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

Yz series
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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
or D
"T" = (Maximum 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.
4-28
CYLINDER AND PISTON
a.
9 mm (0.35 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"
f. If out of specification, replace the
cylinder, and replace the piston
in)
and piston rings as set.
in)

CHECKING THE PISTON RING

1. Measure:
• Ring side clearance
) - (Max-
2
Use a feeler gauge "1".
Out of specification→Replace the
or D
) -
5
piston and rings as a set.
)
6
Clean carbon from the piston ring
grooves and rings before measuring
the side clearance.
Top
ring
2nd
ring
Piston size "P"
76.955–76.970
mm (3.0297–
3.0303 in)
Piston-to-cylinder clear-
ance:
0.030–0.055 mm
(0.0012–0.0022 in)
<Limit>:0.1 mm (0.004
in)
Side clearance:
Standard
<Limit>
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)

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Yz250f b 2012Yz250f 20122013 yz250f2013 yz250fd

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