Yamaha YZ450F, Owner'S Service Manual

Page: 318 / 650

4 - 39
ENG
VALVES AND VALVE SPRINGS
9. Lap:
• Valve face
• Valve seat
NOTE:
After refacing the valve seat or replacing the
valve and valve guide, the valve seat and
valve face should be lapped.
Lapping steps:
• Apply a coarse lapping compound to the
valve face.
CAUTION:
Do not let the compound enter the gap
between the valve stem and the guide.
• 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.
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.