Yamaha 2009 YFM25RY Service Manual page 185

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a. Apply Mechanic's blueing dye (Dykem) "b"
onto the valve face.
b. Install the valve into the cylinder head.
c. Press the valve through the valve guide
and onto the valve seat to make a clear
impression.
d. Measure the valve seat width.
TIP
Where the valve seat and valve face contacted
one another, the blueing will have been
removed.
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4. Lap:
• Valve face
• Valve seat
TIP
After replacing the cylinder head or replacing
the valve and valve guide, the valve seat and
valve face should be lapped.
Valve lapper
90890-04101
Valve lapping tool
YM-A8998
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a. Apply a coarse lapping compound "a" to
the valve face.
VALVES AND VALVE SPRINGS
ECA13790
Do not let the lapping compound enter the
gap between the valve stem and the valve
guide.
b. Apply molybdenum disulfide oil onto the
valve stem.
c. Install the valve into the cylinder head.
d. Turn the valve until the valve face and valve
seat are evenly polished, then clean off all
of the lapping compound.
TIP
For the best lapping results, lightly tap the
valve seat while rotating the valve back and
forth between your hands.
e. Apply a fine lapping compound to the valve
face and repeat the above steps.
f. After every lapping procedure, be sure to
clean off all of the lapping compound from
the valve face and valve seat.
g. Apply Mechanic's blueing dye (Dykem) "b"
onto the valve face.
h. Install the valve into the cylinder head.
i. Press the valve through the valve guide
and onto the valve seat to make a clear
impression.
j. Measure the valve seat width "c" again. If
the valve seat width is out of specification,
reface and lap the valve seat.
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