2010 YAMAHA YZ250F Owners Manual

4-23
VALVES AND VALVE SPRINGS
4. Measure:• Margin thickness "a"Out of specification →Replace.
5. Measure: • Runout (valve stem)Out of specification →Replace.
• When installing a new valve always
replace the guide.
• If the valve is removed or replaced
always replace the oil seal.
6. Eliminate:• Carbon deposits(from the valve face and valve
seat)
7. Inspect: • Valve seatPitting/wear →Reface the valve
seat.
8. Measure: • Valve seat width "a"
Out of specification →Reface the
valve seat.
Measurement steps:
a. Apply Mechanic's blueing dye (Dykem) "b" to 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 pattern.
d. Measure the valve seat width. Where the valve seat and valve
face made contact, blueing will
have been removed.
e. If the valve seat is too wide, too
narrow, or the seat is not cen-
tered, the valve seat must be
refaced.
9. Lap: • Valve face
• Valve seat
This model uses titanium intake
and exhaust valves. Titanium
valves that have been used to lap
the valve seats must not be used.
Always replace lapped valves with
new valves.
• When replacing the cylinder head, replace the valves without lapping
the valve seats and valve faces.
• When replacing the valves or valve guides, use new valves to lap the
valve seats, and then replace them
with new valves.
Lapping steps:
a. Apply a coarse lapping compound
to the valve face.
Do not let the co mpound enter the
gap between the valve stem and
the guide.
b. Apply molybdenum disulfide oil to the valve stem.
c. Install the valve into the cylinder head.
d. Turn the valve until the valve face
and valve seat are evenly pol-
ished, then clean off all of the
compound.
For best lapping results, lightly tap
the valve seat while rotating the valve
back and forth between your hands.
e. Apply a fine lapping compound to the valve face and repeat the
above steps.
After every lapping operation be sure
to clean off all of the compound from
the valve face and valve seat.
f. Apply Mechanic's blueing dye (Dykem) to the valve face.
g. Install the valve into the cylinder
head.
h. Press the valve through the valve guide and onto the valve seat to
make a clear pattern.
i. Measure the valve seat width again. If the valve seat width is out
of specification, reface and relap
the valve seat.
Margin thickness:Intake:
0.8 mm (0.0315 in)
Exhaust: 0.7 mm (0.0276 in)
Runout limit: 0.01 mm (0.0004 in)
Valve seat width: Intake:
0.9–1.1 mm
(0.0354–0.0433 in)
:1.6 mm
(0.0630 in)
Exhaust: 0.9–1.1 mm
(0.0354–0.0433 in)
:1.6 mm
(0.0630 in)

4-24
VALVES AND VALVE SPRINGS
CHECKING THE VALVE SPRINGS
1. Measure:• Valve spring free length "a"Out of specification →Replace.
2. Measure: • Compressed spring force "a"Out of specification →Replace.
b. Installed length
3. Measure: • Spring tilt "a"Out of specification →Replace. CHECKING THE
VALVE LIFTERS
1. Inspect:
• Valve lifterScratches/damage →Replace
both lifters and cylinder head.
INSTALLING THE VALVES
1. Apply: • Molybdenum disulfide oil
Onto the valve stem and valve
stem seal.
2. Install:
• Valve "1"
• Valve spring seat "2"
• Valve stem seal "3"
• Valve spring "4"
• Valve spring retainer "5"To cylinder head.
• Make sure that each valve is in-
stalled in its original place, also re-
ferring to the painted color as
follows.
Intake (middle) "a": Sky blue
Intake (right/left) "b": not paint
Exhaust "c": Purple
• Install the valve springs with the larger pitch "d" facing upward.
e. Smaller pitch 3. Install:
• Valve cotter
While compressing the valve spring
with a valve spring compressor "1" in-
stall the valve cotters.
4. To secure the valve cotters onto the valve stem, lightly tap the
valve tip with a piece of wood.
Hitting the valve tip with excessive
force could damage the valve.
5. Install:• Adjusting pad "1"
• Valve lifter "2"
• Apply the molybdenum disulfide oil on the valve stem end.
• Apply the engine oil on the valve lift-
ers.
• Valve lifter must turn smoothly when rotated with a finger.
• Be careful to reinstall valve lifters and pads in their original place.
Free length (valve
spring): Intake:39.76 mm (1.57 in)
: 38.76 mm
(1.53 in)
Exhaust:
37.78 mm (1.49 in)
: 36.78 mm
(1.45 in)
Compressed spring
force: Intake:99–114 N at 28.98 mm
(9.9–11.4 kg at 28.98
mm, 22.27–25.57 lb at
1.14 in)
Exhaust: 126–145 N at 28.30
mm (12.6–14.5 kg at
28.30 mm,
28.44–31.97 lb at 1.11
in)
Spring tilt limit: Intake:2.5°/1.7 mm (0.067 in)
Exhaust:
2.5°/1.6 mm (0.063 in)
Valve spring compres-
sor: YM-4019/90890-04019

4-25
CYLINDER AND PISTON
CYLINDER AND PISTON
REMOVING THE CYLINDER AND PISTONOrder Part name Q'ty Remarks Cylinder head Refer to "CYLINDER HEAD" section.
1 Bolt (cylinder) 1
2 Cylinder 1
3 Piston pin clip 2 Refer to removal section.
4 Piston pin 1 Refer to removal section.
5 Piston 1 Refer to removal section.
6 Piston ring set 1 Refer to removal section.

4-26
CYLINDER AND PISTON
REMOVING THE PISTON AND
PISTON RING
1. Remove:• Piston pin clip "1"
• Piston pin "2"
• Piston "3"
• Put identification marks on each piston head for reference during re-
installation.
• Before removing each piston pin, deburr the clip groove and pin hole
area. If the piston pin groove is de-
burred and the piston pin is still dif-
ficult to remove, use the piston pin
puller set "4".
Do not use a hammer to drive the
piston pin out.
2. Remove:
• Piston ring "1"
Spread the end gaps apart while at
the same time lifting the piston ring
over the top of the piston crown, as
shown in the illustration.
CHECKING THE CYLINDER AND
PISTON
1. Inspect:• Cylinder and piston walls
Vertical scratches →Replace cyl-
inder and piston.
2. Measure:
• Piston-to-cylinder clearance
Measurement steps:
a. Measure the cylinder bore "C" with a cylinder bore gauge.
Measure the cylinder bore "C" in par-
allel to and at right angles to the
crankshaft. Then, find the average of
the measurements.
b. If out of specific ation, replace the
cylinder, and replace the piston
and piston rings as set.
c. Measure the piston skirt diameter "P" with a micrometer. a. 8 mm (0.31 in) from the piston
bottom edge
d. If out of specification, replace the piston and piston rings as a set.
e. Calculate the piston-to-cylinder clearance with following formula:
f. If out of specification, replace the cylinder, and replace the piston
and piston rings as set.
CHECKING THE PISTON RING
1. Measure:• Ring side clearance
Use a feeler gauge "1".
Out of specification →Replace the
piston and rings as a set.
Clean carbon from the piston ring
grooves and rings before measuring
the side clearance.
Piston pin puller set:
YU-1304/90890-01304
Cylinder bore "C" 77.00–77.01
mm
(3.0315–3.0319
in)
Taper limit "T" 0.05 mm (0.002
in)
Out of round "R" 0.05 mm (0.002
in)
"C" = Maximum D
"T" = (Maximum D
1 or D2) - (Max-
imum D
5 or D6)
"R" = (Maximum D
1, D3 or D5) -
(Minimum D
2, D4 or D6)
Piston size "P"
Standard 76.955–76.970
mm
(3.0297–3.0303
in)
Piston-to-cylinder clearance = Cylinder bore "C" - Piston skirt
diameter "P"
Piston-to-cylinder clear-
ance:0.030–0.055 mm
(0.0012–0.0022 in)
:0.1 mm (0.004
in)
Side clearance: Standard
Top
ring 0.030–0.065
mm
(0.0012–0.0026
in) 0.12
mm
(0.005
in)
2nd
ring 0.020–0.055
mm
(0.0008–0.0022
in) 0.12
mm
(0.005
in)

4-27
CYLINDER AND PISTON
2. Position:• Piston ring(in cylinder)
Insert a ring into the cylinder and
push it approximately 10 mm (0.39 in)
into the cylinder. Push the ring with
the piston crown so that the ring will
be at a right angle to the cylinder
bore.
a. 10 mm (0.39 in)
3. Measure:
• Ring end gapOut of specification →Replace.
You cannot measure the end gap on
the expander spacer of the oil control
ring. If the oil control ring rails show
excessive gap, replace all three rings.
CHECKING THE PISTON PIN
1. Inspect:
• Piston pinBlue discoloration/grooves →Re-
place, then inspect the lubrication
system.
2. Measure:
• Piston pin-to-piston clearance
Measurement steps:
a. Measure the outside diameter (piston pin) "a".
If out of specification, replace the
piston pin. b. Measure the inside diameter (pis-
ton) "b".
c. Calculate the piston pin-to-piston clearance with the following for-
mula.
d. If out of specific ation, replace the
piston.
INSTALLING THE PISTON RING
AND PISTON
1. Install: •Piston ring
Onto the piston.
• Be sure to install the piston rings so that the manufacturer's marks or
numbers are located on the upper
side of the rings.
• Lubricate the piston and piston rings liberally with engine oil.
2. Position:
•Top ring
•2nd ring
•Oil ringOffset the piston ring end gaps as
shown.
a. Top ring end
b. 2nd ring end
c. Oil ring end (upper)
d. Oil ring
e. Oil ring end (lower)
3. Install: • Piston "1"
• Piston pin "2"
• Piston pin clip "3"
• Apply engine oil onto the piston pin and piston.
• Be sure that the arrow mark "a" on
the piston points to the exhaust side
of the engine.
• Before installing the piston pin clip,
cover the crankcase with a clean
rag to prevent the piston pin clip
from falling into the crankcase.
• Install the piston pin clips with their ends facing downward.
End gap:
Standard
Top
ring 0.15–0.25 mm
(0.006–0.010 in) 0.50
mm
(0.020 in)
2nd
ring 0.30–0.45 mm
(0.012–0.018 in) 0.80
mm
(0.031 in)
Oil
ring 0.10–0.40 mm
(0.004–0.016 in) —
Outside diameter (piston
pin): 15.991–16.000 mm
(0.6296–0.6299 in)
Inside diameter (piston):
16.002–16.013 mm
(0.6300–0.6304 in)
Piston pin-to-pis ton clearance =
Inside diameter (piston) "b" -
Outside diameter (piston pin)
"a"
Piston pin-to-piston
clearance:0.002–0.022 mm
(0.0001–0.0009 in)
:0.07 mm (0.003
in)
New

4-28
CYLINDER AND PISTON
INSTALLING THE CYLINDER
1. Lubricate:•Piston
• Piston ring
• Cylinder
Apply a liberal coating of engine oil.
2. Install:
• Dowel pin "1"
• O-ring "2"
Apply the lithium soap base grease
on the O-ring.
3. Install:• Cylinder gasket "1"
• Cylinder "2"
Install the cylinder with one hand
while compressing the piston rings
with the other hand.
• Pass the timing chain "3" through the timing chain cavity.
• Be careful not to damage the tim-
ing chain guide "4" during instal-
lation.
4. Install:• Bolt (cylinder) "5"
Bolt (cylinder):10 Nm (1.0 m•kg, 7.2
ft•lb)

4-29
CLUTCH
CLUTCH
REMOVING THE CLUTCHOrder Part name Q'ty Remarks Drain the engine oil. Refer to "CHANGING THE ENGINE OIL"
section in the CHAPTER 3.
Brake pedal Refer to "ENGINE REMOVAL" section.
Clutch cable Disconnect at engine side.
1 Clutch cover 1
2 Clutch spring 5
3 Pressure plate 1
4 Push rod 1 1
5 Circlip 1
6 Washer 1
7 Bearing 1
8Ball 1
9 Push rod 2 1
10 Friction plate 9
11 Clutch plate 8
12 Nut (clutch boss) 1 Refer to removal section.
13 Lock washer 1 Refer to removal section.

4-30
CLUTCH
14 Clutch boss1 Refer to removal section.
15 Thrust washer 1
16 Primary driven gear 1
17 Spacer 1
18 Washer 1
19 Push lever shaft 1
Order
Part name Q'tyRemarks