STIHL Chainsaw 021 Service Manual

The STIHL 021 is a lightweight, professional-grade gasoline chainsaw designed for occasional to medium-duty cutting tasks. This service manual covers essential maintenance, troubleshooting, and repair procedures for the STIHL 021 chainsaw, including the engine, fuel system, ignition, chain drive, and safety systems. Below are key sections for safety, specifications, maintenance intervals, troubleshooting, and repair procedures.

1. Safety Information & Symbols 2. Specifications & Technical Data 3. Maintenance Schedule 4. Engine System 5. Fuel System 6. Ignition System 7. Chain & Bar Assembly 8. Carburetor Adjustment 9. Troubleshooting Guide 10. Disassembly & Assembly 11. Parts List & Diagrams

Safety Information & Symbols

Critical safety warnings and symbols found on the chainsaw and in this manual.

SymbolDescription
⚠️General Warning - Indicates potential hazard
🔥Fire Hazard - Keep away from flammable materials
⚡Electrical Hazard (Ignition System)
⛓️Chain-related Danger - Kickback zone, chain tension
🧤Wear Protective Gear - Gloves, eye/ear protection, chainsaw pants
🛑Stop Immediately - Engine stop switch position

WARNING! Always disconnect the spark plug before performing any maintenance. Use only STIHL-approved parts and fuel mixtures.

Specifications & Technical Data

Maintenance Schedule

Regular maintenance intervals to ensure optimal performance and safety.

  1. Before Each Use: Check chain tension, sharpness, and lubrication. Inspect for loose fasteners.
  2. Every 10 Hours: Clean air filter. Check spark plug condition and gap (0.5 mm / 0.020 in).
  3. Every 20 Hours: Clean spark arrestor screen (if equipped). Check fuel filter.
  4. Every 50 Hours: Replace fuel filter. Clean cooling fins. Check carburetor adjustment.
  5. Every 100 Hours: Replace spark plug. Clean exhaust port and muffler. Check anti-vibration elements.
  6. Annually or Every Season: Replace air filter. Inspect clutch, sprocket, and guide bar for wear.

CAUTION! Use only STIHL-approved bar and chain oil. Never use waste oil.

Engine System

Components: Cylinder, piston, crankshaft, connecting rod, bearings.

Compression Test: Normal range: 120-150 psi. Below 100 psi indicates wear.

Top-End Rebuild: Steps for piston/cylinder replacement:

  1. Remove muffler and cylinder cover.
  2. Remove cylinder base nuts.
  3. Lift cylinder, remove piston pin circlips and pin.
  4. Install new piston rings (gap 0.3-0.5 mm).
  5. Reassemble with new gaskets, torque cylinder nuts to 10 Nm (7.4 ft-lb).

Fuel System

Components: Fuel tank, filter, lines, carburetor (Walbro or Zama).

  1. Fuel Line Replacement: Drain tank, remove old lines, install new lines with clamps.
  2. Fuel Filter Replacement: Use hook tool to extract old filter from tank.
  3. Carburetor Removal: Disconnect throttle linkage and fuel lines, remove mounting screws.

Tip: Always use fresh fuel mix (max 30 days old) to prevent carburetor issues.

Ignition System

Components: Flywheel, ignition coil (module), spark plug, stop switch.

Spark Test: Remove spark plug, connect to ground, pull starter - look for strong blue spark.

Coil Air Gap: Set to 0.3 mm (0.012 in) using feeler gauge between coil legs and flywheel magnets.

Resistance Check: Primary coil: 0.5-2.0 Ω; Secondary: 5-15 kΩ.

WARNING! High voltage risk. Do not touch ignition wires while engine is running or being started.

Chain & Bar Assembly

Proper chain installation, tensioning, and bar maintenance.

  1. Install bar: Fit bar onto studs with tensioning pin in bar hole.
  2. Install chain: Direction arrows toward bar tip. Fit drive links into bar groove.
  3. Tension chain: Loosen bar nuts, adjust tensioning screw until chain snaps back when lifted from bar.
  4. Tighten bar nuts: 8-10 Nm (5.9-7.4 ft-lb). Re-check tension after tightening.

Bar Maintenance: File rails if burred, clean oil holes weekly, flip bar monthly for even wear.

Carburetor Adjustment

Standard adjustment screws: L (low speed), H (high speed), LA (idle).

Initial Settings: Turn L and H screws clockwise to gentle stop, then back out 1 turn each.

Fine Tuning:

CAUTION! Over-leaning (turning screws too far clockwise) can cause engine damage.

Troubleshooting Guide

SymptomPossible CauseCorrective Action
Engine won't startNo spark, flooded, fuel issueCheck spark, dry plug, fresh fuel, clean carburetor.
Engine starts then diesClogged fuel filter, ventReplace fuel filter, check tank vent.
Poor performance/lacks powerDirty air filter, exhaust screenClean/replace air filter, clean spark arrestor.
Chain won't turnClutch worn, sprocket damagedInspect clutch shoes and drum, replace if worn.
Excessive vibrationLoose parts, AV mounts wornTighten all fasteners, replace anti-vibration elements.
Chain oil not flowingClogged oil passages, pump failureClean oil holes, check oil pump drive and diaphragm.

Disassembly & Assembly

General sequence for major service.

  1. Remove top cover, air filter, and carburetor.
  2. Remove starter assembly and flywheel (use puller).
  3. Remove cylinder and piston assembly.
  4. Remove clutch and sprocket (use clutch tool).
  5. Remove oil pump and drive assembly.
  6. Reassemble in reverse order with new gaskets/seals.

Torque Values: Cylinder nuts: 10 Nm, Clutch: 40 Nm, Flywheel: 25 Nm, Spark plug: 20 Nm.

Parts List & Diagrams

Reference exploded-view diagrams for part numbers. Common replacement parts:

For complete parts diagrams and ordering, contact an authorized STIHL dealer or visit www.stihlusa.com.

STIHL Chainsaw 021 Service Manual © - All Rights Reserved 2025