Yamaha XG250, Service Manual

Page: 168 / 278

CYLINDER AND PISTON
5-26
EAS24380
REMOVING THE PISTON
1. Remove:
●
Piston pin clips “1”
●
Piston pin “2”
●
Piston “3”
CAUTION:
ECA13810
Do not use a hammer to drive the piston pin
out.
NOTE:
●
Before removing the piston pin clip, cover the
crankcase opening with a clean rag to prevent
the piston pin clip from falling into the crank-
case.
●
Before removing the piston pin, deburr the
piston pin clip’s groove and the piston’s pin
bore area. If both areas are deburred, remove
them.
2. Remove:
●
Top ring
●
2nd ring
●
Oil ring
NOTE:
When removing a piston ring, open the end gap
with your fingers and lift the other side of the ring
over the piston crown.
EAS24400
CHECKING THE CYLINDER AND PISTON
1. Check:
●
Piston wall
●
Cylinder wall
Vertical scratches
→
Rebore or replace the
cylinder, and replace the piston and piston
rings as a set.
2. Measure:
●
Piston-to-cylinder clearance
▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼
a. Measure cylinder bore “C” with the cylinder
bore gauge.
NOTE:
Measure cylinder bore “C” by taking side-to-side
and front-to-back measurements of the cylinder.
Then, find the average of the measurements.
b. If out of specification, rebore or replace the
cylinder, and replace the piston and piston
rings as a set.
c. Measure piston skirt diameter D “a” with the
micrometer.
1
2
3
Bore “C”
74.000
–
74.010 mm (2.9134
–
2.9138
in)
Wear limit
74.100 mm (2.9173 in)
Taper limit “T”
0.050 mm
Out of round limit “R”
0.050 mm
“C” = maximum of D1 – D6
“T” = maximum of D1 or D2
–
maximum of D5
or D6
“R” = maximum of D1 , D3 or D5
–
minimum of
D2 , D4 or D6
A
B
C
D1 D2
D3 D4
D5 D6