Mercury Mariner 30/40 4-Stroke Outboard Service Manual 1998, Page 209Get this manual
CYLINDER HEAD
VALVE SEAT RECONDITIONING Clean the carbon deposits from the combustion chambers and valve seats and check for pittingSeveral different types of equipment are available for reseating valve seatsFollow the equipment manufacturer's instructionsMeasure valve seat width (a)Resurface the valve seat if not in specificationReface valve seat, use 60, 45, 15 valve seat cutterNOTE: When twisting cutter, keep an even downward pressure to prevent chatter marks