Yamaha yz250 2015, Owner'S Manual

Page: 81 / 174

4-17
CYLINDER HEAD, CYLINDER AND PISTON
3. Measure:
• Side clearance
Use a thickness gauge "1".
Out of specification
→
Replace
piston and/or ring.
Check at several points.
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 1 "2"
• Link lever "3"
• Pulley "4"
• Valve shaft "5"
• Link rod "6"
• Washer "7"
Wear/Damage
→
Replace.
• Spring 1, 2 "8"
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.
Side clearance:
Standard <Limit>
0.030–0.065
mm (0.0012–
0.0026 in)
0.100 mm
(0.0039 in)
PISTON CLEARANCE = CYLIN-
DER BORE - PISTON DIAME-
TER
Piston clearance:
Standard <Limit>
0.045–0.050
mm (0.0018–
0.0020 in)
0.100 mm
(0.0039 in)
Cylinder mark
"a"
Cylinder size
A
66.400–66.402
mm (2.61417–
2.61425 in)
B
66.404–66.406
mm (2.61433–
2.61441 in)
C
66.408–66.410
mm (2.61449–
2.61457 in)
D
66.412–66.414
mm (2.61465–
2.61472 in)
Piston mark "a"
(color)
Piston size
A (red)
66.352–66.355
mm (2.61228–
2.61240 in)
B (orange)
66.356–66.359
mm (2.61244–
2.61256 in)
C (green)
66.360–66.363
mm (2.61260–
2.61272 in)
D (purple)
66.364–66.367
mm (2.61276–
2.61287 in)
Cylinder mark
Piston mark
(color)
A A (red)
B B (orange)
C C (green)
D D (purple)