1991 Yamaha Outboard Factory Service Manual 9.9 and 15 HP, Page 65Get this manual
Cylinder body
Hold the crankshaft at right angle to the cylinder body
1Thoroughly clean the bench, and place the cylinder body with the head side downApply the outboard motor oil to the upper and lower cylinders
2Apply the outboard motor oil to the upper and lower cylinder pistons and their ring grooves3Align the piston ring end gaps with the respective locating pins
bBy turning the crankshaft, insert the lower cylinder piston into the lower cylindercPush down the crankshaft so it fits in the cylinder bodyBe sure to fit the upper, center, and lower bearing locating pins, as well as the labyrinth seal circlips, into the cylinder body