Yamaha 2001 YZ426F/LC Owner's Service Manual page 306

VALVES AND VALVE SPRINGS
9. Lap:
NOTE:
After refacing the valve seat or replacing the
valve and valve guide, the valve seat and
valve face should be lapped.
Lapping steps:
Do not let the compound enter the gap
between the valve stem and the guide.
NOTE:
For best lapping results, lightly tap the valve
seat while rotating the valve back and forth
between your hands.
NOTE:
After every lapping operation be sure to
clean off all of the compound from the valve
face and valve seat.
4 - 35
Valve face
Valve seat
Apply a coarse lapping compound to the
valve face.
CAUTION:
Apply molybdenum disulfide oil to the
valve stem.
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.
Apply a fine lapping compound to the
valve face and repeat the above steps.
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.
ENG