2002 YAMAHA YZ125LC Cylinder

CHAPTER 4
ENGINE
SEAT, FUEL TANK AND SIDE
COVERS.....................................................4-1
EXHAUST PIPE AND SILENCER..............4-3
RADIATOR..................................................4-4
CARBURETOR AND REED VALVE...........4-7
CYLINDER HEAD , CYLINDER AND
PISTON.....................................................4-17
CLUTCH AND PRIMARY DRIVEN
GEAR.......................................................4-30
KICK AXLE, SHIFT SHAFT AND
PRIMARY DRIVE GEAR...........................4-38
YPVS GOVERNOR...................................4-46
WATER PUMP..........................................4-49
CDI MAGNETO.........................................4-53
ENGINE REMOVAL..................................4-57
CRANKCASE AND CRANKSHAFT.........4-62
TRANSMISSION, SHIFT CAM AND
SHIFT FORK.............................................4-69
CHAPTER 5
CHASSIS
FRONT WHEEL AND REAR WHEEL........5-1
FRONT BRAKE AND REAR BRAKE.......5-10
FRONT FORK...........................................5-27
HANDLEBAR............................................5-40
STEERING................................................5-47
SWINGARM..............................................5-52
REAR SHOCK ABSORBER.....................5-59
5NY-9-30-0 5/30/01 8:11 PM Page 20

1-3
GEN
INFO
IMPORTANT INFORMATION
EC130000
IMPORTANT INFORMATION
EC131010
PREPARATION FOR REMOVAL AND
DISASSEMBLY
1. Remove all dirt, mud, dust, and foreign ma-
terial before removal and disassembly.
When washing the machine with high pres-
sured water, cover the parts as follows.
9Silencer end
9Air filter intake slit
9Crankcase cover hole at the bottom
9Water pump housing hole at the bottom
2. Use proper tools and cleaning equipment.
Refer to “SPECIAL TOOLS” section.
3. When disassembling the machine, keep
mated parts together. They include gears,
cylinders, pistons, and other mated parts
that have been “mated” through normal
wear. Mated parts must be reused as an as-
sembly or replaced.
4. During the machine disassembly, clean all
parts and place them in trays in the order of
disassembly. This will speed up assembly
time and help assure that all parts are cor-
rectly reinstalled.
5. Keep away from fire.
5NY-9-30-1A 5/24/01 7:32 PM Page 6

1-14
GEN
INFO
STARTING AND BREAK-IN
EC194001
BREAK-IN PROCEDURES
1. Before starting the engine, fill the fuel tank
with a break-in oil-fuel mixture as follows.
2. Perform the pre-operation checks on the
machine.
3. Start and warm up the engine. Check the
idle speed, and check the operation of the
controls and the “ENGINE STOP” button.
4. Operate the machine in the lower gears at
moderate throttle openings for five to eight
minutes. Stop and check the spark plug
condition; it will show a rich condition during
break-in.
5. Allow the engine to cool. Restart the engine
and operate the machine as in the step
above for five minutes. Then, very briefly
shift to the higher gears and check full-throt-
tle response. Stop and check the spark
plug.
6. After again allowing the engine to cool,
restart and run the machine for five more
minutes. Full throttle and the higher gears
may be used, but sustained full-throttle op-
eration should be avoided. Check the spark
plug condition.
7. Allow the engine to cool, remove the top
end, and inspect the piston and cylinder.
Remove any high spots on the piston with
#600 grit wet sandpaper. Clean all compo-
nents and carefully reassemble the top end.
8. Drain the break-in oil-fuel mixture from the
fuel tank and refill with the specified mix.
9. Restart the engine and check the operation
of the machine throughout its entire operat-
ing range. Stop and check the spark plug
condition. Restart the machine and operate
it for about 10 to 15 more minutes. The ma-
chine will now be ready to race.
Mixing oil: Mixing ratio:
Yamalube “2-R” 15:1
5NY-9-30-1B 5/23/01 8:12 PM Page 6

1-15
GEN
INFO
STARTING AND BREAK-IN
cC
9
After the break-in or before each race, you
must check the entire machine for loose fit-
tings and fasteners as per “TORQUE-
CHECK POINTS”.
Tighten all such fasteners as required.
9When any of the following parts have been
replaced, they must be broken in.
CYLINDER AND CRANKSHAFT:
About one hour of break-in operation is
necessary.
PISTON, RING AND GEARS:
These parts require about 30 minutes of
break-in operation at half-throttle or less.
Observe the condition of the engine careful-
ly during operation.
5NY-9-30-1B 5/23/01 8:12 PM Page 8

1-16
GEN
INFO
TORQUE-CHECK POINTS
EC1A0013
TORQUE-CHECK POINTS
Frame construction Frame to rear frame
Combined seat and tank Fuel tank to frame
Engine mounting Frame to engine
Engine bracket to engine
Engine bracket to frame
Steering Steering shaft to Steering shaft to frame
handlebar Steering shaft to handle crown
Handle crown to handlebar
Suspension Front Steering shaft to Front fork to handle crown
front fork Front fork to under bracket
Rear For link type Assembly of links
Link to frame
Link to shock absorber
Link to swingarm
Rear Installation of shock Shock absorber to frame
absorber
Rear Installation of swingarm Tightening of pivot shaft
Wheel Installation of wheel Front Tightening of front axle
Tightening of axle holder
Rear Tightening of rear axle
Wheel to sprocket
Brake Front Caliper to front fork
Brake disc to wheel
Tightening of union bolt
Master cylinder to handlebar
Tightening of air bleeder
Rear Brake pedal to frame
Brake disc to wheel
Tighening of union bolt
Master cylinder to frame
Tightening of air bleeder
Fuel system Fuel tank to fuel cock
NOTE:
Concerning the tightening torque, refer to “
MAINTENANCE SPECIFICATIONS” section in
the CHAPTER 2.
5NY-9-30-1B 5/23/01 8:12 PM Page 10

1-18
GEN
INFO
CLEANING AND STORAGE
EC1B2001
STORAGE
If your machine is to be stored for 60 days or
more, some preventive measures must be taken
to avoid deterioration. After cleaning the ma-
chine thoroughly, prepare it for storage as fol-
lows:
1. Drain the fuel tank, fuel lines, and the carbu-
retor float bowl.
2. Remove the spark plug, pour a tablespoon
of SAE 10W-30 motor oil in the spark plug
hole, and reinstall the plug. With the engine
stop switch pushed in, kick the engine over
several times to coat the cylinder walls with
oil.
3. Remove the drive chain, clean it thoroughly
with solvent, and lubricate it. Reinstall the
chain or store it in a plastic bag tied to the
frame.
4. Lubricate all control cables.
5. Block the frame up to raise the wheels off
the ground.
6. Tie a plastic bag over the exhaust pipe out-
let to prevent moisture from entering.
7. If the machine is to be stored in a humid or
salt-air environment, coat all exposed metal
surfaces with a film of light oil. Do not apply
oil to rubber parts or the seat cover.
NOTE:
Make any necessary repairs before the machine
is stored.
5NY-9-30-1B 5/23/01 8:12 PM Page 16

2-1
SPECGENERAL SPECIFICATIONS
Model name: YZ125LC (EUROPE)
YZ125P1 (USA)
YZ125(P) (CDN, AUS, NZ, ZA)
Model code number: 5NY1 (USA, CDN, ZA)
5NY2 (EUROPE)
5NY4 (AUS, NZ)
Dimensions:USA, ZAAUS, NZ EUROPE CDN
Overall length 2,134 mm←2,130 mm←
(84.0 in) (83.9 in)
Overall width 827 mm←←←
(32.6 in)
Overall height 1,318 mm 1,315 mm 1,311 mm 1,315 mm
(51.9 in) (51.8 in) (51.6 in) (51.8 in)
Seat height 993 mm←992 mm←
(39.1 in) (39.1 in)
Wheelbase 1,438 mm←←←
(56.6 in)
Minimum ground clearance 400 mm←398 mm←
(15.7 in) (15.7 in)
Basic weight:
With oil and full fuel tank 95.0 kg (209.4 lb)
Engine:
Engine type Liquid cooled 2-stroke, gasoline
Cylinder arrangement Single cylinder, forward inclined
Displacement 124 cm
3(4.36 lmp oz, 4.19 US oz)
Bore ×Stroke 54 ×54.5 mm (2.126 ×2.146 in)
Compression ratio 8.7~10.8 : 1
Starting system Kick starter
Lubrication system: Premix (30 : 1)(Yamalube 2-R)
Oil type or grade (2-Cycle):
Transmission oil Yamalube 4 (10W-30) or SAE 10W-30 type SE
motor oil
Periodic oil change 0.65 L (0.57 Imp qt, 0.69 US qt)
Total amount 0.70 L (0.62 Imp qt, 0.74 US qt)
Coolant capacity (including all routes): 0.95 L (0.84 Imp qt, 1.00 US qt)
Air filter: Wet type element
Fuel:
Type Except for ZA: Premium unleaded gasoline
only with a research octane
number of 95 or higher
For ZA: Premium gasoline
Tank capacity 8.0 L (1.76 Imp gal, 2.11 US gal)
EC200000
SPECIFICATIONSEC211000
GENERAL SPECIFICATIONS
2
5NY-9-30-2A 5/24/01 8:03 PM Page 2

2-3
SPECMAINTENANCE SPECIFICATIONS
Item Standard Limit
Cylinder head:
Combustion chamber capacity 10.2 cm
3…
(0.359 Imp oz, 0.345 US oz)
Warp limit…
0.03 mm (0.0012 in)
Cylinder:
Bore size 54.000~54.014 mm 54.1 mm (2.130 in)
(2.1260~2.1265 in)
Taper limit…
0.05 mm (0.0020 in)
Out of round limit…
0.01 mm (0.0004 in)
Piston:
Piston size/ 53.957~53.972 mm…
(2.1243~2.1249 in)
Measuring point* 17.5 mm (0.69 in)…
Piston clearance 0.040~0.045 mm 0.1 mm (0.004 in)
(0.0016~0.0018 in)
Piston offset 1.0 mm (0.039 in)/EX-side…
Piston pin:
Piston pin outside diameter 14.995~15.000 mm 14.975 mm
(0.5904~0.5906 in) (0.5896 in)
Piston ring:
Sectional sketch Plain…
B=1.0 mm (0.039 in)…
T=2.35 mm (0.093 in)…
End gap (installed) 0.5~0.7 mm (0.020~0.028 in) 1.2 mm (0.047 in)
Side clearance (installed) 0.035~0.070 mm 0.1 mm (0.004 in)
(0.0014~0.0028 in)
Crankshaft:
Crank width “A” 55.90~55.95 mm …
(2.201~2.203 in)
Runout limit “C” 0.03 mm (0.0012 in) 0.05 mm (0.0020 in)
Connecting rod big end
side clearance “D” 0.20~0.70 mm (0.008~0.028 in)…
Small end free play “F” 0.8~1.0 mm (0.031~0.039 in) 2.0 mm (0.08 in)
Clutch:
Friction plate thickness 2.9~3.1 mm (0.114~0.122 in) 2.8 mm (0.110 in)
Quantity 8 …
Clutch plate thickness 1.5~1.7 mm (0.059~0.067 in) …
Quantity 7 …
Warp limit … 0.2 mm (0.008 in)
Clutch spring free length 41.2 mm (1.622 in) 39.2 mm (1.543 in)
Quantity 5 …
Clutch housing thrust clearance 0.20~0.25 mm (0.008~0.010 in) …
Clutch housing radial clearance 0.015~0.049 mm …
(0.0006~0.0019 in)
Clutch release method Outer pull, rack and pinion pull …
EC212000
MAINTENANCE SPECIFICATIONSEC212100
ENGINE
5NY-9-30-2A 5/24/01 8:03 PM Page 4