Mercury Mariner 30/40 4-Stroke Outboard Service Manual 1998, Page 212Get this manual
CYLINDER HEAD
3Check the valve face for pittingValves faces that are pitted must be refacedNOTE: Several different types of equipment are available for refacing valvesFollow the equipment manufacturer's instructionsNOTE: After refacing the valve seat or replacing the valve and valve guide, the valve seat and valve face should be lapped4Check the margin thickness (a) of the valves after the valves have been groundAny valve with margin thickness of less than the specification, should be replaced