2002 YAMAHA YZ125LC engine

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)
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4-27
ENGCYLINDER HEAD, CYLINDER AND PISTON
EC475233
Piston ring and piston
1. Install:
9Piston ring 1
NOTE:
9Take care not to scratch the piston or damage
the piston ring.
9Align the piston ring gap with the pin 2.
9After installing the piston ring, check the
smooth movement of it.
2. Install:
9Gasket (cylinder) 1
9Small end bearing 2
9Dowel pin 3
NOTE:
9Apply the engine oil onto the bearing (crank-
shaft and connecting rod) and connecting rod
big end washers.
9Install the gasket with the seal print side
toward the crankcase.
3. Install:
9Piston 1
9Piston pin 2
9Piston pin clip 3
NOTE:
9The arrow aon the piston dome must point to
exhaust side.
9Before installing the piston pin clip, cover the
crankcase with a clean rag to prevent the pis-
ton pin clip from falling into the crankcase cavi-
ty.
cC
Do not allow the clip open ends to meet the
piston pin slot b b
.
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4-28
ENGCYLINDER HEAD, CYLINDER AND PISTON
3. Install:
9Nut (cylinder) 1
NOTE:
Tighten the nuts in stage, using a crisscross pat-
tern.
4. Install:
9Collar 1
9Push rod 2
9Plain washer 3
9Bolt (push rod) 4
NOTE:
9Set the collar 5included in owner's tool kit to
install the bolt (push rod).
9Do not forget to remove the collar.
EC4753A1
Cylinder head and cylinder
1. Apply:
9Engine oil
To piston 1, piston ring 2and cylinder
surface.
2. Install:
9Cylinder 1
cC
Make sure the piston ring is properly posi-
tioned. Install the cylinder with one hand
while compressing the piston ring with the
other hand.
NOTE:
After installing, check the smooth movement of
the piston.
30 Nm (3.0 m•kg, 22 ft•lb)
5 Nm (0.5 m•kg, 3.6 ft•lb)
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4-29
ENGCYLINDER HEAD, CYLINDER AND PISTON
7. Install:
9Cylinder head 1
9Copper washer 2
9Nut (cylinder head) 3
NOTE:
Tighten the nuts (cylinder head) in stage, using a
crisscross pattern.
8. Install:
9Engine bracket 1
9Bolt (engine bracket) 2
9CDI unit bracket 3
9Engine mounting bolt (upper) 4
9. Install:
9Spark plug 1
9Spark plug cap 2
9Radiator hose 2 3
28 Nm (2.8 m•kg, 20 ft•lb)
34 Nm (3.4 m•kg, 24 ft•lb)
34 Nm (3.4 m•kg, 24 ft•lb)
20 Nm (2.0 m•kg, 14 ft•lb)
5. Install:
9Gasket (power valve housing)
9Power valve housing 1
9Bolt (power valve housing) 2
6. Install:
9O-ring 1
NOTE:
Apply the lithium soap base grease on the O-
rings.
5 Nm (0.5 m•kg, 3.6 ft•lb)
1 Nm (0.1 m•kg, 0.7 ft•lb)
5NY-9-30-4B 5/23/01 8:48 PM Page 24

4-30
ENGCLUTCH AND PRIMARY DRIVEN GEAR
Extent of removal Order Part name Q’ty Remarks
CLUTCH PLATE AND
FRICTION PLATE REMOVAL
Preparation for Drain the transmission oil.
Refer to “TRANSMISSION OIL REPLACE-
removalMENT“ section in the CHAPTER 3.
Bolt (brake pedal) Shift the brake pedal downward.
Clutch cable Disconnect at engine side.
1 Clutch cover 1
2 Screw (clutch spring) 5
3 Clutch spring 5
4 Pressure plate 1
5 Push rod 1
6 Plain washer 2
7 Bearing 1
8 Friction plate 8
9 Clutch plate 7
Extent of removal:1Clutch plate and friction plate removal
EC490000
CLUTCH AND PRIMARY DRIVEN GEAREC498000
CLUTCH PLATE AND FRICTION PLATE
1
10 Nm (1.0 m•kg, 7.2 ft•lb)
10 Nm (1.0 m•kg, 7.2 ft•lb)
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4-38
ENG
KICK AXLE, SHIFT SHAFT AND PRIMARY DRIVE
GEAR
Extent of removal Order Part name Q’ty Remarks
RPIMARY DRIVE GEAR REMOVAL
Preparation for Drain the transmission oil.Refer to “TRANSMISSION OIL REPLACE-
removalMENT“ section in the CHAPTER 3.
Clutch cable Disconnect at engine side.
Bolt (brake pedal) Shift the brake pedal downward.
Radiator hose 4 Disconnect at water pump side.
Bolt (push rod)
Refer to “CYLINDER HEAD, CYLINDER
AND PISTON” section.
1 Kick starter 1
2 Crankcase cover (right) 1
3 Bolt (Primary drive gear) 1 Only loosening
Refer to “REMOVAL POINTS”.
4 Primary driven gear 1 Refer to “CLUTCH AND PRIMARY
DRIVEN GEAR”section.
5 Primary drive gear 1
EC4C0000
KICK AXLE, SHIFT SHAFT AND PRIMARY DRIVE GEAREC4C8000PRIMARY DRIVE GEAR
1
Extent of removal:1Primary drive gear removal
10 Nm (1.0 m•kg, 7.2 ft•lb)
10 Nm (1.0 m•kg, 7.2 ft•lb)
55 Nm (5.5 m•kg, 40 ft•lb)
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4-57
ENGENGINE REMOVAL
Extent of removal Order Part name Q’ty Remarks
ENGINE REMOVAL
Preparation for
Hold the machine by placing the
removalsuitable stand under the engine.
Seat and fuel tankRefer to “SEAT, FUEL TANK AND
SIDE COVERS” section.
Carburetor Refer to “CARBURETOR AND REED
VALVE” section.
Exhaust pipe and silencer Refer to “EXHAUST PIPE AND
SILENCER” section.
Clutch cable Disconnect at engine side.
Radiator hose 2 Disconnect at cylinder head side.
Radiator hose 4 Disconnect at water pump side.
Bolt (radiator) Refer to “RADIATOR” section.
Radiator hose 3 Disconnect at right side radiator,
then shift the radiators forward.
Spark plug cap
Disconnect the CDI magneto lead.
EC4M0000
ENGINE REMOVAL
w
Support the machine securely so there is no dan-
ger of it falling over.
26 Nm (2.6 m•kg, 19 ft•lb)
34 Nm (3.4 m•kg, 24 ft•lb)
69 Nm (6.9 m•kg, 50 ft•lb)
69 Nm (6.9 m•kg, 50 ft•lb)
85 Nm (8.5 m•kg, 61 ft•lb)
60 Nm (6.0 m•kg, 43 ft•lb)
5 Nm (0.5 m•kg, 3.6 ft•lb)
5NY-9-30-4D 5/24/01 8:33 PM Page 2

4-58
ENGENGINE REMOVAL
Extent of removal:1Engine removal
Extent of removal Order Part name Q’ty Remarks
1 Chain cover 1
2 Nut (drive sprocket) 1
3 Lock washer 1 Refer to “REMOVAL POINTS”.
4 Drive sprocket 1
5 Clip 1
6 Bolt (brake pedal) 1
7 Brake pedal 1
8 Engine bracket 2
9 Engine mounting bolt 3
10 Pivot shaft 1
Refer to “REMOVAL POINTS”.
11 Engine 1
1







26 Nm (2.6 m•kg, 19 ft•lb)
34 Nm (3.4 m•kg, 24 ft•lb)
69 Nm (6.9 m•kg, 50 ft•lb)
69 Nm (6.9 m•kg, 50 ft•lb)
85 Nm (8.5 m•kg, 61 ft•lb)
60 Nm (6.0 m•kg, 43 ft•lb)
5 Nm (0.5 m•kg, 3.6 ft•lb)
5NY-9-30-4D 5/24/01 8:33 PM Page 4