2014 YAMAHA YZ250F Owners Manual

CYLINDER AND PISTON
5-32
b. If out of specification, rebore or replace the
cylinder, and replace the piston and the pis-
ton rings as a set.
c. Measure the piston outside diameter D “a” at
the measurement position H “b” with the mi-
crometer.
d. If out of specification, replace the cylinder,
the piston, and the piston rings as a set.
e. Calculate the piston-to-cylinder clearance
with the following formula.
f. If out of specification, replace the cylinder,
the piston, and the piston rings as a set.
▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲EAS1SM1229CHECKING THE PISTON RINGS
1. Measure:
• Piston ring side clearance
Out of specification  Replace the piston
and piston rings as a set.
TIP
Before measuring the piston ring side clear-
ance, eliminate any carbon deposits from the
piston ring grooves and piston rings.2. Install:
• Piston rings
TIP
Level the piston ring into the cylinder with the
piston.
3. Measure:
• Piston ring end gap
Out of specification  Replace the piston
ring.
TIP
The oil ring expander’s end gap cannot be
measured. If the oil ring rail’s gap is excessive,
replace all three oil rings.
EAS1SM1230
Diameter D
76.955–76.970 mm (3.0297–
3.0303 in)
Measurement position H
9.0 mm (0.35 in)
Piston-to-cylinder clearance =
Cylinder bore - Piston diameter
Piston-to-cylinder clearance
0.030–0.055 mm (0.0012–0.0022
in)
Limit
0.15 mm (0.006 in)
ba
Ring side clearance
0.030–0.065 mm (0.0012–0.0026
in)
Limit
0.120 mm (0.0047 in)
a. 10 mm (0.39 in)
Piston ring end gap
Top ring
End gap (installed)
0.15–0.25 mm (0.0059–0.0098 in)
Limit
0.50 mm (0.0197 in)
Oil ring
End gap (installed)
0.10–0.35 mm (0.0039–0.0138 in)

CYLINDER AND PISTON
5-33
CHECKING THE PISTON PIN
1. Check:
• Piston pin
Blue discoloration/grooves  Replace the
piston pin and then check the lubrication
system.
2. Measure:
• Piston pin outside diameter “a”
Out of specification  Replace the piston
pin.
3. Measure:
• Piston pin bore inside diameter “b”
Out of specification  Replace the piston.
EAS1SM1231
INSTALLING THE PISTON AND CYLINDER
1. Install:
• Oil ring expander “1”
• Lower oil ring rail “2”
• Upper oil ring rail “3”
• Top ring “4”
TIP
Be sure to install the piston ring so that the
manufacturer’s marks or numbers face up.
2. Install:
• Piston “1”
• Piston pin “2”
• Piston pin clip “3”
TIP
• Apply the engine oil to the piston pin.
• Install the piston with the F mark “a” on it
pointing to its intake (front) side.
• Before installing the piston pin clip, cover the
crankcase opening with a cloth to prevent the
clip from falling into the crankcase.
• Install the piston pin clips, so that the clip ends
are 3 mm (0.12 in) “c” or more from the cutout
in the piston. Piston pin outside diameter
15.991–16.000 mm (0.6296–
0.6299 in)
Limit
15.971 mm (0.6288 in)
Piston pin bore inside diameter
16.002–16.013 mm (0.6300–
0.6304 in)
Limit
16.043 mm (0.6316 in)b
4
New

CYLINDER AND PISTON
5-34
3. Lubricate:
•Piston
• Piston rings
• Cylinder
4. Offset:
• Piston ring end gap
5. Install:
• Cylinder gasket
• Dowel pin
• Cylinder
TIP
• While compressing the piston rings with one
hand, install the cylinder with the other hand.
• Pass the timing chain and timing chain guide
(exhaust side) through the timing chain cavity.
a. Top ring end
b. Upper oil ring end
c. Oil ring end
d. Lower oil ring end
Cylinder bolt
10 Nm (1.0 m·kgf, 7.2 ft·lbf)
1
3
2 a
3 c
a
db
c
15
1515
New
T R..

CLUTCH
5-35
EAS1SM1232
CLUTCH
Removing the clutch
Order Part name Q’ty Remarks
Engine oilDrain.
Refer to “CHANGING THE ENGINE OIL” on
page 3-15.
Brake pedal Refer to “ENGINE REMOVAL” on page 5-1.
1 Clutch cable 1 Disconnect.
2 Clutch cover 1
3Gasket 1
4 Clutch spring 5
5 Pressure plate 1
6 Push rod 1 1
7 Circlip 1
8Washer 1
9 Bearing 1
10 Ball 1
11 Push rod 2 1
LS
New
New
New
2
3 45
(5)
(6)789
61011
12
21
1
12
13
17
1819 16
15
14
13
T.R.
T.R.
T.R.
T.R.
LT
20

CLUTCH
5-36
12 Friction plate 9
13 Clutch plate 8
14 Cushion spring 1
15 Clutch boss 1
16 Thrust washer 1
17 Primary driven gear 1
18 Spacer 1
19 Bearing 1
20 Washer 1
21 Push lever shaft 1
For installation, reverse the removal proce-
dure.
Removing the clutch
Order Part name Q’ty Remarks
LS
New
New
New
2
3 45
(5)
(6)789
61011
12
21
1
12
13
17
1819 16
15
14
13
T.R.
T.R.
T.R.
T.R.
LT
20

CLUTCH
5-37
Removing the right crankcase cover
Order Part name Q’ty Remarks
Right engine guard Refer to “ENGINE REMOVAL” on page 5-1.
Engine oilDrain.
Refer to “CHANGING THE ENGINE OIL” on
page 3-15.
CoolantDrain.
Refer to “CHANGING THE COOLANT” on
page 3-8.
Brake pedal Refer to “ENGINE REMOVAL” on page 5-1.
Clutch cover Refer to “CLUTCH” on page 5-35.
1 Oil filter element cover 1
2 Oil filter element 1
3 Radiator pipe 2 1
4 Kickstarter lever 1
5 Right crankcase cover 1
6 Gaskets 1
7 Dowel pin 2
8Collar 1
For installation, reverse the removal proce-
dure.
T.R
.
E
E
LS
New
New
67
2
5
4
1
7 8
New
3
LS
New
New
LS
NewNew
LT

CLUTCH
5-38
EAS1SM1233REMOVING THE CLUTCH
1. Remove:
• Clutch boss nut “1”
• Lock washer “2”
• Clutch boss “3”
TIP
• Straighten the lock washer tab.
• While holding the clutch boss with the clutch
holder “4”, loosen the clutch boss nut.
EAS1SM1234CHECKING THE FRICTION PLATES
1. Check:
• Friction plate
Damage/wear  Replace the friction plates
as a set.
2. Measure:
• Friction plate thickness
Out of specification  Replace the friction
plates as a set.
TIP
Measure it at four points on the friction plate.
EAS1SM1235
CHECKING THE CLUTCH PLATES
1. Check:
• Clutch plate
Damage  Replace the clutch plates as a
set.
2. Measure:
• Clutch plate warpage
(with a surface plate and thickness gauge
“1”)
Out of specification  Replace the clutch
plates as a set.
EAS1SM1236CHECKING THE CLUTCH SPRINGS
1. Check:
• Clutch spring
Damage  Replace the clutch springs as a
set.
2. Measure:
• Clutch spring free length
Out of specification  Replace the clutch
springs as a set.
EAS1SM1237
Clutch holder
90890-04086
YM-91042
Friction plate thickness
2.90–3.10 mm (0.114–0.122 in)
Wear limit
2.80 mm (0.110 in)
34
12
Warpage limit
0.10 mm (0.0039 in)
Clutch spring free length
45.00 mm (1.77 in)
Limit
44.00 mm (1.73 in)

CLUTCH
5-39
CHECKING THE CLUTCH HOUSING
1. Check:
• Clutch housing dogs “a”
Damage/pitting/wear  Deburr the clutch
housing dogs or replace the clutch housing.
TIP
Pitting on the clutch housing dogs will cause er-
ratic clutch operation.
EAS1SM1238CHECKING THE CLUTCH BOSS
1. Check:
• Clutch boss splines
Damage/pitting/wear  Replace the clutch
boss.
TIP
Pitting on the clutch boss splines will cause er-
ratic clutch operation.
EAS1SM1239CHECKING THE PRESSURE PLATE
1. Check:
• Pressure plate
Crack/damage  Replace.
EAS1SM1240
CHECKING THE PUSH LEVER SHAFT
1. Check:
• Push lever shaft
Wear/damage  Replace.
EAS1SM1241CHECKING THE CLUTCH PUSH RODS
1. Check:
• Push rod 1 “1”
• Bearing “2”
• Washer “3”
• Push rod 2 “4”
•Ball “5”
Cracks/damage/wear  Replace.
2. Measure:
• Push rod 2 bending limit
Out of specification  Replace.
EAS1SM1242CHECKING THE PRIMARY DRIVE GEAR
1. Check:
• Primary drive gear
Damage/wear  Replace the primary drive
and primary driven gears as a set.
Excessive noise during operation  Re-
place the primary drive and primary driven
gears as a set.
2. Check:
• Primary-drive-gear-to-primary-driven-gear
free play
Free play exists  Replace the primary
drive and primary driven gears as a set.
EAS1SM1243
a
Push rod bending limit
0.10 mm (0.0039 in)