1983 Yamaha 30EN Outboards Service Manual, Page 66Get this manual
3 Remove score marks and lacquer deposits from sides of piston using 600 800 grit wet sandpaperSand in crisscross patternDo not sand excessively
Piston rings1Measure the end gap of each piston ringInsert ring into the cylinder, and push it approximately 20 mm (0.8 in) into the cylinderPush the ring with the piston crown so the ring will be at right angle to the cylinder bore2Measure the ring end gap with feeler gaugeIf the end gap exceeds tolerance, replace the whole set of ringsRing end gap installed: TopSecond Minimum: 0.2 mm (0.008 in) Maximum: 0.4 mm (0.016 in)
Piston clearance: 1Measure the outside diameter of the piston at the piston skirtMeasurement should be made at point 10 mm (0.4 in) above the bottom edge of the piston Place the micrometer at right angles to the piston pin