Yamaha WR250FM 2021, Owner'S Service Manual

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CYLINDER AND PISTON
5-35
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.
EAM30098
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 clearance,
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 piston rings.
EAM30099
CHECKING THE PISTON PIN
1. Check:
• Piston pin
Blue discoloration/grooves

Replace the
piston pin and then check the lubrication sys-
tem.
2. Measure:
•
Piston pin outside diameter “a”
Out of specification

Replace the piston pin.
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 “C” - 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 limit
0.50 mm (0.0197 in)
b. Upper of cylinder
a
b