Yamaha 2004 YP400, Service Manual, Page: 239 / 417

5 - 26
ENG
VALVES AND VALVE SPRINGS
NOTE:
_
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
NOTE:
_
After replacing the cylinder head or replacing
the valve and valve guide, the valve seat and
valve face should be lapped.
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a. Apply a coarse lapping compound
a
to the
valve face.
CAUTION:
_
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.
NOTE:
_
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 impres-
sion.