Yamaha RS90GTZ Service Manual page 263

VALVES AND VALVE SPRINGS
10.Lap:
• Valve face
• Valve seat
TIP
After replacing the cylinder head or replacing
the valves and valve guides, the valve seat and
valve face should be lapped.
Lapping steps:
• Apply a coarse lapping compound
valve face.
NOTICE
Do not let the lapping compound enter the
gap between the valve stem and the valve
guide.
• Apply molybdenum disulfide oil onto 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 lapping compound.
TIP
For the 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.
• After every lapping procedure, be sure to
clean off all of the lapping compound from
the valve face and valve seat.
• Apply Mechanic's blueing dye (Dykem)
onto 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 impres-
sion.
• Measure the valve seat width
valve seat width is out of specification, reface
and lap the valve seat.
5-38
ENG
to the
again. If the