Yamaha YZ 2018 Series, Owner'S Service Manual

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CYLINDER AND PISTON
5-31
d. If out of specification, replace the cylinder,
the piston, and the piston rings as a set.
e. Calculate the piston-to-cylinder clearance
with the following formula.
f. If out of specification, replace the cylinder,
the piston, and the piston rings as a set.
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EASB29B232
CHECKING THE PISTON RINGS
1. Measure:
• Piston ring side clearance
Out of specification

Replace the piston
and piston rings as a set.
TIP
Before measuring the piston ring side clear-
ance, eliminate any carbon deposits from the
piston ring grooves and piston rings.
2. Install:
• Piston ring
(into the cylinder)
TIP
Use the piston crown to level the piston ring
near bottom of cylinder “a”, where cylinder wear
is lowest.
3. Measure:
• Piston ring end gap
Out of specification

Replace the piston
ring.
TIP
The oil ring expander spacer’s end gap cannot
be measured. If the oil ring rail’s gap is exces-
sive, replace all three piston rings.
EASB29B233
CHECKING THE PISTON PIN
1. Check:
• Piston pin
Blue discoloration/grooves

Replace the
piston pin and then check the lubrication
system.
Diameter
76.955–76.970 mm (3.0297–
3.0303 in)
Measuring point (from piston
skirt bottom)
4.0 mm (0.16 in)
Piston-to-cylinder clearance =
Cylinder bore - Piston diameter
Piston-to-cylinder clearance
0.030–0.055 mm (0.0012–0.0022
in)
Ring side clearance
0.030–0.065 mm (0.0012–0.0026
in)
Side clearance limit
0.115 mm (0.0045 in)
b
a
Piston ring
Top ring
End gap (installed)
0.15–0.25 mm (0.0059–0.0098 in)
End gap limit
0.50 mm (0.0197 in)
b. Upper of cylinder
a
b