Mercury Mariner 25HP 4-Stroke Outboard Service Manual 1997, Page 140Get this manual
Valves 1Clean the carbon deposits from the valveDis card any cracked, warped, or burned valves2Measure the valve stem diameter to check for wearReplace valves jf not within specification55810
5Measure valve stem runoutreplace if out 01 specification55811
Valve Stem Runout Limit (Max Intake Valve Exhaust Valve 0.0006 in(0.016 mm)
Valve Stem Diameter
Intake Valve Exhaust Valve
0.21560.2161 in(5.4755.490 mm) 0.21500.2156 in(5.4605.475 mm)
3Check the valve face for pittingValves faces that are pitted must be relaced