Yamaha YZ250F(D) Owner's Service Manual page 103

2. Position:
• Piston ring
(in cylinder)
Insert a ring into the cylinder and
push it approximately 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 expander spacer of the oil control
ring. If the oil control ring rails show
excessive gap, replace all three rings.
End gap:
Standard
Top 0.15–0.25 mm
ring (0.006–0.010 in)
2nd 0.30–0.45 mm
ring (0.012–0.018 in)
Oil 0.10–0.40 mm
ring (0.004–0.016 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.
Outside diameter (piston
pin):
15.991–16.000 mm
(0.6296–0.6299 in)
b. Measure the inside diameter (pis-
ton) "b".
Inside diameter (piston):
c. Calculate the piston pin-to-piston
clearance with the following for-
mula.
Piston pin-to-piston clearance =
Inside diameter (piston) "b" -
Outside diameter (piston pin)
"a"
d. If out of specification, replace the
<Limit>
piston.
0.50
Piston pin-to-piston
mm
clearance:
(0.020
in)
0.80
mm
(0.031
in)
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.
a. Top ring
b. 2nd ring
CYLINDER AND PISTON
16.002–16.013 mm
(0.6300–0.6304 in)
0.002–0.022 mm
(0.0001–0.0009 in)
<Limit>:0.07 mm (0.003
in)
4-29
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)
3. Install:
• Piston "1"
• Piston pin "2"
• Piston pin clip "3"
• Apply engine oil onto the piston pin
and piston.
• Install the piston with the valve re-
cess "a" (at two points) facing to the
engine exhaust side.
• Before installing the piston pin clip,
cover the crankcase with a clean
rag to prevent the piston pin clip
from falling into the crankcase.