Stihl 021 Service Manual

The Stihl 021 is a lightweight, professional-grade gasoline chainsaw designed for occasional to frequent use in cutting, limbing, and felling small to medium trees. This manual covers essential safety procedures, technical specifications, routine maintenance, troubleshooting, and repair instructions for the Stihl 021 chainsaw. Always refer to this guide before performing any service or maintenance to ensure safe and proper operation.

Safety Information & Warnings

Before operating or servicing the Stihl 021, read and understand all safety instructions. Failure to follow warnings can result in serious injury or death.

Hazard	Precaution
Kickback	Always use a reduced-kickback chain, maintain proper chain tension, and hold saw firmly with both hands.
Fuel Handling	Use only clean, fresh fuel mix (gasoline and 2-stroke oil). Refuel in well-ventilated area, engine off and cool.
Hot Surfaces	Allow engine to cool before servicing muffler, cylinder, or spark plug.
Chain Contact	Ensure chain brake is functional. Wear cut-resistant gloves and chaps during operation.
Electrical Shock	Disconnect spark plug wire before performing any maintenance to prevent accidental starting.

Specifications & Identification

Key technical data for the Stihl 021 chainsaw.

• Engine Type: Single cylinder, 2-stroke, air-cooled.
• Displacement: 31.8 cc (1.94 cu. in.).
• Power Output: 1.8 kW (2.4 hp).
• Fuel/Oil Ratio: 50:1 (Stihl HP Ultra or equivalent).
• Idle Speed: 2,800 RPM.
• Max Engine Speed: 13,500 RPM (no load).
• Ignition System: Electronic, breakerless.
• Spark Plug: Bosch WSR6F or equivalent, gap 0.5 mm (0.020 in.).
• Guide Bar Length: Typically 12" to 16".
• Chain Pitch/Gauge: 3/8" PMM, 0.050" (1.3 mm).
• Weight (powerhead): Approximately 4.4 kg (9.7 lbs).

Model Identification: Serial number located on crankcase. Use for ordering correct parts.

Components & Controls

Familiarize yourself with the main parts and controls of the Stihl 021.

1. Front Handle: Equipped with throttle trigger lockout and hand guard.
2. Rear Handle: Contains throttle trigger, master control lever (stop switch, choke, fast idle).
3. Chain Brake: Manual activation via front hand guard; inertia-activated.
4. Carburetor Adjustment Screws: LA (idle speed), L (low speed), H (high speed).
5. Fuel Tank: Capacity approx. 0.5 L (16.9 fl. oz.).
6. Oil Tank: For automatic chain lubrication, capacity approx. 0.26 L (8.8 fl. oz.).
7. Muffler: Equipped with spark arrestor screen (requires periodic cleaning).
8. Decompression Valve: Reduces pulling force for easier starting.

Fuel & Lubrication System

Proper fuel mix and lubrication are critical for engine life.

1. Fuel Preparation: Mix 89 octane (or higher) unleaded gasoline with a high-quality 2-stroke engine oil at a 50:1 ratio. Shake well.
2. Fuel Filter: Replace annually or if clogged. Access via fuel line in tank.
3. Fuel Line: Inspect for cracks or leaks. Replace if damaged.
4. Chain Oil System: Check oil delivery by running saw at high speed over light-colored surface; look for oil sling.
5. Oil Pump: Adjustable via screw on bottom of saw (if equipped). Turn clockwise to increase flow.
6. Oil Filter: Located in oil tank; clean or replace if clogged.

WARNING! Never use straight gasoline or automotive motor oil. This will cause severe engine damage.

Ignition System

Testing and servicing the ignition components.

Component	Test/Service Procedure
Spark Plug	Check gap (0.5 mm). Clean electrodes or replace if worn, fouled, or cracked.
Ignition Module	Check for spark with tester. Air gap between module and flywheel: 0.3 mm (0.012 in.).
Stop Switch	Test continuity. Wire should break circuit when switch is pressed.
Flywheel	Inspect for damage or sheared key. Torque nut to 40 Nm (30 ft-lb).

Note: Use a Stihl diagnostic tool or multimeter for accurate module testing.

Engine & Cylinder

Inspection and service of major engine components.

1. Compression Test: Should be a minimum of 120 psi. Low compression indicates worn piston/rings or cylinder damage.
2. Cylinder Inspection: Remove muffler to view piston and cylinder through exhaust port. Look for scoring or aluminum transfer.
3. Piston & Rings: Replace if rings are stuck, broken, or piston is scored.
4. Crankcase Seals: Check for leaks with pressure/vacuum test. Leaking seals cause lean run condition.
5. Crankshaft Bearings: Check for play or roughness. Replace if defective.

CAUTION! Engine disassembly requires special tools and knowledge. Consult a Stihl dealer if unsure.

Chain & Bar Maintenance

Proper care of the cutting system ensures safety and performance.

1. Chain Sharpening: Use correct round file size (typically 4.0 mm). Maintain proper filing angle (30°). File all cutters evenly.
2. Chain Tension: Adjust so chain snaps back when pulled from bar. Tension when chain is warm.
3. Bar Maintenance: Clean bar groove and oil holes. Flip bar regularly for even wear. Dress rails with flat file if burred.
4. Sprocket: Inspect drive sprocket (rim or spur type) for wear. Replace if teeth are hooked or worn.
5. Clutch: Inspect clutch drum and shoes for wear or overheating. Clean centrifugal clutch of debris.

Carburetor Adjustment

Standard adjustment procedure for the Walbro or Zama carburetor.

1. Initial Settings: Turn H and L screws gently clockwise until seated, then back out 1 turn each. Set LA screw to achieve a steady idle.
2. Final Adjustment (Warm Engine):
   • Adjust L screw for fastest, smoothest idle.
   • Adjust H screw for maximum RPM (do not exceed 13,500 RPM).
   • Re-adjust LA screw for correct idle speed (2,800 RPM) and ensure chain does not move.
3. Acceleration Test: Saw should accelerate smoothly without hesitation. If it bogs, open L screw slightly.

WARNING! Excessive H screw opening causes lean condition, leading to engine seizure.

Troubleshooting Guide

Problem	Possible Cause	Solution
Engine will not start	No spark, flooded, fuel issue, clogged air filter	Check spark, dry plug, ensure fresh fuel, clean filter.
Engine starts then dies	Clogged fuel filter, tank vent, carburetor issues	Replace fuel filter, check vent, clean carburetor.
Engine lacks power	Clogged spark arrestor, dirty air filter, incorrect carb adjustment	Clean spark arrestor screen, air filter, adjust carburetor.
Chain does not oil	Empty oil tank, clogged oil holes, faulty pump	Fill tank, clean bar/chain oil holes, check pump.
Excessive vibration	Loose components, unbalanced chain, damaged AV mounts	Tighten bolts, balance chain, inspect anti-vibration mounts.
Chain turns at idle	Idle speed too high, clutch sticking	Adjust idle speed (LA screw), clean clutch.

Disassembly & Assembly

General sequence for major service. Refer to detailed Stihl technical documentation.

1. Remove top cover, air filter, and carburetor.
2. Remove starter assembly and flywheel.
3. Remove cylinder cover and cylinder.
4. Separate crankcase halves (requires heat and special puller).
5. Remove piston, crankshaft, and bearings.
6. Reassembly is reverse order. Use new gaskets and seals. Apply thread locker where specified.

Note: This is a complex procedure. Improper assembly can destroy the engine.

Torque Specifications

Critical fastener torque values (use a calibrated torque wrench).

Fastener	Torque (Nm)	Torque (ft-lb)
Cylinder bolts	10	7.5
Muffler bolts	8	5.9
Flywheel nut	40	30
Clutch nut (left-hand thread)	40	30
Guide bar nuts	9-10	6.5-7.5
Spark plug	20	15

Service Support: For genuine Stihl parts and technical assistance, contact an authorized Stihl dealer or visit www.stihlusa.com.