engine YAMAHA YZ426F 2001 Repair Manual

4 - 4
ENG

RADIATOR
EC450001
RADIATOR
Extent of removal:
1
Radiator removal
Extent of removal Order Part name Q’ty Remarks
RADIATOR REMOVAL
Preparation for removal Drain the coolant. Refer to “COOLANT REPLACEMENT”
section in the CHAPTER 3.
Seat and fuel tank Refer to “SEAT, FUEL TANK AND SIDE
COVERS” section.
Exhaust pipe Refer to “EXHAUST PIPE AND
SILENCER” section.
1 Engine guard 1
2 Panel 2
3 Clamp 8
4 Radiator (right) 1
5 Hose 2 1
6 Hose 3 1
7 Hose 4 1
8 Pipe 2/O-ring 1/1
9 Radiator breather hose 1
10 Radiator (left) 1
11 Hose 1 1
12 Pipe 1/O-ring 1/1
1

4 - 5
ENG

RADIATOR
EC456000
HANDLING NOTE
WARNING

Do not remove the radiator cap when the
engine and radiator are hot. Scalding hot
fluid and steam may be blown out under
pressure, which could cause serious
injury.
When the engine has cooled, open the radi-
ator cap by the following procedure:
Place a thick rag, like a towel, over the radi-
ator cap, slowly rotate the cap counter-
clockwise to the detent. This procedure
allows any residual pressure to escape.
When the hissing sound has stopped,
press down on the cap while turning coun-
terclockwise and remove it.
EC454000
INSPECTION
EC444100
Radiator
1. Inspect:
l
Radiator core
1

Obstruction
®
Blow out with com-
pressed air through rear of the radiator.
Bent fin
®
Repair/replace.
EC455000
ASSEMBLY AND INSTALLATION
Radiator
1. Install:
l
Pipe 1
1

l
Hose 1
2

lPipe 2 3
lHose 3 4
lHose 4 5
2. Install:
lHose 2 1
lRadiator (left) 2
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)
T R..2 Nm (0.2 m · kg, 1.4 ft · lb)
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)
T R..2 Nm (0.2 m · kg, 1.4 ft · lb)
T R..2 Nm (0.2 m · kg, 1.4 ft · lb)
T R..2 Nm (0.2 m · kg, 1.4 ft · lb)
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)

4 - 10
ENGCARBURETOR
EC466020
HANDLING NOTE
CAUTION:
Do not loosen the screws {TPS (throttle
position sensor)} 1 except when changing
the TPS (throttle position sensor) due to
failure because it will cause a drop in
engine performance.
REMOVAL POINTS
Pilot screw
1. Remove:
lPilot screw 1
NOTE:
To optimize the fuel flow at a small throttle
opening, each machine’s pilot screw has been
individually set at the factory. Before removing
the pilot screw, turn it in fully and count the
number of turns. Record this number as the
factory-set number of turns out.
INSPECTION
Carburetor
1. Inspect:
lCarburetor body
Contamination ® Clean.
NOTE:
lUse a petroleum based solvent for cleaning.
Blow out all passages and jets with com-
pressed air.
lNever use a wire.

4 - 19
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 - 38
ENGVALVES AND VALVE SPRINGS
3. Install:
lValve 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-04019/90890-04019
5. Install:
lAdjusting pad 1
lValve lifter 2
NOTE:
lApply the engine oil on the valve lifters.
lApply the molybdenum disulfide oil on the
valve stem end.
lValve lifter must turn smoothly when rotated
with a finger.
lBe careful to reinstall valve lifters and pads
in their original place.

4 - 43
ENG

CYLINDER AND PISTON
Piston pin
1. Inspect:
l
Piston pin
Blue discoloration/grooves
®
Replace,
then inspect the lubrication system.
2. Measure:
l
Piston pin-to-piston clearance
ASSEMBLY AND INSTALLATION
Piston
1. Install:
l
Piston rings
Onto the piston.
NOTE:

l
Be sure to install the piston rings so that the
manufacturer’s marks or numbers are
located on the upper side of the rings.
l
Lubricate the piston and piston rings liberally
with engine oil.
Measurement steps:
l
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)
l
Measure the inside diameter (piston)
b
.
Inside diameter (piston):
19.004 ~ 19.015 mm
(0.7482 ~ 0.7486 in)
l
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

l
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 - 44
ENG

CYLINDER AND PISTON
2. Position:
l
Top ring
l
2nd ring
l
Oil ring
Offset 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:
l
Piston
1

l
Piston pin
2

l
Piston pin clips
3

NOTE:

l
Apply engine oil onto the piston pin and pis-
ton.
l
Be sure that the arrow mark
a
on the piston
points to the exhaust side of the engine.
l
Before installing the piston pin clip, cover the
crankcase with a clean rag to prevent the
piston pin clip from falling into the crankcase.
l
Install the piston pin clips with their ends fac-
ing downward.
4. Lubricate:
l
Piston
l
Piston rings
l
Cylinder
NOTE:

Apply a liberal coating of engine oil.
Cylinder
1. Install:
l
Dowel pins
l
O-ring
l
Gasket
1

l
Cylinder
2

NOTE:

Install the cylinder with one hand while com-
pressing the piston rings with the other hand.
CAUTION:

l
Pass the timing chain
3 through the tim-
ing chain cavity.
lBe careful not to damage the timing chain
guide 4 during installation.
45¡
135¡135¡
a
b ced
3New
New
New

4 - 45
ENG

CLUTCH
CLUTCH
CLUTCH
Extent of removal:
1
Push rod and push lever removal
2
Push pod 1 disassembly
3
Friction plate and clutch plate removal
4
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

4 - 49
ENGCLUTCH
EC4A5000
ASSEMBLY AND INSTALLATION
Push lever
1. Install:
lPush lever 1
lBolt (push lever)
NOTE:
lApply the lithium soap base grease on the oil
seal lip.
lApply the engine oil on the push lever.
lFit the seat plate 2 in the groove a of the
push lever 1 and tighten the bolt (seat
plate).
2
1
a
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)
Clutch
1. Install:
lSeat plate 1
lClutch damper spring 2
lFriction plate 1 [d = ø128 mm (5.04 in)]
3
lClutch plate 4
To clutch boss 5.
NOTE:
lInstall the clutch damper spring with the
“OUT SIDE” mark a facing out.
lInstall the clutch plate with its sharp-edged
corner side b facing the engine.
4 3 2 1 5
a
b
3
5
1
243
1
2
4
2. Install:
lPrimary driven gear 1
lThrust washer 2
lClutch boss 3
NOTE:
Apply the engine oil on the primary driven gear
inner circumference.

4 - 50
ENGCLUTCH
3. Install:
lLock washer 1
lNut (clutch boss) 2
NOTE:
Use the clutch holding tool 3 to hold the clutch
boss.
For USA and CDN
õExcept for USA and CDN
Clutch holding tool:
YM-91042/90890-04086
New
T R..75 Nm (7.5 m · kg, 54 ft · lb)

õ
4. Bend the lock washer 1 tab.
5. Install:
lFriction plate 2 [d = ø119 mm (4.69 in)]
1
lClutch plate 2
NOTE:
lInstall the clutch plates and friction plates 2
alternately on the clutch boss, starting with a
friction plate and ending with a friction plate.
lApply the engine oil on the friction plates and
clutch plates.
6. Install:
lBearing 1
lPlain washer 2
lCirclip 3
To push rod 1 4.
NOTE:
Apply the engine oil on the bearing and plain
washer.
New