2009 Yamaha F40 Outboard Service Manual, Page 219Get this manual
Power unit 2Cut the surface of the valve seat with 45 cutter by turning the cutter clockwise until the valve seat face has become smoothNOTICE: Do not overcut the valve seatMake sure to turn the cutter evenly downward at pressure of 4050 (4.05.0 kgf, 8.811.0 lbf) to prevent chatter marks
Valve seat contact width a: 1.301.50 mm (0.0510.059 in)
Refacing the valve seat
After every lapping procedure, make sure to clean off any remaining lapping compound from the cylinder head and the valves1Reface the valve seats with the valve seat cutters