Yamaha YFM660FS Grizzly 4x4 Supplementary Service Manual page 240

VALVES AND VALVE SPRINGS
3.Check:
Valve face
G
Pitting/wear → Grind the face.
Valve stem end
G
Mushroom shape or diameter larger than the
body of the stem → Replace.
4.Measure:
Margin thickness a
G
Out of specification → Replace.
5.Measure:
Runout (valve stem)
G
Out of specification → Replace.
NOTE:
When installing a new valve always replace
G
the guide.
If the valve is removed or replaced always
G
replace the oil seal.
6.Eliminate:
Carbon deposits
G
(from the valve face and valve seat)
7.Check:
Valve seats
G
Pitting/wear → Reface the valve seat.
4 - 27
ENG
Margin thickness:
Intake:
0.85 ~ 1.15 mm
(0.0335 ~ 0.0452 in)
Exhaust:
0.85 ~ 1.15 mm
(0.0335 ~ 0.0452 in)
Runout limit:
0.01 mm (0.0004 in)