1991 Yamaha Outboard Factory Service Manual 9.9 and 15 HP, Page 59Get this manual
Check valve replacement aPull out the check valve using pliersNOTE: _ Do not pry the valve outRemove it by rotatingbThroughly clean the hole of sealing compoundcCoat new check valve, insert with LOCKING AGENT and drive it inNOTE: The insert should be driven in so that the delivery pipe is inclined 30 toward the cyliner headPiston 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 pistonPlace the micrometer at right angles to the piston pin Piston 1Remove any carbon deposits from the piston crown2Carefully remove any carbon deposits from the ring grooves with the filed end of ring3Remove all score marks and lacquer deposits from the sides of the piston using 600 800 grit wet sandpaperSand in crisscross patternDo not sand excessively