2002 YAMAHA YZ125LC Cylinder

3-17
INSP
ADJ
CHASSIS/BRAKE SYSTEM AIR BLEEDING
EC360000
CHASSIS
EC361012
BRAKE SYSTEM AIR BLEEDING
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Bleed the brake system if:
9The system has been disassembled.
9A brake hose has been loosened or
removed.
9The brake fluid is very low.
9The brake operation is faulty.
A dangerous loss of braking performance may
occur if the brake system is not properly
bleed.
1. Remove:
9Master cylinder cap
9Diaphragm
9Protector (rear brake)
2. Bleed:
9Brake fluid
ÅFront
ıRear
Air bleeding steps:
a. Add proper brake fluid to the reservoir.
b. Install the diaphragm. Be careful not to
spill any fluid or allow the reservoir to
overflow.
c. Connect the clear plastic tube 2tightly to
the caliper bleed screw 1.
d. Place the other end of the tube into a
container.
e. Slowly apply the brake lever or pedal
several times.
f. Pull the lever in or push down on the ped-
al. Hold the lever or pedal in position.
g. Loosen the bleed screw and allow the
lever or pedal to travel towards its limit.
h. Tighten the bleed screw when the lever
or pedal limit has been reached; then re-
lease the lever or pedal.
i. Repeat steps (e) to (h) until of the air
bubbles have been removed from the
system.
Bleed screw:
6 Nm (0.6 m•kg, 4.3 ft•lb)
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3-18
INSP
ADJ
FRONT BRAKE ADJUSTMENT
3. Install:
9Protector (rear brake)
9Diaphragm
9Master cylinder cap
EC362041
FRONT BRAKE ADJUSTMENT
1. Check:
9Brake lever position a
2. Remove:
9Lever cover
3. Adjust:
9Brake lever position
NOTE:
If bleeding is difficult, it may be necessary
to let the brake fluid system stabilize for a
few hours. Repeat the bleeding procedure
when the tiny bubbles in the system have
disappeared.
j. Add brake fluid to the level line on the
reservoir.
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Check the operation of the brake after
bleeding the brake system.
Brake lever position a a
:
Standard position Extent of adjutment
95 mm (3.74 in)76 ~ 97 mm
(2.99 ~ 3.82 in)
Brake lever position adjustment steps:
9Loosen the locknut 1.
9Turn the adjusting bolt 2until the lever
position a is within specified position.
9Tighten the locknut.
cC
Be sure to tighten the locknut, as it will
cause poor brake performance.
Locknut:
5 Nm (0.5 m•kg, 3.6 ft•lb)
4. Install:
9Lever cover
5NY-9-30-3A 5/23/01 8:25 PM Page 44

3-23
REAR BRAKE PAD INSULATOR INSPECTION
BRAKE FLUID LEVEL INSPECTIONINSP
ADJ
EC36b000
REAR BRAKE PAD INSULATOR
INSPECTION
1. Remove:
9Brake pad
Refer to
“REAR BRAKE PAD INSPEC-
TION AND REPLACEMENT” section.
2. Inspect:
9Rear brake pad insulator 1
Damage Replace.
EC367001
BRAKE FLUID LEVEL INSPECTION
1. Place the master cylinder so that its top is in
a horizontal position.
2. Inspect:
9Brake fluid level
Fluid at lower level Fill up.
aLower level
ÅFront
ıRear
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9
Use only designated quality brake fluid to
avoid poor brake performance.
9Refill with same type and brand of brake
fluid; mixing fluids could result in poor
brake performance.
9Be sure that water or other contaminants
do not enter master cylinder when refilling.
9Clean up spilled fluid immediately to avoid
erosion of painted surfaces or plastic parts.
Recommended brake fluid:
DOT #4
5NY-9-30-3B 5/23/01 8:33 PM Page 10

4-17
ENGCYLINDER HEAD, CYLINDER AND PISTON
EC470000
CYLINDER HEAD, CYLINDER AND PISTONEC478000
CYLINDER HEAD AND CYLINDER
Extent of removal Order Part name Q’ty Remarks
CYLINDER HEAD AND
CYLINDER REMOVAL
Preparation for Seat and fuel tank Refer to “SEAT, FUEL TANK AND
removal SIDE COVERS” section.
Exhaust pipe and silencer Refer to ”EXHAUST PIPE AND
SILENCER” section.
Radiator hose 2 Disconnect at cylinder head side.
Bolt (radiator) Refer to “RADIATOR” section.
Radiator hose 3 Disconnect at right side radiator,
then shift the radiators forward.
1 Spark plug 1
2 Engine bracket 2
3 Nut (cylinder head) 5
Loosen each nut 1/4 turn, and remove
them after all nuts are loosened.
4 Cylinder head 1
5 Power valve housing 1
6 Push rod 1 Refer to “REMOVAL POINTS”.
7 Nut (cylinder) 4
8 Cylinder 1
Extent of removal:1Cylinder head removal2Cylinder removal
1
2
34 Nm (3.4 m•kg, 24 ft•lb)
20 Nm (2.0 m•kg, 14 ft•lb)
28 Nm (2.8 m•kg, 20 ft•lb)
5 Nm (0.5 m•kg, 3.6 ft•lb)
5 Nm (0.5 m•kg, 3.6 ft•lb)
30 Nm (3.0 m•kg, 22 ft•lb)
10 Nm (1.0 m•kg, 7.2 ft•lb)
5NY-9-30-4A 5/23/01 8:40 PM Page 34

4-18
ENGCYLINDER HEAD, CYLINDER AND PISTON
EC478100
PISTON AND POWER VALVE
Extent of removal Order Part name Q’ty Remarks
PISTON AND POWER VALVE
REMOVAL
1 Piston pin clip 2
2 Piston pin 1
3 Piston 1 Refer to “REMOVAL POINTS”.
4 Small end bearing 1
5 Piston ring 1
6 Power valve cover 1
7 Thrust plate 1
8 Bolt (link lever) 1
9 Valve shaft 1
10 Collar 2
11 Link lever 1
12 Spring 2
13 Power valve 1 1
14 Power valve 2 1
Extent of removal:1Piston and piston ring removal2Power valve removal
2
1






 






4 Nm (0.4 m•kg, 2.9 ft•lb)
4 Nm (0.4 m•kg, 2.9 ft•lb)
4 Nm (0.4 m•kg, 2.9 ft•lb)
8 Nm (0.8 m•kg, 5.8 ft•lb)
5NY-9-30-4B 5/23/01 8:48 PM Page 2

4-19
ENGCYLINDER HEAD, CYLINDER AND PISTON
EC473000
REMOVAL POINTSEC473210
Push rod
1. Remove:
9Bolt (push rod) 1
9Push rod 2
NOTE:
Set the collar 3included in owner’s tool kit to
remove the bolt (push rod).
EC473402
Piston and piston ring
1. Remove:
9Piston pin clip 1
NOTE:
Before removing the piston pin clip, cover the
crankcase with a clean rag to prevent the pis-
ton pin clip from falling into the crankcase cav-
ity.
2. Remove:
9Piston pin 1
9Piston 2
9Small end bearing 3
NOTE:
Before removing the piston pin, deburr the clip
groove and pin hole area. If the piston pin
groove is deburred and piston pin is still difficult
to remove, use the piston pin puller 4.
cC
Do not use a hammer to drive the piston
pin out.
3. Remove:
9Piston ring 1
NOTE:
Take care not to scratch the piston or damage
the piston ring by expanding it more than neces-
sary.
Piston pin puller:
YU-1304/90890-01304
5NY-9-30-4B 5/23/01 8:48 PM Page 4

4-20
ENGCYLINDER HEAD, CYLINDER AND PISTON
EC474000
INSPECTION
EC474101
Cylinder head
1. Remove:
9Carbon deposits
Use a rounded scraper 1.
NOTE:
Take care to avoid damaging the spark plug
threads. Do not use a sharp instrument. Avoid
scratching the aluminum.
2. Inspect:
9Cylinder head water jacket
Crust of minerals/Rust Remove.
9Cylinder head warpage
Out of specification Re-surface.
Warpage measurement and re-surface-
ment steps:
9Attach a straightedge 1and a thickness
gauge 2on the cylinder head.
9Measure the warpage.
9If the warpage is out of specification, resur-
face the cylinder head.
9Place # 400~600 grit wet sandpaper 3on
the surface plate, and re-surface the head
4using a figure-eight sanding pattern.
NOTE:
Rotate the cylinder head several times to
avoid removing too much material from one
side.
EC474200
Cylinder
1. Remove:
9Carbon deposits
Use a rounded scraper 1.
NOTE:
Do not use a sharp instrument. Avoid scratching
the aluminum.
Warpage limit:
0.03 mm(0.0012 in)
5NY-9-30-4B 5/23/01 8:48 PM Page 6

4-21
ENGCYLINDER HEAD, CYLINDER AND PISTON
EC474301
Piston
1. Remove:
9Carbon deposits
From the piston crown aand ring groove
b.
2. Inspect:
9Piston wall
Score marks Repair or replace.
3. Measure:
9Piston skirt diameter
Use micrometer 1.
Measure the specific distance afrom the
bottom edge.
Out of specification Replace.
2. Inspect:
9Cylinder inner surface
Score marks Repair or replace.
Use #400~600 grit wet sandpaper.
cC
Do not rebore the cylinder.
3. Measure:
9Cylinder bore “C”
Use cylinder gauge 1.
Out of limit Replace.
NOTE:
Measure the cylinder bore “C” in parallel (A, B,
C) to and at right angles to the crankshaft (a, b).
Then, find the average of the measurements.
17.5 mm 53.957~53.972 mm
(0.69 in) (2.1243~2.1249 in)
Standard Wear limit
Cylinder 54.000~54.014mm 54.1 mm
bore “C” (2.1260~2.1265 in) (2.130 in)
Taper “T” —0.05 mm
(0.0020 in)
C=Maximum Aa~Cb
T=(Maximum Aa, or Ab) —
(Maximum Ba, or Bb)
Distance a a
Piston dia.
5NY-9-30-4B 5/23/01 8:48 PM Page 8