Yamaha UMAX, Service Manual

Page: 153 / 320

VALVES AND VALVE SPRINGS
5-14
CHECKING THE VALVE SEATS
The following procedure applies to all of the
valves and valve seats.
1. Eliminate:
• Carbon deposits
(from the valve face and valve seat)
2. Check:
• Valve seat
Pitting/wear
→
Replace the cylinder head.
3. Measure:
• Valve seat width “a”
Out of specification
→
Replace the cylinder
head.
a. Apply blue layout fluid “b” onto the valve
face.
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 width.
TIP
Where the valve seat and valve face contacted
one another, the blueing will have been re-
moved.
4. Lap:
• Valve face
• Valve seat
TIP
After replacing the cylinder head or replacing the
valve and valve guide, the valve seat and valve
face should be lapped.
a. Apply a coarse lapping compound “a” to
the valve face.
NOTICE
Do not let the lapping compound enter the
gap between the valve stem and the valve
guide.
b. Apply engine oil onto the valve stem.
c. Install the valve into the cylinder head.
d. Turn the valve until the valve face and
valve seat are evenly polished, then clean
off all of the lapping compound.
Valve seat contact width (intake)
0.7–0.9 mm (0.0276–0.0354 in)
Valve seat contact width (intake)
limit
1.6 mm (0.063 in)
Valve seat contact width (ex-
haust)
0.7–0.9 mm (0.0276–0.0354 in)
Valve seat contact width (ex-
haust) limit
1.6 mm (0.063 in)