STIHL MS 170 Service Manual

The STIHL MS 170 is a lightweight, easy-to-handle chainsaw designed for occasional users and homeowners. This service manual provides detailed instructions for maintenance, troubleshooting, and repair of the MS 170 chainsaw, including engine specifications, fuel system, ignition system, cutting equipment, safety features, and routine service procedures. Below are key sections for safety, specifications, maintenance, troubleshooting, and repair.

Safety Precautions

Always prioritize safety when servicing the STIHL MS 170. Wear appropriate personal protective equipment (PPE).

Precaution	Description
Disconnect Spark Plug	Always disconnect the spark plug boot before performing any maintenance to prevent accidental starting.
Use Genuine Parts	Use only STIHL-approved replacement parts to ensure safety and performance.
Work in Well-Ventilated Area	Service in an area with good airflow to avoid inhalation of fuel vapors or exhaust fumes.
Secure the Chainsaw	Clamp the saw securely in a vise when working on the cutting equipment or engine.
Handle Fuel Safely	Drain fuel tank before certain repairs; no smoking or open flames nearby.
Chain Brake Check	Test the chain brake functionality regularly as part of maintenance.
Proper Tool Use	Use correct tools as specified in this manual to avoid damage.

Specifications

• Engine Type: Single-cylinder, two-stroke, air-cooled.
• Displacement: 30.1 cc.
• Power Output: 1.5 kW (2.0 hp).
• Fuel/Oil Mix: 50:1 (STIHL HP Ultra or equivalent).
• Fuel Tank Capacity: 0.25 liters.
• Oil Tank Capacity: 0.145 liters.
• Guide Bar Lengths: 12", 14", 16" (30 cm, 35 cm, 40 cm).
• Chain Pitch: 3/8" PMM3.
• Ignition System: Electronic, capacitor discharge (CDI).
• Weight (powerhead): 3.9 kg (8.6 lbs).
• Sound Pressure Level: Approx. 105 dB(A).

Maintenance Schedule

Regular maintenance ensures optimal performance and longevity.

1. Before Each Use: Check chain tension, sharpness, and lubrication. Inspect for loose fasteners.
2. After Each Use: Clean air intake, cooling fins, and guide bar. Wipe down exterior.
3. Every 10 Hours: Clean or replace air filter. Check spark plug.
4. Every 20 Hours: Clean spark arrestor screen (if equipped). Check fuel filter.
5. Every 50 Hours: Replace fuel filter. Inspect clutch and sprocket.
6. Annually or Every 100 Hours: Replace spark plug. Clean carburetor (if needed). Check all rubber mounts and AV elements.

WARNING! Failure to maintain the chainsaw can lead to premature wear, poor performance, or safety hazards.

Engine System

Cylinder, piston, crankshaft, and cooling system components.

Compression Check: Use compression gauge. Minimum acceptable compression: 110 psi (7.6 bar). Low compression indicates worn piston/rings or cylinder damage.

Cooling System: Keep cooling fins and air intake clean. Blocked fins cause overheating.

Muffler/Spark Arrestor: Remove and clean carbon deposits regularly to maintain engine performance and reduce fire risk.

Fuel System

Fuel tank, lines, filter, and carburetor (typically a Zama C1Q-S series).

1. Fuel Lines: Inspect for cracks or brittleness. Replace if damaged.
2. Fuel Filter: Located in tank. Replace by pulling out with wire hook.
3. Carburetor Adjustment: Idle speed (LA) and high speed (H) screws. Factory settings are pre-set; fine-tuning may be required for altitude/temperature.
4. Diaphragms: Check pump and metering diaphragms for stiffness or tears.

Tip: Use fresh, properly mixed fuel to prevent carburetor issues.

Ignition System

Spark plug, ignition module, and stop switch.

Spark Plug: NGK BPMR7A or equivalent. Gap: 0.5 mm (0.020"). Check electrode condition and replace if fouled or worn.

Ignition Module (CDI): Test with multimeter for proper resistance if engine has no spark. Refer to resistance values in wiring diagram.

Stop Switch: Ensure wire is intact and switch grounds the ignition when pressed.

Cutting Equipment

Guide bar, chain, sprocket, and clutch.

1. Chain Tension: Adjust so chain snaps back when pulled from guide bar. Should not sag from bar.
2. Chain Sharpening: Use correct round file size (typically 4.0 mm). Maintain correct filing angle (30°).
3. Guide Bar Maintenance: Clean bar groove. Flip bar regularly for even wear. Dress rails with flat file if burred.
4. Sprocket Inspection: Replace rim sprocket or drum if teeth are hooked or worn.
5. Clutch Inspection: Shoes should move freely and not be excessively worn.
6. Oil Pump: Check output by running saw at idle over light-colored surface; should see oil line.

WARNING! Always wear cut-resistant gloves when handling the chain.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, flooded, fuel issue	Check spark plug, ignition. Wait if flooded. Check fuel lines/filter/carb.
Engine starts then dies	Clogged fuel filter, carb adjustment, air leak	Replace fuel filter. Adjust carburetor. Check crankcase seals.
Lacks power under load	Dirty air filter, clogged muffler, carb adjustment	Clean/replace air filter. Clean spark arrestor. Adjust H screw.
Chain does not turn	Clutch worn, sprocket damaged, brake engaged	Inspect clutch/sprocket. Disengage chain brake.
Excessive vibration	Loose components, unbalanced chain, AV mounts damaged	Tighten fasteners. Balance chain. Inspect anti-vibration elements.
Chain does not oil	Oil hole clogged, pump defective, tank empty	Clean oil passages. Check oil pump drive. Fill oil tank.

Reset: For electronic ignition issues, ensure all connections are secure.

Disassembly & Assembly

General sequence for major service.

1. Remove top cover and air filter assembly.
2. Remove starter assembly.
3. Remove clutch (using clutch tool) and sprocket.
4. Remove muffler and cylinder cover.
5. Separate crankcase halves (requires special tool for bearing separation).
6. Replace piston, rings, or crankshaft seals as needed.
7. Reassemble in reverse order, applying sealant to crankcase mating surfaces.

CAUTION! Mark components and organize parts during disassembly. Use a service manual with exploded views.

Torque Specifications

Critical fasteners and their torque values (Nm / in-lbs).

Component	Torque (Nm)	Torque (in-lbs)
Cylinder Cover Nuts	9-10	80-89
Spark Plug	20-25	177-221
Clutch Nut	40-45 (Left-hand thread)	354-398
Guide Bar Nuts	9-10	80-89
Muffler Bolts	8-10	71-89
Crankcase Screws	10-12	89-106

Warranty & Support

STIHL Limited Warranty: Refer to your original purchase documentation for terms and duration. For technical support, genuine parts, and authorized service, contact:

STIHL USA: 1-800-467-8445 | www.stihlusa.com

STIHL Canada: 1-800-565-3444 | www.stihl.ca

Always provide the model number (MS 170) and serial number when seeking assistance.