Yamaha YZ125 2015 Owner's Service Manual page 78

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 → Replace.
Distance Piston diame-
"a"
53.957–53.972
17.5 mm
mm (2.1243–
(0.69 in)
2.1249 in)
CYLINDER HEAD, CYLINDER AND PISTON
Wear limit
54.100
mm
(2.1299 in)
0.050 mm
CHECKING THE PISTON PIN AND
(0.0020 in)
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.
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 → Replace pis-
ton pin and/or piston.
ter
4. Install:
• Small end bearing
• Piston pin
Into the small end of connecting
rod.
Piston pin outside diam-
eter:
<Limit>
14.975 mm
(0.5896 in)
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 specification → Replace.
Ring end gap (installed):
Standard <Limit>
0.50–0.70 mm
1.20 mm (0.0472
(0.0197–0.0276
in)
3. Measure:
• Side clearance
Use a thickness gauge "1".
Out of specification → Replace
piston and/or ring.
Side clearance:
Standard <Limit>
0.035–0.070
0.100 mm
mm (0.0014–
(0.0039 in)
0.0028 in)
Check at several points.
in)