Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 266Get this manual
POWERHEAD
Piston Installation
1Before installing new piston rings, check gap between ring ends by placing each ring in its respective cylinder, then pushing ring about 12 in(12.7 mm) into cylinder using piston to assure proper position2Check end gap of each new piston ring with feeler gaugeEnd gap must be within 0.010 into 0.018 in(0.25 mm to 0.45 mm)If end gap is greater, check other piston rings in cylinder bore, until rings (within tolerance) are foundIMPORTANT: Piston ring side with dot or letter must be facing up