1991 Yamaha Outboard Factory Service Manual 9.9 and 15 HP, Page 60Get this manual
PistQn rings 1Measure the end gap of each piston ringInsert ring into the cylinder and push it approximately 20 mm (0.8 in) into the cylinderPush 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: 0.15 mm (0.006 in) Maximum: 0.35 mm (0.014 in)
3Piston ringring groove fit must have correct clearanceIf the piston and ring have already been used in the engine, the ring must be removed, the ring groove cleimed of carbon, then the ring should be reinstalledUse feeler gauge to measure the gap between the ring and the ring groove landTop