Stihl MS381 Service Manual

The Stihl MS381 is a professional-grade chainsaw featuring a powerful engine, advanced anti-vibration system, and robust construction for demanding forestry and cutting applications. This service manual provides detailed instructions for maintenance, troubleshooting, repair, and safety procedures. Below are key sections covering specifications, safety, maintenance schedules, component repair, fuel system, ignition system, and troubleshooting.

Features at a Glance

Key components: High-performance engine, STIHL Quickstop® inertia chain brake, Ematic™ lubrication system, anti-vibration system, side-access chain tensioner, and decompression valve.

Safety Information

Always wear appropriate PPE: helmet, eye protection, hearing protection, gloves, chainsaw chaps, and sturdy boots.

1. Before servicing: Ensure engine is off, spark plug wire disconnected, and chain brake engaged.
2. Fuel handling: Use only STIHL MotoMix® or mix 50:1 with high-quality 2-stroke oil and 89+ octane gasoline. Work in well-ventilated area.
3. Chain safety: Always engage chain brake when not cutting. Keep chain sharp and properly tensioned.
4. Hot surfaces: Allow engine to cool before touching muffler or cylinder.

WARNING! Failure to follow safety procedures can result in serious injury or death. Read operator's manual before use.

Maintenance Schedule

Regular maintenance ensures optimal performance and longevity.

1. Daily/Before Use: Check chain tension, sharpness, and lubrication. Inspect for loose fasteners. Check fuel and oil levels.
2. Every 10 Hours: Clean air filter. Check spark plug condition and gap (0.5 mm / .020"). Clean cooling fins and external surfaces.
3. Every 20 Hours: Clean spark arrestor screen (if equipped). Inspect fuel filter and replace if clogged.
4. Every 50 Hours: Clean carburetor strainer. Check anti-vibration elements. Inspect clutch and sprocket.
5. Every 100 Hours: Replace fuel filter. Check cylinder compression. Inspect ignition system components.
6. Annually/Seasonally: Replace spark plug. Clean fuel tank and lines. Check all safety devices function.

CAUTION! Use only genuine STIHL replacement parts. Improper maintenance voids warranty.

Specifications

• Model: MS 381
• Engine Power: 4.6 kW / 6.3 bhp at 9,500 rpm
• Max Engine Speed: 13,500 rpm
• Idle Speed: 2,800 rpm
• Torque: 5.2 Nm at 6,500 rpm
• Sound Pressure Level: 112 dB(A)
• Sound Power Level: 124 dB(A)
• Vibration Levels (Front/Rear Handle): 4.5/2.5 m/s²
• Spark Plug: NGK BPMR 7A (gap 0.5 mm)
• Carburetor: Zama C1Q-S 267
• Fuel/Oil Ratio: 50:1 (STIHL HP Ultra or equivalent)

Engine Components

Cylinder & Piston: Aluminum cylinder with Nikasil plating. Piston with two rings. Crankshaft: Roller bearings on both ends. Crankcase: Magnesium alloy. Muffler: With spark arrestor screen. Decompression Valve: Reduces pulling force during starting.

Disassembly Notes: Mark components for reassembly. Use proper pullers for bearings and seals. Clean all parts in solvent and inspect for wear.

Fuel System Service

Components: Fuel tank, cap, filter, lines, carburetor, and primer bulb.

1. Fuel Filter Replacement: Remove tank cap and filter. Insert new filter with fuel line attached.
2. Fuel Lines: Inspect for cracks. Replace if hardened or leaking.
3. Carburetor Removal: Disconnect throttle linkage and fuel lines. Remove mounting nuts.
4. Primer Bulb: Test for cracks; replace if not returning to shape.

Tip: Always purge air from fuel system after service by pressing primer bulb until firm.

Ignition System

CDI module, spark plug, stop switch, and flywheel.

1. Spark Test: Remove plug, ground to cylinder, pull starter. Look for strong blue spark.
2. Air Gap: Check flywheel to coil gap (0.3 mm / .012"). Adjust by loosening coil screws.
3. Resistance Check: Ignition coil primary: 0.18-0.27 Ω; secondary: 8-12 kΩ.
4. Stop Switch Test: Disconnect wire from coil. If spark returns, switch is faulty.

WARNING! Never test for spark near fuel or flammable materials.

Chain & Bar Maintenance

Chain: Sharpen with round file matching chain pitch. Maintain correct filing angle (30°). Depth gauges should be filed to 0.65 mm (.025") below cutter top plate.

Bar: Clean groove weekly. Flip bar regularly for even wear. Dress rails with flat file if burred. Lubricate sprocket nose.

Tension: Correct when chain snaps back after pulling from bar. Adjust with side tensioner when cold.

Carburetor Adjustment

Factory settings (screws turned out from lightly seated): L: 1 turn, H: 1 turn, LA: 1.5 turns.

1. Preliminary: Warm up engine for 5 minutes.
2. Idle (LA): Adjust so chain does not move at idle, but engine accelerates smoothly.
3. Low Speed (L): Turn for fastest idle, then rich 1/4 turn.
4. High Speed (H): With bar in wood, adjust for maximum power without 4-stroking at no-load. Final setting requires tachometer: Max 13,500 rpm.

CAUTION! Over-leaning causes engine damage. Use tachometer for final H setting.

Disassembly & Assembly

General sequence: Remove external components (handle, clutch, chain brake). Separate crankcase halves. Remove crankshaft, piston, bearings, seals.

Torque Specifications (Nm): Cylinder nuts: 10, Crankcase screws: 8, Clutch: 40 (left-hand thread), Flywheel: 35, Muffler: 8, Spark plug: 20.

Use sealant on crankcase mating surfaces (STIHL Dirko®). Press bearings with proper tools.

Troubleshooting

Reset: For electronic ignition modules, disconnect battery (if equipped) for 30 seconds to reset.

Service: For technical support or parts, contact STIHL authorized service center or visit www.stihlusa.com.