Stihl 066 Service Manual

The Stihl 066 is a professional-grade chainsaw known for its power, durability, and performance in demanding forestry and logging applications. This service manual provides comprehensive information for maintenance, troubleshooting, repair, and overhaul procedures. It covers the engine, fuel system, ignition, clutch, chain brake, bar and chain, and all other major components. Below are key sections for safety, specifications, maintenance intervals, disassembly, assembly, and troubleshooting.

Features at a Glance

Key components: High-performance two-stroke engine, Anti-Vibration System (AV), Decompression valve, Electronic ignition (CDI), Adjustable automatic oiler, Chain brake, Quickstop inertia system.

Safety Information

Always prioritize safety when servicing the Stihl 066. Disconnect the spark plug boot before any work.

1. Personal Protective Equipment (PPE): Wear safety glasses, gloves, and hearing protection.
2. Work Area: Work in a clean, well-ventilated area away from sparks or open flame.
3. Fuel Safety: Drain fuel tank before service. No smoking.
4. Sharp Tools: Use proper tools; keep hands clear of chain and sharp edges.
5. Pressurized Systems: Relieve fuel pressure before opening carburetor or fuel lines.

WARNING! Failure to follow safety procedures can result in serious injury or death. This manual is for qualified technicians.

Specifications

• Model: Stihl 066 (MS 660 predecessor).
• Engine Type: Single cylinder, two-stroke, air-cooled.
• Bore x Stroke: 54 mm x 40 mm.
• Compression Ratio: Approx. 8.5:1.
• Ignition: Electronic CDI (Bosch or SEM).
• Carburetor: Walbro HD or Zama C3M.
• Fuel/Oil Mix: 50:1 with Stihl HP Ultra or equivalent ISO-L-EGD certified oil.
• Fuel Tank Capacity: 825 ml (0.87 US qt).
• Oil Tank Capacity: 360 ml (0.38 US qt).
• Recommended Bar Length: 20" to 36".
• Spark Plug: Bosch WSR 6 F or NGK BPMR 7A (gap 0.5 mm / 0.020").

Maintenance Schedule

Regular maintenance is critical for performance and longevity.

Engine Service

Disassembly and assembly procedures for the engine block.

1. Cylinder & Piston: Remove muffler, decomp valve, cylinder bolts. Inspect cylinder scoring, piston ring end gap, piston pin bearing.
2. Crankshaft & Bearings: Requires splitting the crankcase. Check crankshaft runout, main bearing play, and seal surfaces.
3. Seals: Replace crankshaft seals during major service. Use proper seal driver.
4. Compression Test: Should be above 120 psi (8.3 bar) for a healthy engine.
5. Reassembly: Use new gaskets. Apply appropriate sealant to crankcase halves. Torque fasteners to specification.

CAUTION! Label all parts and fasteners. Take photos during disassembly for reference.

Fuel System

Carburetor adjustment and fuel line replacement.

1. Carburetor (Walbro HD): Adjust H (high speed), L (low speed), and LA (idle) screws. Initial settings: H - 1 turn open, L - 1 turn open, LA - 1 turn open. Fine-tune for smooth acceleration and top speed.
2. Fuel Lines: Replace if cracked or hardened. Use Stihl fuel line kit. Ensure proper routing to avoid kinks.
3. Fuel Filter: Located in tank. Replace by pulling through filler neck with wire hook.
4. Impulse Line: Connects crankcase to carburetor pump diaphragm. Check for cracks and secure fit.

Tip: Use a pressure/vacuum tester to check carburetor diaphragms and inlet needle.

Ignition System

Testing and replacement of ignition components.

1. Spark Plug: Check for fouling, correct gap. Replace if electrodes are worn.
2. Ignition Module (CDI): Test with multimeter for primary and secondary coil resistance. Check air gap between flywheel and coil (0.3 mm / 0.012").
3. Flywheel: Check for sheared key if timing is off. Remove with proper puller.
4. Stop Switch: Test for continuity. Wire should break connection when switch is pressed.

WARNING! Never crank engine with spark plug removed and boot unattended – risk of CDI damage from open circuit.

Clutch & Chain Brake

Service of the centrifugal clutch and inertia brake system.

1. Clutch Removal: Hold piston at Top Dead Center (TDC) with rope, remove clutch nut (left-hand thread).
2. Clutch Shoes: Inspect for wear, cracking, or glazing. Replace as a set.
3. Clutch Drum & Sprocket: Check for groove wear from chain. Replace if grooves are deep.
4. Chain Brake Band: Inspect for wear and heat discoloration. Replace if thickness is below spec.
5. Brake Mechanism: Clean and lubricate pivot points. Test engagement by striking front hand guard.

Bar, Chain & Sprocket

Maintenance of cutting components.

1. Bar: Clean bar groove and oil holes. Dress rails with flat file if burred. Check for straightness and wear.
2. Chain: Sharpen with correct file size (typically 7/32" for .404" pitch, 5/32" for 3/8" pitch). Maintain correct depth gauge setting.
3. Sprocket (Rim or Spur): Replace rim sprocket when worn. Inspect spur sprocket for hooking.
4. Chain Tension: Adjust so chain snaps back when pulled from bar rail. Never tighten on a hot chain.
5. Oiling: Verify oil flow by running saw at high speed pointed at a light surface; look for oil spray.

Troubleshooting

Torque Specifications

Critical fastener torque values (use calibrated torque wrench).

Note: Apply thread locker where specified. Replace self-locking nuts.

Service Support: For genuine Stihl parts and technical bulletins, contact an authorized Stihl servicing dealer.