Yamaha YZ450FY 2009 Owner's Service Manual page 142

c. Measure the piston skirt diameter "P" with a
micrometer.
a. 8 mm (0.31 in) from the piston bottom
edge
Standard
d. If out of specification, replace the piston and
piston rings as a set.
e. Calculate the piston-to-cylinder clearance
with following formula:
Piston-to-cylinder clearance = Cylinder
bore "C" - Piston skirt diameter "P"
Piston-to-cylinder clearance:
0.020–0.045 mm (0.0008–0.0018 in)
<Limit>:0.1 mm (0.004 in)
f. If out of specification, replace the cylinder,
and replace the piston and piston rings as
set.
CHECKING THE PISTON RING
1. Measure:
• Ring side clearance
Use a feeler gauge "1".
Out of specification → Replace the piston
and rings as a set.
Clean carbon from the piston ring grooves and
rings before measuring the side clearance.
Side clearance:
Standard
0.030–0.065 mm
Top ring
(0.0012–0.0026 in)
0.020–0.055 mm
2nd ring
(0.0008–0.0022 in)
Piston size "P"
94.965–94.980 mm
(3.7388–3.7394 in)
<Limit>
0.12 mm
(0.005 in)
0.12 mm
(0.005 in)
4-41
CYLINDER AND PISTON
2. Position:
• Piston ring
(in cylinder)
Insert a ring into the cylinder and push it ap-
proximately 10 mm (0.39 in) into the cylinder.
Push the ring with the piston crown so that the
ring will be at a right angle to the cylinder bore.
a. 10 mm (0.39 in)
3. Measure:
• Ring end gap
Out of specification → Replace.
You cannot measure the end gap on the ex-
pander spacer of the oil control ring. If the oil
control ring rails show excessive gap, replace
all three rings.
End gap:
Standard
0.20–0.30 mm
Top ring
(0.008–0.012 in)
0.35–0.50 mm
2nd ring
(0.014–0.020 in)
0.20–0.50 mm (0.01–
Oil ring
0.02 in)
<Limit>
0.55 mm
(0.022 in)
0.85 mm
(0.033 in)
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