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Stihl MS 251 Instruction Manual - Page 43

Piston, approx. 150°C 300°F.

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Installing Before installing, clean the crankshaft, b 14 1 1 2710RA117 TG 2710RA119 TG 2310RA425 TG Heated ball bearings must be installed quickly because the crankshaft stubs absorb heat and expand. 2 1 : Position the ball bearing (1) with its open side facing the crankshaft. - Heat the inner bearing race to approx. 150°C (300°F). : Push the ball bearing (1) onto the straight crankshaft stub (clutch side) as far as stop. 1 : Lubricate the needle cage (1) with oil and push it into the connecting rod. - Install the piston, b 6.8 - Install the crankshaft and new oil seals, b 6.5 - Install the engine, b 6.5 6.8 Piston - Remove the engine, b 6.5 - Remove the crankshaft, b 6.6 The assembly drift passes through the installed snap ring. Therefore, only one snap ring needs to be removed. - Remove the snap at the side with the recess. : Slide the assembly drift (1) 1110 893 4700 through the installed snap ring and push the piston pin (2) out of the piston. If the piston pin is stuck, release it by tapping the end of the drift lightly with a hammer. Hold the piston steady during this process to ensure that no jolts are transmitted to the connecting rod. - Remove the piston. - Inspect the piston rings and replace if necessary, b 6.9 Installing - Pull out the needle cage , check it and replace if necessary. 1 2710RA118 TG 2710RA418 TG 2710RA123 TG : Position the ball bearing (1) with its open side facing the crankshaft. - Heat the inner bearing race to approx. 150°C (300°F). : Push the ball bearing (1) onto the tapered crankshaft stub (ignition side) as far as stop. : Use a suitable tool to grip the hookless snap ring at the recess (arrow) and ease it out. : Lubricate the needle cage (1) with oil and push it into the connecting rod. 42 MS 231, MS 231 C, MS 251, MS 251 C

Section	Page
Contents	2
1. Introduction and Safety Precautions	4
1.1 Introduction	4
1.2 Safety Precautions	5
2. Specifications	6
2.1 Engine	6
2.2 Fuel System	6
2.3 Ignition System	6
2.4 Chain Lubrication	6
2.5 Tightening Torques	7
3. Troubleshooting	9
3.1 Clutch	9
3.2 Chain Drive, Chain Brake, Chain Tensioner	10
3.3 Chain Lubrication	12
3.4 Rewind Starter	13
3.5 Ignition System	15
3.6 Carburetor	16
3.7 Engine	19
4. Clutch	20
4.1 Clutch Drum	20
4.2 Clutch	20
5. Chain Brake	21
5.1 Checking Operation	21
5.2 Brake Band	21
5.3 Brake Lever	22
5.4 Brake Lever on Machines with QuickStop Super	25
5.4.1 Adjusting the brake cable	27
5.4.2 Brake cable Removing and Installing	28
5.5 Chain Tensioner	30
5.5.1 Quick Chain Tensioner	31
5.5.2 Chain Catcher	31
5.6 Bar Mounting Stud	32
6. Engine	33
6.1 Muffler	33
6.2 Leakage Test	34
6.2.1 Preparations	34
6.2.2 Vacuum Test	35
6.2.3 Pressure Test	35
6.3 Oil Seals	36
6.3.1 Ignition side	36
6.3.2 Clutch side	37
6.4 Shroud	37
6.5 Engine	38
6.6 Cylinder / Crankshaft	39
6.7 Bearings / Crankshaft	42
6.8 Piston	43
6.9 Piston Rings	45
7. Ignition System	46
7.1 Ignition Timing	46
7.2 Preseparator	46
7.3 Install new ignition module	46
7.4 Testing the Ignition Module	48
7.5 Spark Plug Boot / Ignition Lead	49
7.6 Flywheel	50
7.7 Short Circuit Wire	51
7.7.1 Testing	51
7.7.2 Removing and Installing	51
7.7.3 Ground Wire	54
7.7.4 Contact Spring	54
7.8 Ignition System Troubleshooting	56
8. Rewind Starter	59
8.1 General	59
8.2 Fan housing	59
8.3 Pawls	60
8.4 ErgoStart	61
8.5 Rope Rotor	62
8.6 Starter Rope / Grip	62
8.7 Tensioning the Rewind Spring	63
8.8 Replacing the Rewind Spring	64
9. Servicing the AV System	67
9.1 AV Spring on Oil Tank	67
9.2 AV Spring on Fuel Tank	67
9.3 AV Spring on Handlebar	68
9.3.1 Stop Buffers	69
9.3.2 Buffers on Filter Base	70
9.4 Handlebar	70
10. Control Levers	72
10.1 Master Control Lever	72
10.1.1 Removing and Installing	72
10.2 Throttle Trigger/Lockout Lever	73
10.3 Throttle Trigger/Lockout Lever - QuickStop Super	74
10.3.1 Switch Lever QuickStop Super	75
10.3.2 Lockout Lever - QuickStop Super	76
10.3.3 Choke Rod	77
10.3.4 Throttle Rod	77
11. Chain Lubrication	79
11.1 Pickup Body	79
11.2 Oil Suction Hose	79
11.3 Oil Pump	79
11.4 Valve	81
12. Fuel System	82
12.1 Air Filter	82
12.2 Baffle	82
12.3 Filter Base	82
12.4 Air Guide Shroud	84
12.4.1 Air Guide Shroud - Models with Manual Fuel Pump	86
12.5 Carburetor	89
12.5.1 Leakage Test	90
12.6 Servicing the Carburetor	91
12.6.1 Metering Diaphragm	91
12.6.2 Inlet Needle	91
12.6.3 Pump Diaphragm	92
12.6.4 Lever on Throttle Shaft	93
12.6.5 Adjusting Screws	94
12.7 Adjusting the Carburetor	96
12.7.1 Basic Setting	96
12.7.2 Standard setting	97
12.8 Carburetor Carrier	98
12.9 Intake Manifold	99
12.10 Tank Vent	100
12.10.1 Testing	100
12.10.2 Removing and Installing	101
12.11 Fuel Intake	102
12.11.1 Pickup Body	102
12.11.2 Fuel Hose	102
12.11.3 Fuel Hoses - Manual Fuel Pump	106
12.11.4 Manual Fuel Pump	108
12.11.5 Tank Housing	109
13. Special Servicing Tools	111

Match case Limit results 1 per page

MS 231, MS 231 C, MS 251, MS 251 C

Installing

Before installing, clean the

crankshaft,

Heated ball bearings must be

installed quickly because the

crankshaft stubs absorb heat and

expand.

Position the ball bearing (1) with

its open side facing the

crankshaft.

–

Heat the inner bearing race to

approx. 150°C (300°F).

Push the ball bearing

(1) onto the

straight crankshaft stub (clutch

side) as far as stop.

Position the ball bearing (1) with

its open side facing the

crankshaft.

–

Heat the inner bearing race to

approx. 150°C (300°F).

Push the ball bearing

(1) onto the

tapered crankshaft stub (ignition

side) as far as stop.

2710RA117 TG

2710RA118 TG

Lubricate the needle cage (1)

with oil and push it into the

connecting rod.

–

Install the piston,

6.8

–

Install the crankshaft and new oil

seals,

6.5

–

Install the engine,

6.5

6.8

Piston

–

Remove the engine,

6.5

–

Remove the crankshaft,

6.6

The assembly drift passes through

the installed snap ring. Therefore,

only one snap ring needs to be

removed.

– Remove the snap at the side with

the recess.

Use a suitable tool to grip the

hookless snap ring at the

recess (arrow) and ease it out.

2710RA119 TG

2710RA418 TG

Slide the assembly drift (1)

1110 893 4700 through the

installed snap ring and push the

piston pin (2) out of the piston.

If the piston pin is stuck, release it

by tapping the end of the drift

lightly

with a hammer.

Hold the piston steady during this

process to ensure that no jolts are

transmitted to the connecting rod.

–

Remove the piston.

–

Inspect the piston rings and

replace if necessary,

6.9

Installing

–

Pull out the needle cage

, check it

and replace if necessary.

Lubricate the needle cage (1)

with oil and push it into the

connecting rod.

2310RA425 TG

2710RA123 TG