2007-2009 Yamaha F15/F20 Outboard Service Manual, Page 133Get this manual
Power unit
9If 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 seat, and then use 45 cutter to center the area and set its width
Checking the rocker arm and rocker arm shaft
1Check the rocker arms, rocker arm shaft, and rocker arm contact surface aReplace if worn2Measure the rocker arm inside diameter and rocker arm shaft outside diameter cReplace if out of specification
b Previous contact width
10After refacing the valve seat to the specified contact width, apply thin, even layer of lapping compound onto the valve seat, and then lap the valve using valve lapper