2002 YAMAHA YZ426F engine oil

3 - 32
INSP
ADJ

DRIVE CHAIN SLACK ADJUSTMENT
5. Install:

Drive chain
1


Joint
2


Master link clip
3

CAUTION:

Be sure to install the master link clip to the
direction as shown.
a
Turning direction
6. Lubricate:

Drive chain
Drive chain lubricant:
SAE 10W-30 motor oil or suit-
able chain lubricants
New

EC36A060
DRIVE CHAIN SLACK ADJUSTMENT
1. Elevate the rear wheel by placing the
suitable stand under the engine.
2. Check:

Drive chain slack
a

Above the seal guard installation bolt.
Out of specification
→
Adjust.
NOTE:

Before checking and/or adjusting, rotate the
rear wheel through several revolutions and
check the slack several times to find the tight-
est point. Check and/or adjust chain slack with
rear wheel in this “tight chain” position.
Drive chain slack:
40 ~ 50 mm (1.6 ~ 2.0 in)

3 - 33
INSP
ADJ

FRONT FORK INSPECTION
3. Adjust:

Drive chain slack
Drive chain slack adjustment steps:

Loosen the axle nut
1
and locknuts
2
.

Adjust chain slack by turning the adjusters
3
.
To tighten
→
Turn adjuster
3
counter-
clockwise.
To loosen
→
Turn adjuster
3
clockwise
and push wheel forward.

Turn each adjuster exactly the same
amount to maintain correct axle alignment.
(There are marks
a
on each side of chain
puller alignment.)
NOTE:

Turn the adjuster so that the chain is in line
with the sprocket, as viewed from the rear.
CAUTION:

Too small chain slack will overload the
engine and other vital parts; keep the
slack within the specified limits.

Tighten the axle nut while pushing down
the drive chain.
T R..

Axle nut:
125 Nm (12.5 m • kg, 90 ft • lb)

Tighten the locknuts.
EC36C000
FRONT FORK INSPECTION
1. Inspect:

Front fork smooth action
Operate the front brake and stroke the
front fork.
Unsmooth action/oil leakage
→
Repair
or replace.

3 - 34
INSP
ADJ

FRONT FORK OIL SEAL AND DUST SEAL CLEANING/
FRONT FORK INTERNAL PRESSURE RELIEVING/
FRONT FORK REBOUND DAMPING FORCE ADJUSTMENT
EC36D001
FRONT FORK OIL SEAL AND DUST SEAL
CLEANING
1. Remove:

Protector

Dust seal
1

NOTE:

Use a thin screw driver, and be careful not to
damage the inner fork tube and dust seal.
2. Clean:

Dust seal
a


Oil seal
b

NOTE:


Clean the dust seal and oil seal after every
run.

Apply the lithium soap base grease on the
inner tube.
FRONT FORK INTERNAL PRESSURE
RELIEVING
NOTE:

If the front fork initial movement feels stiff during
a run, relieve the front fork internal pressure.
1. Elevate the front wheel by placing a suit-
able stand under the engine.
2. Remove the air bleed screw
1
and
release the internal pressure from the
front fork.
3. Install:

Air bleed screw
EC36H002
FRONT FORK REBOUND DAMPING FORCE
ADJUSTMENT
1. Adjust:
Rebound damping force
By turning the adjuster 1.
Stiffer a →Increase the rebound damp-
ing force. (Turn the adjuster
1 in.)
Softer b →Decrease the rebound damp-
ing force. (Turn the adjuster
1 out.)
T R..1 Nm (0.1 m · kg, 0.7 ft · lb)

4 - 20
ENGCAMSHAFTS
CAMSHAFTS
CYLINDER HEAD COVER
Extent of removal:1 Cylinder head cover removal
Extent of removal Order Part name Q’ty Remarks
CYLINDER HEAD COVER
REMOVAL
Preparation for removal Seat and fuel tank Refer to “SEAT, FUEL TANK AND SIDE
COVERS” section.
Carburetor Refer to “CARBURETOR” section.
1 Spark plug 1
2 Engine upper bracket (right) 1
3 Engine upper bracket (left) 1
4 Cylinder head breather hose 1
5 Oil tank breather hose 1
6 Bolt (cylinder head cover) 2
7 Cylinder head cover 1
8 Gasket 1
9 Timing chain guide (upper) 1
1

4 - 39
ENGVALVES AND VALVE SPRINGS
3. Install:
Valve cotters
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
5. Install:
Adjusting pad 1
Valve lifter 2
NOTE:
Apply the engine oil on the valve lifters.
Apply the molybdenum disulfide oil on the
valve stem end.
Valve lifter must turn smoothly when rotated
with a finger.
Be careful to reinstall valve lifters and pads
in their original place.

4 - 44
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
1. Install:
Piston rings
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):
18.991 ~ 19.000 mm
(0.7477 ~ 0.7480 in)
Measure the inside diameter (piston) b.
Inside diameter (piston):
19.004 ~ 19.015 mm
(0.7482 ~ 0.7486 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:
0.004 ~ 0.024 mm
(0.00016 ~ 0.00094 in)
: 0.07 mm (0.003 in)

4 - 45
ENGCYLINDER AND PISTON
2. Position:
Top ring
2nd ring
Oil ring
Offset the piston ring end gaps as
shown.
aTop ring end
b2nd ring end
cOil ring end (upper)
dOil ring
eOil ring end (lower)
3. Install:
Piston 1
Piston pin 2
Piston pin clips 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.
4. Lubricate:
Piston
Piston rings
Cylinder
NOTE:
Apply a liberal coating of engine oil.
Cylinder
1. Install:
Dowel pins
O-ring
Gasket 1
Cylinder 2
NOTE:
Install the cylinder with one hand while com-
pressing the piston rings with the other hand.
CAUTION:
Pass the timing chain 3 through the tim-
ing chain cavity.
Be careful not to damage the timing chain
guide 4 during installation.
45°
135°135°
a
b ced
3New
New
New

4 - 46
ENGCLUTCH
CLUTCH
CLUTCH
Extent of removal:1 Push rod and push lever removal2 Push pod 1 disassembly
3 Friction plate and clutch plate removal4 Clutch housing 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 Gasket 1
3 Dowel pin 2
4 Clutch spring 6
5 Pressure plate 1
6 Push rod 1 1
7 Circlip 1
8 Plain washer 1
9 Bearing 1
10 Ball 1
11Push rod 2
1
41
2
3
1