Yamaha YFM660RN Service Manual page 207

VALVES AND VALVE SPRINGS
3.Check:
Valve face
Pitting/wear → Grind the face.
Valve stem end
Mushroom shape or diameter larger than the
body of the stem → Replace.
4.Measure:
Margin thickness a
Out of specification → Replace.
5.Measure:
Runout (valve stem)
Out of specification → Replace.
NOTE:
When installing a new valve always replace
the guide.
If the valve is removed or replaced always
replace the oil seal.
6.Eliminate:
Carbon deposits
(from the valve face and valve seat)
7.Check:
Valve seats
Pitting/wear → Reface the valve seat.
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ENG
Margin thickness:
Intake:
0.85 ~ 1.15 mm
(0.0335 ~ 0.0453 in)
Exhaust:
0.85 ~ 1.15 mm
(0.0335 ~ 0.0453 in)
Runout limit:
0.01 mm (0.0004 in)