2000 Yamaha YFM400FWA(M) Factory Service workshop Manual, Page 144Get this manual

CAMSHAFT, ROCKER ARMS AND VALVES 9.Lap: Valve face Valve seat

NOTE: After refacing the valve seat or replacing the valve and valve guide, the valve seat and valve face should be lapped

Lapping steps: Apply coarse lapping compound to the valve face
CAUTION:

• ATV
• Jet Ski
• Outboard Motors
• Personal Watercraft
• Snowmobiles
• Stern Drives