap, replace all three ringsEnd gap Standard Top ring 2nd ring Oil ring 0.20.4 mm (0.0080.016 in) 0.40.6 mm (0.0160.024 in) 0.20.7 mm (0.0080.028 in) Limit 0.65 mm (0.026 in) 0.95 mm (0.037 in)
PISTON PIN INSPECTION 1.Inspect: Piston pin Blue discolorationgrooves Replace, then inspect the lubrication system