Yamaha YZ450F Owner's Service Manual page 106

2. Position:
• Piston ring
(in cylinder)
Insert a ring into the cylinder and
push it approximately 40mm (1.57 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. 40 mm (1.57 in)
3. Measure:
• Ring end gap
Out of specification → Replace.
You cannot measure the end gap on
the expander 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
Top mm
ring (0.008–0.01
2 in)
0.35–0.50
2nd mm
ring (0.014–0.02
0 in)
0.20–0.50
Oil mm
ring (0.01–0.02
in)
CHECKING THE PISTON PIN
1. Inspect:
• Piston pin
Blue discoloration/grooves → Re-
place, 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.
b. Measure the inside diameter (pis-
ton) "b".
<Limit>
0.55 mm
(0.022 in)
c. Calculate the piston pin-to-piston
clearance with the following for-
mula.
0.85 mm
(0.033 in)
Piston pin-to-piston clearance =
Inside diameter (piston) "b" -
Outside diameter (piston pin)
"a"
—
d. If out of specification, replace the
piston.
CYLINDER AND PISTON
Outside diameter (piston
pin):
17.991–18.000 mm
(0.7083–0.7087 in)
Inside diameter (piston):
18.004–18.015 mm
(0.7088–0.7093 in)
Piston pin-to-piston
clearance:
0.004–0.024 mm
(0.00016–0.00094 in)
<Limit>:0.07 mm (0.003
in)
4-25
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.
25 25
115
25
25
a. Top ring end
b. 2nd ring end
c. Oil ring end (upper)
d. Oil ring
e. Oil ring end (lower)