Stihl 025 Chainsaw Service Manual

The Stihl 025 is a professional-grade gasoline chainsaw designed for demanding cutting tasks. This service manual provides comprehensive information for maintenance, troubleshooting, and repair. It covers safety procedures, technical specifications, engine components, fuel system, ignition system, cutting equipment, and step-by-step repair instructions. Always refer to this manual and use proper tools and Stihl-approved parts for all service work.

1. Safety Information 2. Specifications & Identification 3. Maintenance Schedule 4. Engine System 5. Fuel System 6. Ignition System 7. Chain & Bar Assembly 8. Clutch & Sprocket 9. Troubleshooting Guide 10. Disassembly & Assembly 11. Torque Specifications

Safety Information

Working on a chainsaw involves significant risks. Always prioritize personal safety and the safety of others.

Hazard	Precaution
Fuel & Fire	Work in well-ventilated area; no smoking or open flames.
Sharp Components	Wear cut-resistant gloves when handling chain, bar, or cutter.
Hot Surfaces	Allow engine and muffler to cool completely before servicing.
Springs under Tension	Use proper tools; be aware of stored energy in clutch springs, starter springs.
Battery Disconnect	Disconnect spark plug wire before any service to prevent accidental start.

WARNING! Failure to follow safety instructions can result in serious injury or death. Always wear appropriate PPE (safety glasses, gloves).

Specifications & Identification

Key identification and technical data for the Stihl 025 chainsaw.

Engine Type: Single-cylinder, two-stroke, air-cooled.
Displacement: 45.4 cc (2.77 cu. in.).
Power Output: 2.5 kW (3.4 hp).
Fuel/Oil Mix: 50:1 (Stihl HP Ultra or equivalent).
Idle Speed: 2,800 RPM.
Max Power Speed: 12,500 RPM.
Ignition System: Electronic, breakerless.
Spark Plug: Bosch WSR6F or equivalent, gap 0.5 mm (0.020 in).
Guide Bar Lengths: 40 cm (16") to 50 cm (20") recommended.
Chain Pitch/Gauge: 3/8" PM / .050" (1.3 mm).
Serial Number Location: Stamped on crankcase, near rear handle.

Maintenance Schedule

Regular maintenance is critical for performance, longevity, and safety.

Before Each Use: Check chain tension, sharpness, and lubrication. Inspect for loose fasteners, fuel leaks, and general condition.
Every 10 Hours: Clean air filter, check spark plug condition and gap, clean cooling fins and cylinder area.
Every 20 Hours: Clean or replace fuel filter. Inspect clutch drum and sprocket for wear.
Every 50 Hours: Clean spark arrestor screen (if equipped). Check anti-vibration elements. Clean carburetor compartment.
Annually or 100 Hours: Replace fuel line and impulse line (if cracked). Clean and inspect muffler. Check piston/cylinder through exhaust port for scoring.

CAUTION! Use only genuine Stihl parts and recommended lubricants. Improper parts can cause damage or unsafe operation.

Engine System

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

Compression Test: Minimum 110 psi (7.6 bar) for proper operation. Low compression indicates worn rings, piston, or cylinder.

Cylinder & Piston Inspection: Remove muffler to inspect piston skirt and cylinder walls for scoring, scratches, or aluminum transfer. Light scoring can be cleaned with fine emery cloth. Deep scoring requires replacement.

Crankcase Seals: Critical for engine vacuum/pressure. Test with pressure/vacuum tester. Leaking seals cause lean run condition and engine damage.

Fuel System

Components: Fuel tank, cap, filter, fuel line, impulse line, carburetor (Walbro or Zama).

Fuel Line Replacement: Drain tank. Remove old line from tank grommet and carburetor. Install new line with filter into tank. Connect to carburetor inlet.
Impulse Line: Connects crankcase to carburetor pump diaphragm. Cracked line causes poor fuel delivery. Replace if hardened or cracked.
Carburetor Adjustment (H, L, LA): Initial settings: Turn H and L screws gently clockwise to stop, then back out 1 turn each. Fine-tune for smooth acceleration and top RPM within limits.
Carburetor Rebuild: Kit includes diaphragms, gaskets, inlet needle. Clean all passages with compressed air. Do not use wire.

Tip: Always use fresh, properly mixed fuel. Stale fuel is a common cause of starting and running issues.

Ignition System

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

Spark Test: Remove spark plug, connect to plug wire, hold against cylinder. Pull starter; look for strong blue spark. No spark indicates faulty module, broken wire, or flywheel key sheared.

Air Gap: Between ignition module legs and flywheel magnets. Set to 0.3 mm (0.012") using a business card as a feeler gauge.

Flywheel Key: A sheared key causes incorrect ignition timing and hard starting. Replace if damaged.

Chain & Bar Assembly

Proper chain maintenance is essential for safe and efficient cutting.

Tensioning: Loosen bar nuts. Lift bar tip. Adjust tensioning screw until chain fits snugly against bar but can be pulled around by hand. Tighten bar nuts securely.
Sharpening: Use correct round file size (typically 4.0 mm or 5/32"). Maintain factory filing angle (usually 30°). File all cutters to equal length. File depth gauges (rakers) with flat file and gauge.
Bar Maintenance: Clean bar groove. Flip bar regularly for even wear. Dress bar rails with flat file if burred. Check for uneven wear ("hooking").
Oiler System: Check oil delivery by running saw at high idle over light-colored surface; look for oil sling. Clean oil passages in bar and saw.

WARNING! A loose or dull chain can derail or cause kickback. Always stop engine before adjusting or servicing chain.

Clutch & Sprocket

The centrifugal clutch engages the chain at engine speed.

Clutch Inspection: Remove cover and bar/chain. Inspect clutch shoes for wear, cracking, or glazing. Springs must be intact and secure.

Clutch Removal: Hold piston at Top Dead Center (TDC) using piston stop or rope in spark plug hole. Use clutch tool to hold drum and remove retaining nut (left-hand thread). Remove drum and clutch assembly.

Sprocket Wear: Replace rim sprocket or spur sprocket if teeth are hooked or worn more than 0.5 mm. A worn sprocket accelerates chain wear.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, no fuel, flooded	Check spark, fuel freshness, carburetor. Pull with throttle wide open if flooded.
Engine starts then dies	Clogged fuel filter, carb issue, air leak	Replace fuel filter, rebuild carb, pressure test crankcase.
Lacks power, bogs down	Clogged air filter, muffler screen, improper H adjustment	Clean/replace filter, clean spark arrestor, adjust H screw.
Chain does not turn at high RPM	Worn clutch springs, glazed shoes	Replace clutch springs and/or shoes.
Excessive vibration	Loose components, worn AV mounts, unbalanced chain	Tighten fasteners, replace AV buffers, sharpen chain evenly.
Chain oil not dispensing	Empty oil tank, clogged oil passages, faulty oil pump	Fill tank, clean bar groove and outlet hole, inspect/replace pump.

Disassembly & Assembly

Major service requires disassembly. Organize parts and note orientation.

Remove Top Cover/Air Filter Cover: Remove screws, lift off cover.
Remove Fuel Tank/Handle Assembly: Disconnect throttle linkage and fuel line. Remove mounting screws.
Separate Crankcase Halves: Requires removing all engine components, then splitting case with a puller tool. Note bearing and seal locations.
Reassembly: Use new gaskets and seals. Apply sealant to crankcase mating surfaces as specified. Torque crankcase screws in a crisscross pattern to specified value.

CAUTION! This is complex work. If unsure, consult a Stihl-trained technician.

Torque Specifications

Critical fasteners must be tightened to the correct torque.

Cylinder Nuts: 20 Nm (15 ft-lb)
Muffler Bolts: 10 Nm (7.5 ft-lb)
Clutch Nut (Left-hand thread): 40 Nm (30 ft-lb)
Flywheel Nut: 40 Nm (30 ft-lb)
Spark Plug: 25 Nm (18 ft-lb)
Guide Bar Nuts: 9-10 Nm (6.5-7.5 ft-lb)
Crankcase Screws: 10 Nm (7.5 ft-lb)

Note: Use a calibrated torque wrench. Overtightening can strip threads; undertightening can cause failures.