2016 YAMAHA YZ250F oil

VALVES AND VALVE SPRINGS
5-26
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a. Apply Mechanic’s blueing dye (Dykem) “b”
onto the valve face.
b. Install the valve into the cylinder head.
c. Press the valve through the valve guide and
onto the valve seat to make a clear impres-
sion.
d. Measure the valve seat contact width “c”.
TIP
Where the valve seat and the valve face are in
contact with each other, the blueing will have
been removed.
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4. Lap:
• Valve face
• Valve seat
ECANOTICE
This model uses titanium intake and ex-
haust valves.
Do not use the valves used for lapping the
valve seat. Always replace the valves used
for lapping with new ones.
TIP
• When replacing the cylinder head, replace
also the valves with new ones without them.
• When replacing the valves or the valve
guides, use new valves to lap the valve seats,
and then replace them with new valves.
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a. Apply a coarse lapping compound “a” to the
valve face.
ECA
NOTICE
Do not let the lapping compound enter the
gap between the valve stem and the valve
guide.
b. Apply molybdenum disulfide oil onto the
valve stem.
c. Install the valve into the cylinder head.
d. Turn the valve until the valve face and the
valve seat are evenly polished, then clean
off all of the lapping compound.
TIP
While turning the valve lapper, tap and lap the
valve seat.
e. Apply a fine lapping compound to the valve
face, and repeat the above steps.
f. After every lapping step, be sure to clean off
all of the lapping compound from the valve
face and the valve seat.
g. Apply Mechanic’s blueing dye (Dykem) “b”
onto the valve face.
c

VALVES AND VALVE SPRINGS
5-28
EAS1SM9222CHECKING THE VALVE LIFTERS
1. Check:
• Valve lifter
Damage/scratches  Replace the valve lift-
ers and cylinder head.
EAS1SM9223INSTALLING THE VALVES
1. Clean:
• Valve stem end
2. Lubricate:
• Valve stem “1”
• Valve stem seal “2”
3. Install:
• Spring seat “1”
• Valve stem seal “2”
•Valve “3”
• Valve spring “4”
• Valve spring retainer “5”
(to the cylinder head)
TIP
• Make sure each valve is installed in its original
place.
• Install the valve springs with the larger pitch
“a” facing up.
4. Install:
• Valve cotter “1”
TIP
Install the valve cotters by compressing the
valve spring with the valve spring compressor
“2” and the valve spring compressor adapter
“3”.
5. To fasten the valve cotters onto the valve
stem, lightly tap the valve tip with a soft-face
hammer.
ECA
NOTICE
Hitting the valve tip with excessive force
could damage the valve. Recommended lubricant
Molybdenum disulfide oil
New
b. Smaller pitch
Valve spring compressor
90890-04019
YM-04019
Valve spring compressor adapter
22 mm
90890-04108
YM-04108

CYLINDER AND PISTON
5-31
EAS1SM9225REMOVING THE PISTON
1. Remove:
• Piston pin clip “1”
• Piston pin “2”
• Piston “3”
ECA
NOTICE
Do not use a hammer to drive the piston pin
out.
TIP
• Before removing the piston pin clip, cover the
crankcase opening with a cloth to prevent the
piston pin clip from falling into the crankcase.
• Before removing the piston pin, deburr the
piston pin clip’s groove and the piston pin’s
bore area. If the piston pin groove is deburred
and the piston pin is still difficult to remove,
use the piston pin puller set “4”.
2. Remove:
• Top ring
• Oil ring
TIP
When removing a piston ring, open the end gap
with your fingers and lift the opposite end gap
of the piston ring over the piston crown.
EAS1SM9226CHECKING THE CYLINDER AND PISTON
1. Check:
• Piston wall (Sidewall)
• Cylinder wall
Vertical scratches  Replace the cylinder,
and replace the piston and piston rings as a
set.
2. Measure:
• Piston-to-cylinder clearance
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a. Measure the cylinder bore with the cylinder
bore gauge.
TIP
Measure cylinder bore by taking side-to-side
and front-to-back measurements of the cylin-
der. Piston pin puller set
90890-01304
Piston pin puller
YU-01304
1
Bore
77.000–77.010 mm (3.0315–
3.0319 in)
Taper limit
0.050 mm (0.0020 in)
Out of round limit
0.050 mm (0.0020 in)
Cylinder bore = maximum of D
1, D2, D3, D4,
D
5, D6
Taper limit (front-to-back) = maximum differ-
ence between D
1, D3, D5Taper limit (side-to-side) = maximum differ-
ence between D
2, D4, D6
Out of round limit (top)= difference between
D
1, D2Out of round limit (middle)= difference be-
tween D
3, D4Out of round limit (bottom)= difference be-
tween D
5, D6

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 “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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EAS1SM9227CHECKING 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)
4.0 mm (0.16 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
5-33
EAS1SM9228CHECKING THE PISTON PIN
1. Check:
• Piston pin
Brown or silver discoloration/excessive
wear/pitting/scratches/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.EAS1SM9229INSTALLING 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
5-34
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..

CLUTCH
5-35
EAS1SM9230
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
New
New
New
2
3 45
(5)
(6)78961011
20
1
12
13
16
1718 15
14
T.R.
T.R.
T.R.
T.R.
LT
19

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.
LS
New
New
67
2
5
4
1
7 8
New
3
LS
New
New
LS
NewNew
LT
LS