Yamaha YZ125 2015 Owner's Service Manual page 79

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".
PISTON CLEARANCE = CYLIN-
DER BORE - PISTON DIAME-
TER
Piston clearance:
Standard <Limit>
0.040–0.045
0.100 mm
mm (0.0016–
(0.0039 in)
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"
Piston size
(color)
53.957–53.960
A (red) mm (2.1243–
53.961–53.964
B (orange) mm (2.1244–
53.965–53.968
C (green) mm (2.1246–
53.969–53.972
D (purple) mm (2.1248–
3. Combination:
Combine the piston and cylinder
by the following chart.
Piston mark
Cylinder mark
A
B B (orange)
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 → Replace.
Carbon deposits → Remove.
• Valve holder "2"
• Link lever "3"
• Valve shaft "4"
• Collar "5"
Wear/Damage → Replace.
• Spring "6"
Broken → Replace.
4-16
CHECKING THE POWER VALVE
HOLE ON CYLINDER
1. Remove:
• Carbon deposits
From power valve hole surface
"a".
2.1244 in)
Do not use a sharp instrument. Avoid
scratching the aluminum.
2.1246 in)
2.1247 in)
2.1249 in)
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.
(color)
A (red)
C (green)
D (purple)
2. Install:
• Spring "1"
• Link lever "2"
• Collar "3"
• Valve shaft "4"
• Valve holder "5"
• Bolt (link lever) "6"
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)