2007-2009 Yamaha F15/F20 Outboard Service Manual, Page 131Get this manual
Power unit
Checking the valve seat
1Eliminate carbon deposits from the valve2Apply thin, even layer of Mechanic's blueing dye (Dykem) onto the valve seat3Press the valve slowly against the valve seat with valve lapper as shown
Valve seat contact width a: Intake and exhaust: 1.21.6 mm (0.0470.063 in)
Refacing the valve seat
1Reface the valve seat with the valve seat cutters