Yamaha YZ125X1 Owner's Service Manual page 71

CHECKING THE PISTON
CLEARANCE
1. Calculate:
• Piston clearance
→
Out of limit Replace piston, and
piston ring and/or cylinder.
Refer to "Cylinder" and "Piston".
PISTON CLEARANCE = CYLIN-
DER BORE - PISTON DIAME-
TER
Piston clearance:
Standard <Limit>
0.040–0.045
0.1 mm (0.004
mm (0.0016–
0.0018 in)
CHECKING THE COMBINATION
OF PISTON AND CYLINDER
1. Check:
• Cylinder mark "a"
Cylinder mark
Cylinder size
"a"
54.000–54.002
A mm (2.1260–
2.1261 in)
54.004–54.006
B mm (2.1261–
2.1262 in)
54.008–54.010
C mm (2.1263–
2.1264 in)
54.012–54.014
D mm (2.1265–
2.1265 in)
CYLINDER HEAD, CYLINDER AND PISTON
2. Check:
• Piston mark "a"
Piston mark "a"
(color)
A (red)
B (orange)
C (green)
D (purple)
3. Combination:
Combine the piston and cylinder
by the following chart.
Cylinder mark
A
in)
B
C
D
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
Carbon deposits
• Valve holder "2"
• Link lever "3"
• Valve shaft "4"
• Collar "5"
Wear/Damage
• Spring "6"
Broken
Piston size
53.957–53.960
mm (2.1243–
2.1244 in)
53.961–53.964
mm (2.1244–
2.1246 in)
53.965–53.968
mm (2.1246–
2.1247 in)
53.969–53.972
mm (2.1248–
2.1249 in)
Piston mark
(color)
A (red)
B (orange)
C (green)
D (purple)
→
Replace.
→
Remove.
→
Replace.
→
Replace.
4-16
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"
Bolt (power valve):
8 Nm (0.8 m•kg, 5.8
ft•lb)
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"
Bolt (link lever):
4 Nm (0.4 m•kg, 2.9
ft•lb)
• Install the spring to the link lever,
and then to the cylinder.
• Install the spring with its stopper
portion "a" facing inward.
• Apply the lithium soap base grease
on the oil seal lip.