Yamaha YZ450FY 2009 Owner's Service Manual page 143

CHECKING THE PISTON PIN
1. Inspect:
• Piston pin
Blue discoloration/grooves → Replace, then
inspect the lubrication system.
2. Measure:
• Piston pin-to-piston clearance
Measurement steps:
a. Measure the outside diameter (piston pin)
"a".
If out of specification, replace the piston pin.
Outside diameter (piston pin):
17.991–18.000 mm (0.7083–0.7087
in)
b. Measure the inside diameter (piston) "b".
Inside diameter (piston):
18.004–18.015 mm (0.7088–0.7093
in)
c. Calculate the piston pin-to-piston clearance
with the following formula.
Piston pin-to-piston clearance = Inside di-
ameter (piston) "b" - Outside diameter
(piston pin) "a"
d. If out of specification, replace the piston.
Piston pin-to-piston clearance:
0.004–0.024 mm (0.00016–0.00094
in)
<Limit>:0.07 mm (0.003 in)
CYLINDER AND PISTON
INSTALLING THE PISTON RING AND
PISTON
1. Install:
• Piston ring
Onto the piston.
• Be sure to install the piston rings so that the
manufacturer's marks or numbers are located
on the upper side of the rings.
• Lubricate the piston and piston rings liberally
with engine oil.
2. Position:
• Top ring
• 2nd ring
• Oil ring
Offset the piston ring end gaps as shown.
a. Top ring end
b. 2nd ring end
c. Oil ring end (upper)
d. Oil ring
e. Oil ring end (lower)
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