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Stihl MS 251 Instruction Manual - Page 7

Tightening Torques - what size bar

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2.5 Tightening Torques DG and P (Plastoform) screws are used in polymer and light metal components. These screws form a permanent thread when they are installed for the first time. They can be removed and installed as often as necessary without impairing the strength of the screwed assembly, providing the specified tightening torque is observed. For this reason it is essential to use a torque wrench. Fastener Thread size For component Torque Nm Remarks Screw Screw Screw Screw Screw Collar screw Collar screw Collar screw Screw Nut Screw Screw Screw Screw Screw Screw Screw Screw Screw Carrier Screw Screw Screw Nut Screw P 4x12 Chain tensioner cover 1.5 P 5x18 Antivibration element / engine housing 4.0 P 5x34 Antivibration element / tank housing 4.0 P 4x12 Brake band/engine housing 2.0 P 4x10 Brake cable retainer / tank housing 1.0 D 8x18 Collar stud for bar 16.0 D 8x18 Collar stud for bar 16.0 D 9x18 Collar stud for bar / engine housing 16.0 (repair solution) P 5x18 Cover, chain brake / engine housing 4.0 M 5 Filter base / baffle / carburetor 3.5 P 6x26.5 Handlebar / tank housing, right 6.0 M 5x16 Handlebar / tank housing, bottom 5.0 M 5x20 Hand guard / fan housing / engine housing 6.0 M 5x14 Shroud / engine housing 5.0 P 5x16 Spiked bumper / engine housing 4.0 D 4x12 Manifold / cylinder 4.0 D 5x18 Bearing plug / cylinder 8.0 P 5x18 Fan housing / engine housing 4.0 P 4x12 Air baffle / engine housing 2.0 M 12x1 L Carrier / crankshaft 50.0 P 6x28 Engine housing / bearing plug 6.0 D 4x16 Oil pump 4.0 M 5x16 Muffler / cylinder 10.0 M 8x1 Flywheel / crankshaft 28.0 P 4x10 Retainer 1.0 Q B 2), 3) 2) 1), 3) 1), 3) 1), 3) 2), 3) 4) 6 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

2.5

Tightening Torques

DG and P (Plastoform) screws are used in polymer and light metal components. These screws form a

permanent thread when they are installed for the first time. They can be removed and installed as often as

necessary without impairing the strength of the screwed assembly, providing the specified tightening torque is

observed.

For this reason it is

essential to use a torque wrench.

Fastener

Thread size

For component

Torque

Remarks

Screw

P 4x12

Chain tensioner cover

1.5

Screw

P 5x18

Antivibration element / engine housing

4.0

Screw

P 5x34

Antivibration element / tank housing

4.0

Screw

P 4x12

Brake band/engine housing

2.0

Screw

P 4x10

Brake cable retainer / tank housing

1.0

Collar screw

D 8x18

Collar stud for bar

16.0

Collar screw

D 8x18

Collar stud for bar

16.0

Collar screw

D 9x18

Collar stud for bar / engine housing

(repair solution)

16.0

Screw

P 5x18

Cover, chain brake / engine housing

4.0

Nut

M 5

Filter base / baffle / carburetor

3.5

Screw

P 6x26.5

Handlebar / tank housing, right

6.0

Screw

M 5x16

Handlebar / tank housing, bottom

5.0

2), 3)

Screw

M 5x20

Hand guard / fan housing / engine housing

6.0

Screw

M 5x14

Shroud / engine housing

5.0

Screw

P 5x16

Spiked bumper / engine housing

4.0

Screw

D 4x12

Manifold / cylinder

4.0

1), 3)

Screw

D 5x18

Bearing plug / cylinder

8.0

1), 3)

Screw

P 5x18

Fan housing / engine housing

4.0

Screw

P 4x12

Air baffle / engine housing

2.0

Carrier

M 12x1 L

Carrier / crankshaft

50.0

Screw

P 6x28

Engine housing / bearing plug

6.0

Screw

D 4x16

Oil pump

4.0

1), 3)

Screw

M 5x16

Muffler / cylinder

10.0

2), 3)

Nut

M 8x1

Flywheel / crankshaft

28.0

Screw

P 4x10

Retainer

1.0