Checking The Piston Ring; Removing The Piston And Piston Ring; Checking The Cylinder And Piston - Yamaha YZ250F 2008 Owner's Service Manual

REMOVING THE PISTON AND
PISTON RING
1. Remove:
• Piston pin clip "1"
• Piston pin "2"
• Piston "3"
• Put identification marks on each
piston head for reference during re-
installation.
• Before removing each piston pin,
deburr the clip groove and pin hole
area. If the piston pin groove is de-
burred and the piston pin is still dif-
ficult to remove, use the piston pin
puller set "4".
Piston pin puller set:
YU-1304/90890-01304
Do not use a hammer to drive the
piston pin out.
2. Remove:
• Piston ring "1"
Spread the end gaps apart while at
the same time lifting the piston ring
over the top of the piston crown, as
shown in the illustration.
CHECKING THE CYLINDER AND
PISTON
1. Inspect:
• Cylinder and piston walls
→
Vertical scratches Replace cyl-
inder and piston.
2. Measure:
• Piston-to-cylinder clearance
Measurement steps:
a. Measure the cylinder bore "C"
with a cylinder bore gauge.
Measure the cylinder bore "C" in par-
allel to and at right angles to the
crankshaft. Then, find the average of
the measurements.
77.00–77.01
Cylinder bore
mm (3.0315–
"C"
3.0319 in)
0.05 mm (0.002
Taper limit "T"
Out of round 0.05 mm (0.002
"R"
"C" = Maximum D
"T" = (Maximum D or D
1
imum D or D )
5 6
"R" = (Maximum D , D
1 3
(Minimum D , D or D
2 4
b. If out of specification, replace the
cylinder, and replace the piston
and piston rings as set.
c. Measure the piston skirt diameter
"P" with a micrometer.
a. 8 mm (0.31 in) from the piston
bottom edge
Piston size "P"
76.955–76.970
Standard mm (3.0297–
3.0303 in)
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"
4-26
CYLINDER AND PISTON
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
in)
the piston and rings as a set.
Clean carbon from the piston ring
in)
grooves and rings before measuring
the side clearance.
) - (Max-
2
or D ) -
5
)
6
Top
ring
2nd
ring
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)
Piston-to-cylinder clear-
ance:
0.030–0.055 mm
(0.0012–0.0022 in)
<Limit>:0.1 mm (0.004
in)
→
Replace
Side clearance:
<Lim-
Standard
it>
0.12
0.030–0.065
mm
mm (0.0012–
(0.005
0.0026 in)
in)
0.12
0.020–0.055
mm
mm (0.0008–
(0.005
0.0022 in)
in)