STIHL MS 660 Workshop Manual

The STIHL MS 660 is a professional-grade chainsaw featuring a high-performance engine, robust construction, and advanced safety systems. This workshop manual provides detailed instructions for maintenance, repair, and troubleshooting. Below are key sections covering safety, specifications, engine systems, fuel system, ignition system, cutting equipment, troubleshooting, and assembly/disassembly procedures.

1. Safety Precautions 2. Technical Specifications 3. Engine Overview 4. Fuel System 5. Ignition System 6. Chain & Bar Assembly 7. Cooling & Exhaust System 8. Maintenance Schedule 9. Disassembly & Reassembly 10. Troubleshooting Guide 11. Parts List & Diagrams

Safety Precautions

Always wear appropriate PPE: safety glasses, hearing protection, gloves, and chainsaw chaps. Ensure the chainsaw is on a stable surface before servicing. Disconnect the spark plug boot to prevent accidental starting. Use only STIHL-approved parts and tools.

Hazard	Precaution
Fuel Handling	Work in well-ventilated area; no smoking or open flames.
Sharp Components	Wear cut-resistant gloves when handling chain or bar.
Hot Surfaces	Allow engine and muffler to cool before touching.
Electrical	Disconnect spark plug before any internal work.
Chemical Exposure	Use gloves when handling fuel, oil, or solvents.

Technical Specifications

Engine Type: 2-stroke, single cylinder, air-cooled.
Displacement: 91.6 cm³ (5.59 cu. in.).
Power Output: 6.4 kW (8.7 HP) at 9,500 rpm.
Fuel/Oil Ratio: 50:1 (STIHL HP Ultra or equivalent).
Fuel Tank Capacity: 825 ml (0.22 US gal).
Oil Tank Capacity: 360 ml (0.10 US gal).
Guide Bar Lengths: 50 cm (20") to 105 cm (41") recommended.
Chain Pitch/Gauge: .404" / .063" (standard).
Weight (powerhead): 7.4 kg (16.3 lbs).
Ignition System: Electronic, capacitor discharge.

Engine Overview

Cylinder and piston assembly, crankshaft, connecting rod, bearings, and seals. Torque specifications for critical fasteners.

Cylinder: Remove cylinder head; inspect for scoring or carbon buildup.
Piston: Check ring end gap and piston skirt wear.
Crankshaft: Inspect bearings and seals for wear or leakage.
Decompression Valve: Test operation; replace if leaking.

WARNING! Use a torque wrench for reassembly. Incorrect torque can cause engine failure.

Fuel System

Components: Fuel tank, filter, lines, carburetor (HD-12 or equivalent), and fuel pump.

Drain fuel before servicing.
Replace fuel filter every 3 months or as needed.
Clean carburetor: Remove diaphragm, jets, and gaskets; use compressed air.
Adjust carburetor: Set idle (L) and high-speed (H) screws to factory specs.

CAUTION! Use only ethanol-free fuel to prevent carburetor damage.

Ignition System

Spark plug (Bosch WSR6F or equivalent), ignition module, flywheel, and stop switch.

Testing: Check spark plug gap (0.5 mm / 0.020"). Use an ignition tester to verify spark strength. Inspect flywheel key for shearing. Ensure stop switch circuit is functional.

Replacement: Align flywheel timing marks; torque nut to 40 Nm (30 ft-lbs).

Chain & Bar Assembly

Guide bar, saw chain, sprocket, clutch, and chain tensioner.

Bar Maintenance: Clean bar groove; flip bar regularly for even wear.
Chain Sharpening: Maintain correct filing angle (30°) and depth gauge height.
Sprocket: Replace if worn (diameter increase > 0.5 mm).
Clutch: Inspect shoes for wear; replace if cracked or glazed.

Tip: Always tension chain with bar nuts loose, then tighten.

Cooling & Exhaust System

Air filter, cooling fins, muffler, and spark arrestor.

Air Filter: Clean daily in dusty conditions; replace if damaged.
Cooling Fins: Remove debris from cylinder fins for proper cooling.
Muffler: Clean carbon deposits; replace gasket during reassembly.
Spark Arrestor: Clean screen every 10 hours of operation.

WARNING! A clogged spark arrestor can cause overheating.

Maintenance Schedule

Interval	Task
Daily	Check chain tension, sharpness; clean air filter.
Weekly	Inspect spark plug; clean cooling fins.
Monthly	Replace fuel filter; clean carburetor; check clutch.
Annually	Replace spark plug; inspect piston/cylinder; check all seals.

Disassembly & Reassembly

Step-by-step guide for major components.

Remove top cover, air filter, and carburetor.
Remove cylinder cover and muffler.
Remove clutch and flywheel.
Separate crankcase halves (requires case splitter tool).
Reassembly in reverse order using new gaskets and seals.

CAUTION! Mark components for correct reassembly. Use sealant on crankcase mating surfaces.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, fuel issue, flooded	Check spark plug, fuel lines, carburetor; clear flood.
Engine runs poorly	Dirty air filter, clogged fuel filter, carb adjustment	Clean/replace filters; adjust carburetor.
Loss of power	Clogged muffler, worn piston/cylinder	Clean muffler; inspect compression (should be > 120 psi).
Chain doesn't turn	Worn clutch, broken drive spring	Inspect clutch assembly; replace worn parts.
Excessive vibration	Unbalanced flywheel, loose components	Check flywheel key; tighten all fasteners.

Compression Test: Minimum 120 psi for proper operation.

Parts List & Diagrams

Refer to STIHL parts catalog 5910 890 3601 for exploded views and part numbers. Common replacement parts: Piston kit (1128 030 2005), Cylinder (1128 020 1201), Carburetor kit (1128 007 1060), Ignition module (1128 400 1401).

Ordering: Use genuine STIHL parts for warranty and performance.

Support: Contact STIHL at 1-800-467-8445 or visit www.stihlusa.com.