Checking The Piston Rings - Yamaha YZ250F Owner's Service Manual

b. If out of specification, rebore or replace the
cylinder, and replace the piston and the pis-
ton rings as a set.
c. Measure the piston outside diameter D "a" at
the measurement position H "b" with the mi-
crometer.
Diameter D
76.955–76.970 mm (3.0297–
3.0303 in)
Measurement position H
9.0 mm (0.35 in)
b
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.
Piston-to-cylinder clearance =
Cylinder bore - Piston diameter
Piston-to-cylinder clearance
0.030–0.055 mm (0.0012–0.0022
in)
Limit
0.15 mm (0.006 in)
f. If out of specification, replace the cylinder,
the piston, and the piston rings as a set.
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CHECKING THE PISTON RINGS

EAS1SM5230
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.
a
CYLINDER AND PISTON
Ring side clearance
0.030–0.065 mm (0.0012–0.0026
in)
Limit
0.120 mm (0.0047 in)
2. Install:
• Piston rings
TIP
Level the piston ring into the cylinder with the
piston.
a. 10 mm (0.39 in)
3. Measure:
• Piston ring end gap
Out of specification  Replace the piston
ring.
TIP
The oil ring expander's end gap cannot be
measured. If the oil ring rail's gap is excessive,
replace all three oil rings.
Piston ring end gap
Top ring
End gap (installed)
0.15–0.25 mm (0.0059–0.0098 in)
Limit
0.50 mm (0.0197 in)
Oil ring
End gap (installed)
0.10–0.35 mm (0.0039–0.0138 in)
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