2005 YAMAHA YZ250F oil

4 - 38
ENGVALVES AND VALVE SPRINGS
9. Lap:
Valve face
Valve seat
NOTE:
After refacing the valve seat or replacing the
valve and valve guide, the valve seat and
valve face should be lapped.
Lapping steps:
Apply a coarse lapping compound to the
valve face.
CAUTION:
Do not let the compound enter the gap
between the valve stem and the guide.
Apply molybdenum disulfide oil to the
valve stem.
Install the valve into the cylinder head.
Turn the valve until the valve face and
valve seat are evenly polished, then clean
off all of the compound.
NOTE:
For best lapping results, lightly tap the valve
seat while rotating the valve back and forth
between your hands.
Apply a fine lapping compound to the
valve face and repeat the above steps.
NOTE:
After every lapping operation be sure to
clean off all of the compound from the valve
face and valve seat.
Apply Mechanic’s blueing dye (Dykem) to
the valve face.
Install the valve into the cylinder head.
Press the valve through the valve guide
and onto the valve seat to make a clear
pattern.
Measure the valve seat width again. If the
valve seat width is out of specification,
reface and relap the valve seat.

4 - 40
ENGVALVES AND VALVE SPRINGS
ASSEMBLY AND INSTALLATION
Valve and valve spring
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.
NOTE:
Make sure that each valve is installed in its
original place, also referring to the painted
color as follows.
Intake (middle) a: yellow
Intake (right/left) b: white
Exhaust: no paint
Install the valve springs with the larger pitch
c facing upward.
d
Smaller pitch
3. Install:
Valve cotter
NOTE:
While compressing the valve spring with a
valve spring compressor 1 install the valve
cotters.
4. To secure the valve cotters onto the valve
stem, lightly tap the valve tip with a piece of
wood.
CAUTION:
Hitting the valve tip with excessive force
could damage the valve.
Valve spring compressor:
YM-4019/90890-04019

4 - 41
ENGVALVES AND VALVE SPRINGS
5. Install:
Adjusting pad 1
Valve lifter 2
NOTE:
Apply the molybdenum disulfide oil on the
valve stem end.
Apply the engine oil on the valve lifters.
Valve lifter must turn smoothly when rotated
with a finger.
Be careful to reinstall valve lifters and pads in
their original place.

4 - 45
ENGCYLINDER AND PISTON
Piston ring
1. Measure:
Ring side clearance
Use a feeler gauge 1.
Out of specification → Replace the piston
and rings as a set.
NOTE:
Clean carbon from the piston ring grooves and
rings before measuring the side clearance.
2. Position:
Piston ring
(in cylinder)
NOTE:
Insert a ring into the cylinder and push it
approximately 10 mm (0.39 in) into the cylin-
der. Push the ring with the piston crown so that
the ring will be at a right angle to the cylinder
bore.
a10 mm (0.39 in)
Side clearance:
Standard
Top ring0.030 ~ 0.065 mm
(0.0012 ~ 0.0026 in)0.12 mm
(0.005 in)
2nd ring0.020 ~ 0.055 mm
(0.0008 ~ 0.0022 in)0.12 mm
(0.005 in)
3. Measure:
Ring end gap
Out of specification → Replace.
NOTE:
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.
End gap:
Standard
Top ring0.15 ~ 0.25 mm
(0.006 ~ 0.010 in)0.50 mm
(0.020 in)
2nd ring0.30 ~ 0.45 mm
(0.012 ~ 0.018 in)0.80 mm
(0.031 in)
Oil ring0.10 ~ 0.40 mm
(0.004 ~ 0.016 in)—

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ENGCYLINDER AND PISTON
Piston pin
1. Inspect:
Piston pin
Blue discoloration/grooves → Replace,
then inspect the lubrication system.
2. Measure:
Piston pin-to-piston clearance
ASSEMBLY AND INSTALLATION
Piston ring and piston
1. Install:
Piston ring
Onto the piston.
NOTE:
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. Measurement steps:
Measure the outside diameter (piston pin)
a.
If out of specification, replace the piston
pin.
Outside diameter (piston pin):
15.991 ~ 16.000 mm
(0.6296 ~ 0.6299 in)
Measure the inside diameter (piston) b.
Inside diameter (piston):
16.002 ~ 16.013 mm
(0.6300 ~ 0.6304 in)
Calculate the piston pin-to-piston clear-
ance with the following formula.
Piston pin-to-piston clearance =
Inside diameter (piston)
b –
Outside diameter (piston pin)
a
If out of specification, replace the piston.
Piston pin-to-piston clearance:
0.002 ~ 0.022 mm
(0.0001 ~ 0.0009 in)
: 0.07 mm (0.003 in)

4 - 47
ENGCYLINDER AND PISTON
2. Position:
Top ring
2nd ring
Oil ring
Offset the piston ring end gaps as shown.
a
Top ring end
b2nd ring end
cOil ring end (upper)
dOil ring
eOil ring end (lower)
3. Install:
Piston 1
Piston pin 2
Piston pin clip 3
NOTE:
Apply engine oil onto the piston pin and pis-
ton.
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 fac-
ing downward.
45
135 135
a
b ced
3
Cylinder
1. Lubricate:
Piston
Piston ring
Cylinder
NOTE:
Apply a liberal coating of engine oil.
2. Install:
Dowel pin 1
O-ring 2
NOTE:
Apply the lithium soap base grease on the O-
ring.

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ENGCLUTCH
EC4A0000
CLUTCH
Extent of removal:
1 Push rod 1, 2 and push lever shaft removal
2 Push pod 1 disassembly
3 Friction plate and clutch plate removal
4 Primary driven gear removal
Extent of removal Order Part name Q’ty Remarks
CLUTCH REMOVAL
Preparation for removal Drain the engine oil. Refer to “ENGINE OIL REPLACEMENT”
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
1
4
1
3
2

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ENGCLUTCH
EC4A5000
ASSEMBLY AND INSTALLATION
Push lever shaft
1. Install:
Push lever shaft 1
Bolt (push lever shaft) 2
NOTE:
Apply the lithium soap base grease on the oil
seal lip.
Apply the engine oil on the push lever shaft.
Fit the seat plate 3 in the groove a of the
push lever shaft and tighten the bolt (seat
plate).
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)
Clutch
1. Install:
Primary driven gear 1
Thrust washer 2
Clutch boss 3
NOTE:
Apply the engine oil on the primary driven gear
inner circumference.
2. Install:
Lock washer 1
Nut (clutch boss) 2
NOTE:
Use the clutch holding tool 3 to hold the clutch
boss.
È
For USA and CDN
ÉExcept for USA and CDN
Clutch holding tool:
YM-91042/90890-04086
T R..60 Nm (6.0 m · kg, 43 ft · lb)
È
É