Yamaha YZ125 2015, Owner'S Service Manual

Page: 78 / 166

4-15
CYLINDER HEAD, CYLINDER AND PISTON
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
→
Replace.
CHECKING THE PISTON PIN AND
SMALL END BEARING
1. Inspect:
• Piston pin
• Small end bearing
Signs of heat discoloration
→
Re-
place.
2. Measure:
• Piston pin outside diameter
Use micrometer "1".
Out of specification
→
Replace.
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
→
Replace pis-
ton pin and/or piston.
4. Install:
• Small end bearing
• Piston pin
Into the small end of connecting
rod.
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 specification
→
Replace.
3. Measure:
• Side clearance
Use a thickness gauge "1".
Out of specification
→
Replace
piston and/or ring.
Check at several points.
Standard Wear limit
Cylin-
der bore
"C"
54.000–
54.014 mm
(2.1260–
2.1265 in)
54.100
mm
(2.1299 in)
Taper
"T"
—
0.050 mm
(0.0020 in)
"C" = Maximum Aa–Cb
"T" = (Maximum Aa, or Ab) - (Max-
imum Ba, or Bb)
Distance
"a"
Piston diame-
ter
17.5 mm
(0.69 in)
53.957–53.972
mm (2.1243–
2.1249 in)
Piston pin outside diam-
eter:
Standard <Limit>
14.995–15.000
mm (0.5904–
0.5906 in)
14.975 mm
(0.5896 in)
Ring end gap (installed):
Standard <Limit>
0.50–0.70 mm
(0.0197–0.0276
in)
1.20 mm (0.0472
in)
Side clearance:
Standard <Limit>
0.035–0.070
mm (0.0014–
0.0028 in)
0.100 mm
(0.0039 in)