Yamaha YFM660RN Service Manual page 209

VALVES AND VALVE SPRINGS
Install the valve into the cylinder head.
Turn the valve until the valve face and valve
seat are evenly polished, then clean off all of
the compound.
NOTE:
For best lapping results, lightly tap the valve
seat while rotating the valve back and forth
between your hands.
Apply a fine lapping compound to the valve
face and repeat the above steps.
NOTE:
After every lapping operation be sure to clean
off all of the compound from the valve face and
valve seat.
Apply Mechanic's blueing dye (Dykem) to the
valve face.
Install the valve into the cylinder head.
Press the valve through the valve guide and
onto the valve seat to make a clear pattern.
Measure the valve seat width again. If the
valve seat width is out of specification, reface
and relap the valve seat.
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10.Measure:
Valve spring free length a
Out of specification → Replace.
5 - 29
ENG
Free length (valve spring):
Inner:
35.95 mm (1.42 in)
<Limit>: 34.15 mm (1.34 in)
Outer:
37.75 mm (1.49 in)
<Limit>: 35.86 mm (1.41 in)