Mercury Mariner 200, 225 Optimax Outboards Service Manual, 90-855348, Page 249Get this manual
Piston and Piston Ring Combination .078 in
aHalf Keystone (tapered) Piston Ring
IMPORTANT: Piston ring side with dot or letter must be facing up
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(12.7mm) 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(O.2S4mm to 0.4S7mm)If end gap is greater, check other piston rings in cylinder bore, until rings (within tolerance) are found