2000 Yamaha YFM400FWA(M) Factory Service workshop Manual, Page 143Get this manual
CAMSHAFT, ROCKER ARMS AND VALVES
5.Measure: Runout (valve stem) Out of specification ReplaceRunout limit: 0.01 mm (0.0004 in) NOTE: When installing new valve always replace the guideIf the valve is removed or replaced always replace the oil seal6.Eliminate: Carbon deposits (from the valve face and valve seat) 7.Inspect: Valve seats Pittingwear Reface the valve seat