YAMAHA WR 250F 2016 Owners Manual

CYLINDER AND PISTON
6-32
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 “a” at
the measuring point (from piston skirt bot-
tom) “b” with the micrometer.
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.
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EAS2GB7259
CHECKING 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. Diameter
76.955–76.970 mm (3.0297–
3.0303 in)
Measuring point (from piston
skirt bottom)
6.0 mm (0.24 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)
b
a
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)

CYLINDER AND PISTON
6-33
EAS2GB7260CHECKING 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.EAS2GB7261INSTALLING 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” Piston ring
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)
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
6-34
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.
3. Lubricate:
•Piston
• Piston rings
• Cylinder
4. Offset:
• Piston ring end gap5. 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
b. Upper oil ring rail
c. Oil ring expander
d. Lower oil ring rail
1
3
2 a
3 c
a
db
c15
15
15
Cylinder bolt
10 Nm (1.0 m·kgf, 7.2 ft·lbf)
New
T R..

ELECTRIC STARTER
6-35
EAS2GB7262
ELECTRIC STARTER
Removing the starter motor
Order Job/Parts to remove Q’ty Remarks
Exhaust pipe Refer to “ENGINE REMOVAL” on page 6-1.
1 Starter motor 1
For assembly, reverse the disassemble pro-
cedure.
1
12 Nm (1.2 m kgf, 8.7 ft Ibf)T.R.

ELECTRIC STARTER
6-36
Disassembling the starter motor
Order Job/Parts to remove Q’ty Remarks
1 Starter motor front cover 1
2 Starter motor yoke 1
3 Armature assembly 1
4 Starter motor rear cover 1
5 Brush set 2
For assembly, reverse the disassembly pro-
cedure.

ELECTRIC STARTER
6-37
EAS2GB7263CHECKING THE STARTER MOTOR
1. Check:
•Commutator
Dirt  Clean with 600 grit sandpaper.
2. Measure:
• Commutator diameter “a”
Out of specification  Replace the starter
motor.
3. Measure:
• Mica undercut “a”
Out of specification  Scrape the mica to
the proper measurement with a hacksaw
blade that has been grounded to fit the com-
mutator.
TIP
The mica of the commutator must be undercut
to ensure proper operation of the commutator.
4. Measure:
• Armature assembly resistances (commuta-
tor and insulation)
Out of specification  Replace the starter
motor.
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a. Measure the armature assembly resistanc-
es with the pocket tester.
b. If any resistance is out of specification, re-
place the starter motor.
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5. Measure:
• Brush length “a”
Out of specification  Replace the brush
set.
6. Measure:
• Brush spring force
Out of specification  Replace the brush
set. Limit
16.6 mm (0.65 in)
Mica undercut (depth)
1.50 mm (0.06 in)
Pocket tester
90890-03112
Analog pocket tester
YU-03112-C
Armature coil
Commutator resistance “1”
0.0189–0.0231 
Insulation resistance “2”
Above 1 M
Limit
3.50 mm (0.14 in)
Brush spring force
3.92–5.88 N (400–600 gf, 14.11–
21.17 oz)

ELECTRIC STARTER
6-38
7. Check:
• Gear teeth
Damage/wear  Replace the starter motor.
8. Check:
• Oil seal
Damage/wear  Replace the defective part
(s).
EAS2GB7264ASSEMBLING THE STARTER MOTOR
1. Install:
• Brush spring “1”
• Brush “2”
2. Install:
• Armature assembly “1”
Install while holding down the brush using a
thin screw driver.
ECA
NOTICE
Be careful not to damage the brush during
installation.
3. Install:
• O-ring “1”
• Starter motor yoke “2”
ECA
NOTICE
Install the part that has a smaller step differ-
ence “a” with the magnet of the starter mo-
tor yoke facing the starter motor front
cover.
TIP
Align the match mark “a” on the starter motor
yoke with the match mark “b” on the starter mo-
tor rear cover.
4. Install:
• Circlip
•Plain washer “1”
• O-ring “2”
• Washer (starter motor front cover) “3”
• Starter motor front cover “4”
TIP
• For installation, align the projections on the
washer with the slots in the front cover.
• Align the match mark “a” on the starter motor
yoke with the match mark “b” on the starter
motor front cover.
New
ab
New

ELECTRIC STARTER
6-39
5. Install:
•Bolt “1”
• O-ring “2”
TIP
Apply the lithium-soap-based grease on the O-
ring.
New

CLUTCH
6-40
EAS2GB7265
CLUTCH
Removing the clutch
Order Part name Q’ty Remarks
Engine oilDrain.
Refer to “CHANGING THE ENGINE OIL” on
page 3-17.
Brake pedal Refer to “ENGINE REMOVAL” on page 6-1.
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
12 Friction plate 1 4
13 Clutch plate 8
New
New
New
2
3 45
(5)
(6)78961011
12
22
1
15
12
13
13
18
1920 17
16
14
13
T.R.
T.R.
T.R.
T.R.
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21

CLUTCH
6-41
14 Cushion spring 1
15 Friction plate 2 5 Identification color (purple)
16 Clutch boss 1
17 Thrust washer 1
18 Primary driven gear 1
19 Spacer 1
20 Bearing 1
21 Washer 1
22 Push lever shaft 1
For installation, reverse the removal proce-
dure.
Removing the clutch
Order Part name Q’ty Remarks
New
New
New
2
3 45
(5)
(6)78961011
12
22
1
15
12
13
13
18
1920 17
16
14
13
T.R.
T.R.
T.R.
T.R.
LT
21