Yamaha YZ 2020 Series, Owner'S Service Manual, Page: 217 / 376

VALVES AND VALVE SPRINGS
5-30
b. Install the valve into the cylinder head.
c. Press the valve through the valve guide
and onto the valve seat to make a clear
impression.
d. Measure the valve seat contact width “c”.
TIP
Where the valve seat and the valve face are in
contact with each other, the blueing will have
been removed.
4. Lap:
• Valve face
•
Valve seat
NOTICE
ECA24640
This model uses titanium intake and exhaust
valves.
Do not use the valves used for lapping the
valve seat. Always replace the valves used
for lapping with new ones.
TIP
• When replacing the cylinder head, replace also
the valves with new ones without them.
• When replacing the valves or the valve guides,
use new valves to lap the valve seats, and then
replace them with new valves.
a. Apply a coarse lapping compound “a” to
the valve face.
NOTICE
ECA13790
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 the
valve seat are evenly polished, then clean
off all of the lapping compound.
TIP
While turning the valve lapper, tap and lap the
valve seat.
e. Apply a fine lapping compound to the
valve face, and repeat the above steps.
f. After every lapping step, be sure to clean
off all of the lapping compound from the
valve face and the valve seat.
g. Apply blue layout fluid “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
impression.
j. Measure the valve seat contact width “c”
again. If the valve seat contact width is out
of specification, reface and lap the valve
seat.
c c c