Yamaha F100B F100C Outboards Factory Service Manual, Page 124Get this manual
Power unit
Checking the camshaft
1Measure the cam lobeReplace if out of specification
8If the valve seat contact area is too narrow and situated near the bottom edge of the valve face, use 60 cutter to cut the bottom edge of the valve seatIf necessary, use 45 cutter to center the area and set its width
b Previous contact width
9Apply thin, even layer of lapping compound onto the valve seat, and then lap the valve using valve lapper (commercially obtainable)