STIHL MS 171 Workshop Manual

The STIHL MS 171 is a lightweight, professional-grade chainsaw designed for occasional users and homeowners. This workshop manual provides detailed technical information, maintenance procedures, troubleshooting guides, and repair instructions for the MS 171 model. It covers engine specifications, fuel system, ignition system, clutch and sprocket, guide bar and chain, carburetor adjustments, and safety precautions for servicing. Below are key sections for specifications, maintenance schedules, disassembly, repair, reassembly, and troubleshooting.

Specifications & Technical Data

Key technical data for the STIHL MS 171 chainsaw engine and components.

Feature	Specification
Engine Type	STIHL single-cylinder two-stroke engine
Displacement	30.1 cm³ (1.84 cu. in.)
Power Output	1.5 kW (2.0 bhp)
Fuel Tank Capacity	250 ml (0.25 l)
Oil Tank Capacity	145 ml (0.145 l)
Guide Bar Length (Recommended)	35 cm (14") / 40 cm (16")
Chain Pitch	3/8" PMM3
Chain Gauge	1.3 mm (0.050")
Ignition System	Electronic, capacitor discharge ignition (CDI)
Weight (Powerhead)	Approx. 3.9 kg (8.6 lbs)

Safety Precautions

Essential safety measures to follow before and during any workshop procedure.

1. Disconnect spark plug boot: Always remove spark plug connector before working to prevent accidental starting.
2. Depressurize systems: Relieve fuel system pressure before opening fuel lines or tank.
3. Use proper tools: Utilize STIHL special tools or equivalents as specified.
4. Work in ventilated area: Avoid fumes; no smoking near fuel or battery.
5. Wear PPE: Safety glasses, gloves, and protective clothing are mandatory.

WARNING! Fuel is highly flammable. Keep away from sparks, open flames, and heat sources. Ensure chainsaw is cool before servicing.

Maintenance Schedule

Recommended maintenance intervals for optimal performance and longevity.

1. Daily/Before Use: Check chain tension, sharpness, and lubrication; inspect for loose fasteners.
2. After 5 Hours: Clean air filter; check spark plug condition.
3. After 20 Hours: Clean spark arrestor screen (if equipped); inspect fuel filter.
4. After 50 Hours: Replace air filter; inspect clutch drum and sprocket.
5. Annually: Clean cooling fins; inspect anti-vibration elements; replace fuel filter.

CAUTION! Use only STIHL-approved lubricants and replacement parts. Incorrect maintenance can void warranty.

Engine & Fuel System

• Cylinder & Piston: Inspect for scoring, carbon buildup, and ring wear.
• Crankcase: Check seals for leaks; inspect bearings for smooth rotation.
• Fuel Tank: Clean sediment; replace fuel filter and lines if cracked.
• Fuel Cap: Ensure vent is clear; replace if leaking.
• Muffler: Remove and clean carbon deposits; inspect for cracks.
• Decompression Valve: Test operation; replace if stuck or leaking.
• Rewind Starter: Inspect rope for fraying; lubricate spring and pulley.

Ignition System

Components: Spark plug, ignition module (coil), stop switch, wiring harness.

Spark Plug: Type BPMR 7A (NGK) or equivalent; gap 0.5 mm (0.020"). Check electrode wear and ceramic insulator for cracks.

Ignition Module: Test with ohmmeter per specifications. Air gap between flywheel and coil: 0.3 mm (0.012").

Stop Switch: Continuity test; should be open circuit when off, closed when on.

Flywheel: Check keyway and magnets; torque nut to 40 Nm (30 ft-lb).

Clutch, Sprocket & Drive

Clutch: Centrifugal type; engages at approx. 2,500 rpm. Sprocket: Spur or rim type (replaceable).

1. Remove clutch cover and bar/chain.
2. Hold piston at TDC using piston stop tool to lock crankshaft.
3. Remove clutch nut (left-hand thread) with appropriate socket.
4. Inspect clutch shoes for wear, springs for tension, and drum for scoring.
5. Check sprocket wear; replace if teeth are hooked or excessively worn.
6. Lubricate sprocket bearing with high-temperature grease.

Tip: Always replace clutch drum when replacing sprocket to ensure proper alignment and wear.

Guide Bar & Chain

Supports STIHL Rollomatic® guide bars and .325" pitch or 3/8" PMM3 chains.

1. Bar Maintenance: Clean bar groove and oil holes; file down burrs on rails; flip bar regularly for even wear.
2. Chain Sharpening: Use round file matching chain pitch (e.g., 4.0 mm for 3/8" PMM3). Maintain 30° cutting angle, 10° top plate angle.
3. Tension Adjustment: Loosen bar nuts, adjust tensioning screw until chain snaps back when pulled from bar.
4. Lubrication: Ensure oiler passage is clear; adjust oil flow via screw (if equipped).
5. Depth Gauges: File down to 0.65 mm (0.025") below cutter top plate.

WARNING! Always wear cut-resistant gloves when handling chain. A dull chain increases kickback risk.

Carburetor Adjustment

STIHL Zama C1Q-S carburetor with H (high), L (low), and LA (idle) adjustment screws.

Initial Settings (screws turned gently clockwise to stop, then back out): H: 1 turn, L: 1 turn, LA: 1 turn.

Fine-tuning: Run engine at operating temperature. Adjust L for smooth idle and acceleration. Adjust H for maximum RPM (do not exceed 13,500 rpm no-load). Adjust LA for stable idle (~2,800-3,200 rpm).

Disassembly Procedures

Systematic teardown for major repairs. Document part placement and orientation.

1. Remove external components: Top cover, air filter, carburetor, muffler, starter assembly.
2. Separate crankcase halves: Remove all screws; gently tap with plastic mallet.
3. Extract crankshaft with bearings and seals; support to avoid damage.
4. Remove cylinder: Unbolt and lift off; may require light prying if stuck.
5. Extract piston and connecting rod from crankshaft; note orientation.

CAUTION! Label all screws and parts. Do not force components; use appropriate pullers.

Reassembly & Torque Specifications

Critical torque values for safe and reliable operation.

Component	Torque Specification
Cylinder bolts	10 Nm (88 in-lb)
Muffler bolts	8 Nm (71 in-lb)
Clutch nut (left-hand thread)	40 Nm (30 ft-lb)
Flywheel nut	40 Nm (30 ft-lb)
Guide bar nuts	9-10 Nm (80-88 in-lb)
Crankcase screws	8 Nm (71 in-lb)
Spark plug	25 Nm (18 ft-lb)

Procedure: Apply sealant to crankcase mating surfaces. Install new gaskets and seals. Lubricate bearings and piston rings during assembly. Tighten bolts in a crisscross pattern.

Troubleshooting & Fault Diagnosis

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, fuel issue, flooded	Check spark, fuel flow, air filter; dry plug if flooded.
Engine starts then dies	Clogged fuel filter, carb adjustment	Replace fuel filter; clean carburetor; adjust L screw.
Loss of power under load	Clogged air filter, muffler screen, carb settings	Clean/replace air filter; clean spark arrestor; adjust H screw.
Excessive vibration	Loose components, unbalanced chain, AV mounts	Tighten all fasteners; balance chain; inspect anti-vibration elements.
Chain does not turn	Clutch worn, sprocket jammed, drive link damage	Inspect clutch shoes and springs; replace sprocket; check chain for binding.
Overheating	Lean fuel mixture, clogged cooling fins	Adjust carburetor richer; clean cylinder fins; check fuel quality.
Oil not lubricating chain	Clogged oiler, empty tank, faulty pump	Clean oil passages; fill tank; inspect/replace oil pump.

Reset: For electronic ignition issues, disconnect battery/spark plug for 10 minutes to reset module.

Service: For genuine parts and technical support, contact authorized STIHL servicing dealers or visit www.stihl.com.