2001 Yamaha YFM660 Raptor Factory Service Manual, Page 201Get this manual
CAMSHAFT AND CYLINDER HEAD
3.Measure: Cylinder head warpage Out of specification ResurfaceCylinder head warpage: Less than 0.05 mm (0.002 in)
Warpage measurement and resurfacing steps: Place straightedge and feeler gauge across the cylinder headUse feeler gauge to measure the warpageIf the warpage is out of specification, resurface the cylinder headPlace 400600 grit wet sandpaper on the surface plate, and resurface the head using figure-eight sanding patternNOTE: To ensure an even surface rotate the cylinder head several times