STIHL 021 Chainsaw Service Manual

The STIHL 021 is a lightweight, professional-grade gasoline chainsaw designed for general cutting, pruning, and felling tasks. This manual provides essential information for safe operation, routine maintenance, troubleshooting, and repair procedures. It covers the engine, fuel system, ignition, chain drive, lubrication, and safety features specific to the 021 model. Always refer to this manual and wear appropriate personal protective equipment (PPE) before operating or servicing the saw.

Safety Information & Warnings

Operating a chainsaw is inherently dangerous. Failure to follow safety instructions can result in serious injury or death.

WARNING! Never operate a chainsaw when fatigued, under the influence, or in poor weather conditions. Always plan an escape path from falling trees.

Specifications & Identification

• Model: STIHL 021
• Engine Type: Single-cylinder, two-stroke, air-cooled gasoline engine.
• Displacement: 31.8 cc (1.94 cu. in.)
• Power Output: Approx. 1.6 kW (2.1 hp).
• Fuel Tank Capacity: 0.47 L (0.12 US gal).
• Oil Tank Capacity: 0.26 L (0.07 US gal).
• Guide Bar Lengths: Typically 12" to 16" (30 cm to 40 cm).
• Chain Pitch: 3/8" PMM or .325".
• Ignition: Electronic, breakerless.
• Weight (powerhead): Approx. 4.2 kg (9.3 lbs).
• Serial Number Location: Stamped on crankcase or rear handle.

Component Overview & Controls

Key components and operator controls for the STIHL 021.

Fuel & Lubricants

Using the correct fuel and oil is critical for engine performance and longevity.

1. Fuel Mix: Use unleaded gasoline with a minimum octane rating of 89 (RON). Mix with high-quality two-stroke engine oil at a 50:1 ratio (2% oil). Use STIHL HP Ultra or equivalent ISO-L-EGD certified oil.
2. Mixing: Add oil to an approved fuel can first, then add gasoline. Shake thoroughly before refueling.
3. Chain & Bar Oil: Use STIHL chain oil or a high-tack, non-foaming bar oil. Do not use waste oil.
4. Storage: For long-term storage (over 30 days), drain fuel system or add fuel stabilizer.

CAUTION! Never use automotive motor oil, diesel, or E85 ethanol blends. Improper fuel will cause rapid engine failure.

Starting & Stopping Procedures

Follow these steps to start the engine safely.

1. Pre-Start Check: Ensure chain brake is engaged (pulled back toward front handle). Check chain tension and sharpness. Verify area is clear.
2. Cold Start: Set master control lever to choke position (full down). Press decompression valve (if equipped). Pull starter rope sharply until engine fires (usually 1-3 pulls). Move lever to half-throttle position (middle). Pull again to start. Immediately squeeze throttle trigger to disengage fast idle.
3. Warm Start: Set master control lever to run position (middle). Pull starter rope.
4. Stopping: Release throttle and allow engine to idle. Push master control lever up to STOP position.

WARNING! Always start the saw on the ground, firmly holding the rear handle with your foot in the handle. Never "drop start."

Routine Maintenance

Regular maintenance ensures safe operation and extends saw life.

Note: Maintenance intervals vary with operating conditions. Increase frequency in dusty environments.

Chain & Guide Bar Care

Proper chain and bar maintenance is essential for cutting performance and safety.

1. Tensioning: Loosen bar nuts. Use adjustment screw to raise bar nose until chain fits snugly against bar but can be pulled around by hand. Tighten bar nuts. Re-check tension after running saw for 30 seconds.
2. Sharpening: Use a round file matching chain pitch (typically 5/32" for 3/8" PMM). Maintain original filing angle (usually 30°). File all cutters to equal length. File depth gauges (rakers) with a flat file and gauge.
3. Bar Maintenance: Regularly clean bar groove and oil holes. Flip bar weekly to ensure even wear. Dress bar rails with a flat file if burred.
4. Lubrication: Ensure oiler port is clear and oil tank is filled. Adjust oil flow via screw (if adjustable) for adequate chain lubrication.

CAUTION! A dull or loose chain increases kickback risk and operator fatigue. Always stop engine before adjusting chain.

Carburetor Adjustment

The STIHL 021 uses a Zama or Walbro carburetor with three adjustment screws: L (Low speed), H (High speed), and LA (Idle).

1. Preliminary: Ensure saw is at operating temperature. Clean air filter. Use a tachometer for precise adjustment.
2. Initial Settings: Turn L and H screws gently clockwise until seated, then back out 1 turn (L) and 1 turn (H) as a baseline.
3. Idle Speed (LA): Adjust so chain does not move at idle but engine runs smoothly.
4. Low Speed (L): Adjust for smooth acceleration from idle without hesitation.
5. High Speed (H): Do not exceed maximum no-load speed of 13,500 RPM. A lean H setting can cause engine seizure.
6. Final Check: Let saw idle for 30 seconds, then accelerate to full throttle. Chain should not move at idle.

WARNING! Improper carburetor adjustment can cause severe engine damage. If unsure, consult an authorized STIHL dealer.

Ignition System

The electronic ignition module requires no timing adjustment. Primary service involves spark plug and wire inspection.

• Spark Plug: Gap: 0.020" (0.5 mm). Replace if electrodes are worn, cracked, or fouled.
• Ignition Wire: Check for cracks or damage to boot. Ensure connection is tight.
• Stop Switch: Test continuity; wire should break circuit when switch is pressed.
• Ignition Coil Air Gap: Set between 0.10 - 0.30 mm (0.004 - 0.012 in.) using a feeler gauge between flywheel magnet and coil leg.

Note: If no spark, check all wiring connections, kill switch, and flywheel key before replacing coil.

Engine & Cylinder

Major engine service procedures (compression test, cylinder/piston inspection).

1. Compression Test: Requires at least 110 psi (7.6 bar) for proper starting. Low compression indicates worn piston rings, cylinder scoring, or leaking crank seals.
2. Cylinder Inspection: Remove muffler to inspect piston and cylinder through exhaust port. Look for scoring, aluminum transfer (seizure), or excessive carbon.
3. Crankcase Seals: Leaking seals cause air leaks, leading to lean run condition and engine damage. Test with pressure/vacuum tester.
4. Fuel System: Inspect fuel lines, filter, and carburetor diaphragm for cracks or stiffness.

CAUTION! Engine disassembly requires special tools and expertise. Consider dealer service for major repairs.

Troubleshooting & Repair

Service Support: For technical assistance, genuine parts, and authorized service, contact STIHL USA at 1-800-467-8445 or visit www.stihlusa.com.