1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 175Get this manual
CYLINDER HEAD
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 replaced or refaced
NOTE: Several different types of equipment are available for refacing valvesFollow the equipment manufacturer's instructionsIMPORTANT: Do not lap the valves4Check the margin thickness (a) of the valves after the valves have been groundAny valve with margin thickness of less than the specification, should be replaced45