Yamaha WR125R 2009, Service Manual

Page: 214 / 372

CYLINDER AND PISTON
5-27
3. Measure:
• Piston ring end gap
Out of specification
→
Replace the piston
ring.
TIP
The oil ring expander end gap cannot be mea-
sured. If the oil ring rail gap is excessive, replace
all three piston rings.
EAS24440
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.
3. Measure:
• Piston pin bore inside diameter “b”
Out of specification
→
Replace the piston.
4. Calculate:
• Piston-pin-to-piston-pin-bore clearance
Out of specification
→
Replace the piston pin
and piston as a set.
EAS24450
INSTALLING THE PISTON AND CYLINDER
1. Install:
• Top ring “1”
• 2nd ring “2”
• Oil ring expander “3”
• Lower oil ring rail “4”
• Upper oil ring rail “5”
TIP
Be sure to install the piston rings so that the
manufacturer marks or numbers face up.
Piston ring
Top ring
End gap (installed)
0.10–0.25 mm (0.0039–0.0098
in)
Limit
0.50 mm (0.0197 in)
2nd ring
End gap (installed)
0.10–0.25 mm (0.0039–0.0098
in)
Limit
0.60 mm (0.0236 in)
Oil ring
End gap (installed)
0.20–0.70 mm (0.0079–0.0276
in)
Piston pin outside diameter
13.995–14.000 mm (0.5510–
0.5512 in)
Limit
13.975 mm (0.5502 in)
Piston pin bore inside diameter
14.002–14.013 mm (0.5513–
0.5517 in)
Limit
14.043 mm (0.5529 in)
• Piston-pin-to-piston-pin-bore clearance =
Piston pin bore inside diameter “b” -
Piston pin outside diameter “a”
Piston-pin-to-piston-pin-bore
clearance
0.002–0.018 mm (0.0001–0.0007
in)
Limit
0.068 mm (0.0027 in)