Yamaha YZ125X1 Owner's Service Manual page 70

Standard
54.000–
Cylin-
54.014 mm
der bore
(2.1260–
"C"
2.1265 in)
Taper
—
"T"
"C" = Maximum Aa–Cb
"T" = (Maximum Aa, or Ab) - (Max-
imum Ba, or Bb)
CHECKING THE PISTON
1. Eliminate:
• Carbon deposits
From the piston crown "a" and
ring groove "b".
2. Inspect:
• Piston wall
→
Score marks Repair or replace.
3. Measure:
• Piston skirt diameter
Use micrometer "1".
Measure the specific distance "a"
from the bottom edge.
→
Out of specification
Distance
Piston dia.
"a"
53.957–53.972
17.5 mm
mm (2.1243–
(0.69 in)
2.1249 in)
CYLINDER HEAD, CYLINDER AND PISTON
Wear limit
54.1 mm
(2.130 in)
0.05 mm
CHECKING THE PISTON PIN AND
(0.0020 in)
SMALL END BEARING
1. Inspect:
• Piston pin
• Small end bearing
Signs of heat discoloration
place.
2. Measure:
• Piston pin outside diameter
Use micrometer "1".
Out of limit
Standard
14.995–15.000
mm (0.5904–
0.5906 in)
3. Check:
• Free play (when the piston pin "1"
is in place in the piston "2")
There should be no noticeable for
the play.
Free play exists
pin and/or piston.
Replace.
4. Install:
• Small end bearing
• Piston pin
Into the small end of connecting
rod.
→
Re-
→
Replace.
Piston pin outside diam-
eter:
<Limit>
14.975 mm
(0.5896 in)
→
Replace piston
4-15
5. Check:
• Free play
There should be no noticeable
free play.
→
Free play exists Inspect the
connecting rod for wear/Replace
the pin and/or connecting rod as
required.
CHECKING THE PISTON RING
1. Install:
• Piston ring
Into the cylinder.
Push the ring with the piston
crown.
2. Measure:
• End gap
Use a thickness gauge "1".
→
Out of limit Replace.
Ring end gap (installed):
Standard <Limit>
0.5–0.7 mm 1.2 mm (0.047
(0.020–0.028 in)
3. Measure:
• Side clearance
Use a thickness gauge "1".
→
Out of limit Replace piston and/
or ring.
Side clearance:
Standard <Limit>
0.035–0.070
0.1 mm (0.004
mm (0.0014–
0.0028 in)
Check at several points.
in)
in)