Checking The Valve Springs - Yamaha MTN850-A 2017 Service Manual

TIP
After replacing the cylinder head or replacing the
valve and valve guide, the valve seat and valve
face should be lapped.
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a. Apply a coarse lapping compound "a" to the
valve face.
ECA13790
NOTICE
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 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 be-
tween your hands.
e. Apply a fine lapping compound to the valve
face and repeat the above steps.
f. After every lapping procedure, be sure to
clean off all of the lapping compound from the
valve face and valve seat.
g. Apply blue layout fluid "b" onto the valve face.
VALVES AND VALVE SPRINGS
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 impres-
sion.
j. Measure the valve seat contact width "c"
again. If the valve seat width is out of specifi-
cation, reface and lap the valve seat.
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EAS30286

CHECKING THE VALVE SPRINGS

The following procedure applies to all of the
valve springs.
1. Measure:
• Valve spring free length "a"
Out of specification  Replace the valve
spring.
Free length (intake)
39.31 mm (1.55 in)
Limit
37.34 mm (1.47 in)
Free length (exhaust)
37.78 mm (1.49 in)
Limit
35.89 mm (1.41 in)
5-26