Stihl 066 Magnum Service Manual

The Stihl 066 Magnum is a professional-grade chainsaw renowned for its high power output, durability, and performance in demanding forestry applications. This service manual provides comprehensive instructions 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 engine, Electronic Ignition (CDI), Decompression Valve, Anti-Vibration System, Quickstop® Chain Brake, Side-Access Chain Tensioner, and advanced filtration systems.

Safety Precautions

Always prioritize safety when servicing power equipment. Disconnect the spark plug boot before performing any maintenance.

1. Work in a clean, well-ventilated area free of flammable materials.
2. Use only genuine Stihl replacement parts and specified lubricants.
3. Secure the saw in a proper holding fixture before disassembly.
4. Wear appropriate personal protective equipment (PPE): safety glasses, gloves.
5. Handle fuel and oil with care; clean up spills immediately.

WARNING! Failure to follow safety procedures can result in serious injury or damage to the equipment. The chainsaw must not be operated until fully and correctly reassembled.

Specifications

• Model: Stihl 066 Magnum.
• Engine Type: Single cylinder, 2-stroke, air-cooled.
• Displacement: 91.6 cm³ (5.6 cu. in.).
• Bore x Stroke: 54 mm x 40 mm.
• Power Output: ~6.4 kW (8.6 HP).
• Idle Speed: 2,500 - 2,800 RPM.
• Maximum Power Speed: ~9,500 RPM.
• Fuel/Oil Mix Ratio: 50:1 with STIHL HP Ultra or equivalent ISO-L-EGD certified oil.
• Fuel Tank Capacity: 0.82 L (27.7 fl. oz).
• Oil Tank Capacity: 0.36 L (12.2 fl. oz).
• Ignition: Electronic, air gap 0.30 mm (0.012 in).
• Spark Plug: Bosch WSR 6 F or equivalent, gap 0.50 mm (0.020 in).

Maintenance Schedule

Access components as needed. Follow intervals based on operating hours or as part of annual servicing.

Daily/Before Use: Check chain tension, sharpness, and lubrication. Inspect for loose fasteners, leaks, and general condition.

Weekly/Every 10 Hours: Clean air filter, check spark plug condition, clean cooling fins and external surfaces.

Monthly/Every 50 Hours: Clean fuel filter, check fuel lines, inspect clutch drum and sprocket, clean spark arrestor screen (if equipped).

Annually/Every 100 Hours: Replace fuel filter, inspect and clean carburetor diaphragm/filter, check cylinder compression, inspect anti-vibration elements, check chain brake function.

CAUTION! Adherence to maintenance intervals is critical for performance, safety, and longevity.

Fuel & Lubrication System

Fuel System: Includes tank, lines, filter, and carburetor. Oil System: Includes oil tank, manual/auto pump, lines, and outlet.

1. Fuel Line Replacement: Remove tank housing, disconnect old lines, install new lines with proper routing to carburetor and primer (if equipped).
2. Fuel Filter: Replace by pulling from pickup line inside tank.
3. Oil Pump Adjustment/Service: Located under clutch drum. Remove clutch to access pump. Check gear engagement and adjust output via control screw (if manual).
4. Oil Line Replacement: Similar to fuel lines; ensure check valve (if present) is oriented correctly.

Tip: Always pressure test fuel and oil systems after reassembly to check for leaks.

Engine & Cylinder

Supports major engine overhaul including piston, cylinder, crankshaft, and bearings.

1. Cylinder Removal: Remove muffler, intake, decomp valve, and cylinder base nuts. Carefully lift cylinder.
2. Piston Inspection: Check for scoring, ring wear, and pin bearing play. Replace if out of specification.
3. Crankshaft & Bearings: Requires splitting the crankcase. Check bearings for smooth rotation and no play.
4. Crankcase Sealing: Use proper sealant (e.g., ThreeBond 1184) on case halves and ensure all seals are replaced during reassembly.
5. Compression Test: Should be a minimum of 120 psi (8.3 bar) for a healthy engine.

WARNING! Use a ring compressor when installing piston. Align ring end gaps with piston pin bosses. Torque cylinder nuts in a crisscross pattern to specified value.

Ignition System

Electronic CDI module, spark plug, stop switch, and wiring.

1. Testing Spark: Remove spark plug, reconnect boot, ground plug base to cylinder, pull starter. Look for strong blue spark.
2. Ignition Module Air Gap: Set between flywheel magnets and module legs to 0.30 mm (0.012 in) using a feeler gauge.
3. Stop Switch Test: Disconnect wire from module; if engine then has spark, switch is faulty.
4. Flywheel Key: Inspect for shearing; a sheared key will cause timing issues and poor starting.

CAUTION! Do not crank engine with spark plug removed and boot ungrounded, as it can damage the CDI module.

Clutch & Chain Brake

Centrifugal clutch and Quickstop® inertia-activated chain brake assembly.

1. Clutch Removal: Use clutch holding tool and appropriate socket to remove left-hand thread nut. Inspect shoes for wear and springs for integrity.
2. Chain Brake Band: Check for wear and proper tension. Replace if glazed or thin.
3. Hand Guard/Activation Lever: Ensure mechanism moves freely and returns to disengaged position.
4. Clutch Drum & Sprocket: Inspect for wear, especially where the chain rides. Replace sprocket if teeth are hooked or worn unevenly.

Tip: Lubricate the clutch bearing (behind the drum) with high-temperature grease during reassembly.

Bar, Chain & Sprocket

Proper maintenance ensures cutting performance and safety.

1. Bar Maintenance: Clean bar groove regularly. Flip bar periodically for even wear. Dress rails with a flat file if burred.
2. Chain Sharpening: Maintain correct filing angle (typically 30°) and depth gauge clearance. Use a consistent file size.
3. Sprocket Inspection: As noted in Clutch section. Use spur or rim sprocket as specified.
4. Chain Tension: Correct tension allows the chain to be pulled around the bar by hand with slight resistance. It should snap back when pulled from the underside of the bar.

WARNING! Always use matched chain pitch and gauge for the bar and sprocket. A mismatched chain is dangerous and will cause rapid wear.

Carburetor Adjustment

Standard carburetor (e.g., Walbro HD/HDA). Adjustments for idle (L), high-speed (H), and idle speed (LA/T).

1. Initial Settings: Turn H and L screws gently clockwise until seated, then back out H 1 turn, L 1 turn as a starting point.
2. Final Adjustment (Warm Engine): Set idle speed screw so chain just does not turn. Adjust L for fastest, smoothest idle, then re-adjust idle speed. Adjust H for maximum RPM without "four-stroking" excessively under load, not exceeding max allowed RPM.
3. Acceleration Test: Throttle should respond instantly without hesitation.

CAUTION! H screw adjustment affects engine speed and temperature. An overly lean H setting can cause severe engine damage. Use a tachometer for accuracy.

Troubleshooting

Reset: For electronic issues, disconnect spark plug boot for 1 minute to reset CDI module.

Service: For technical support and genuine parts, contact an authorized STIHL servicing dealer or visit www.stihlusa.com.