1998-2006 Yamaha F20/F25 Outboards Service Manual, Page 171Get this manual
CAMSHAFT AND VALVES
Valve seat refacing
After refacing the valve seat or replacing the valve and valve guide, lap the valve seat and valve face
To prevent chatter marks, turn the valve seat cutter while applying an even, downward pressure (4kg)
Valve seat cutter set: YM-91043-C90890-06803