Yamaha YZ426F(M)/LC Owner's Service Manual page 296

VALVES AND VALVE SPRINGS
Measurement steps:
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.
4 - 31
Apply Mechanic's blueing dye (Dykem) b
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. Where the
valve seat and valve face made contact,
blueing will have been removed.
If the valve seat is too wide, too narrow, or
the seat is not centered, the valve seat
must be refaced.
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.
ENG