Checking The Piston Rings - Yamaha 2009 YFM25RY Service Manual

b. If out of specification, rebore or replace the
cylinder, and replace the piston and piston
rings as a set.
c. Measure piston skirt diameter D "a" with
the micrometer.
Piston size diameter "D"
Standard
73.983–73.998 mm (2.9127–
2.9133 in)
b. 5 mm (0.20 in) from the bottom edge of the
piston
d. If out of specification, replace the piston
and piston rings as a set.
e. Calculate the piston-to-cylinder clearance
with the following formula.
• Piston-to-cylinder clearance =
Cylinder bore "C" -
Piston skirt diameter "D"
Piston-to-cylinder clearance
0.010–0.025 mm (0.0004–
0.0010 in)
Limit
0.15 mm (0.006 in)
f. If out of specification, rebore or replace the
cylinder, and replace the piston and piston
rings as a set.
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EAS24430

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.
CYLINDER AND PISTON
Piston ring
Top ring
Ring side clearance
0.030–0.065 mm (0.0012–
0.0026 in)
Limit
0.115 mm (0.0045 in)
2nd ring
Ring side clearance
0.020–0.055 mm (0.0008–
0.0022 in)
Limit
0.115 mm (0.0045 in)
2. Install:
• Piston ring
(into the cylinder)
TIP
Level the piston ring into the cylinder with the
piston crown.
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.
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