Yamaha YZ125(D), Owner'S Service Manual

Page: 78 / 168

4-16
CYLINDER HEAD, CYLINDER AND PISTON
CHECKING THE PISTON
CLEARANCE
1. Calculate:
• Piston clearance
Out of specification
→
Replace
piston, and piston ring and/or cyl-
inder.
Refer to "Cylinder" and "Piston".
CHECKING THE COMBINATION
OF PISTON AND CYLINDER
1. Check:
• Cylinder mark "a"
2. Check:
• Piston mark "a"
3. Combination:
Combine the piston and cylinder
by the following chart.
When you purchase a cylinder, you
cannot designate its size. Choose the
piston that matches the above chart.
CHECKING THE POWER VALVE
1. Inspect:
• Power valve 1,2 "1"
Wear/Damage
→
Replace.
Carbon deposits
→
Remove.
• Valve holder "2"
• Link lever "3"
• Valve shaft "4"
• Collar "5"
Wear/Damage → Replace.
• Spring "6"
Broken
→
Replace.
CHECKING THE POWER VALVE
HOLE ON CYLINDER
1. Remove:
• Carbon deposits
From power valve hole surface
"a".
Do not use a sharp instrument. Avoid
scratching the aluminum.
INSTALLING THE POWER VALVE
1. Install:
• Power valve 1 "1"
• Power valve 2 "2"
• Bolt (power valve) "3"
Install the power valve at cut-away
faced "a" for down side.
2. Install:
• Spring "1"
• Link lever "2"
• Collar "3"
• Valve shaft "4"
• Valve holder "5"
• Bolt (link lever) "6"
PISTON CLEARANCE = CYLIN-
DER BORE - PISTON DIAME-
TER
Piston clearance:
Standard <Limit>
0.040–0.045
mm (0.0016–
0.0018 in)
0.1 mm (0.004
in)
Cylinder mark
"a"
Cylinder size
A
54.000–54.002
mm (2.1260–
2.1261 in)
B
54.004–54.006
mm (2.1261–
2.1262 in)
C
54.008–54.010
mm (2.1263–
2.1264 in)
D
54.012–54.014
mm (2.1265–
2.1265 in)
Piston mark "a"
(color)
Piston size
A (red)
53.957–53.960
mm
(2.1243–2.1244
in)
B (orange)
53.961–53.964
mm
(2.1244–2.1246
in)
C (green)
53.965–53.968
mm
(2.1246–2.1247
in)
D (purple)
53.969–53.972
mm
(2.1248–2.1249
in)
Cylinder mark
Piston mark
(color)
A A (red)
B B (orange)
C C (green)
D D (purple)
Bolt (power valve):
8 Nm (0.8 m•kg, 5.8
ft•lb)
Bolt (link lever):
4 Nm (0.4 m•kg, 2.9
ft•lb)