STIHL 025 Workshop Manual

The STIHL 025 is a professional-grade chainsaw designed for demanding forestry and woodcutting applications. This workshop manual provides comprehensive technical information for maintenance, troubleshooting, and repair. It covers engine specifications, fuel system, ignition system, cutting equipment, safety features, and step-by-step repair procedures. Below are key sections for safety, specifications, maintenance, troubleshooting, and repair.

Specifications

Technical data and performance characteristics of the STIHL 025 chainsaw.

Parameter	Specification
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 Tank Capacity	550 ml (18.6 fl. oz.)
Oil Tank Capacity	260 ml (8.8 fl. oz.)
Guide Bar Length (Max)	40 cm (16 inches)
Chain Pitch	3/8" or .325"
Chain Gauge	1.5 mm (.058")
Dry Weight (without cutting equipment)	5.2 kg (11.5 lbs)
Ignition System	Electronic, capacitor discharge (CDI)

Safety Precautions

Essential safety instructions to prevent injury during operation and maintenance.

1. Personal Protective Equipment (PPE): Always wear chainsaw chaps, gloves, eye protection, hearing protection, and steel-toe boots.
2. Work Area Safety: Clear work area of obstacles and ensure no persons or animals are within 15 feet.
3. Fuel Safety: Refuel in a well-ventilated area with engine off and cool. Use fuel/oil mix ratio 50:1 (STIHL HP Ultra oil).
4. Chain Brake: Test chain brake functionality before each use.
5. Kickback Zone: Avoid contact with the upper quadrant of the guide bar tip.

WARNING! Failure to follow safety instructions can result in serious injury or death. Read and understand the operator's manual before use.

Maintenance Schedule

Recommended maintenance intervals to ensure optimal performance and longevity.

1. Daily/Before Use: Check chain tension and sharpness, inspect guide bar for wear, check all fasteners for tightness, clean air filter, check chain brake function.
2. Weekly/Every 10 Hours: Clean spark arrestor screen (if equipped), clean cooling fins, check fuel filter, inspect anti-vibration elements.
3. Monthly/Every 50 Hours: Replace spark plug (NGK BPMR7A), clean carburetor (external), check clutch drum and sprocket for wear.
4. Annually/Every 100 Hours: Replace fuel filter, clean/replace carburetor diaphragm and gaskets, check piston/cylinder through exhaust port for scoring.

CAUTION! Use only genuine STIHL replacement parts. Improper maintenance can void warranty and cause unsafe operation.

Engine System

• Cylinder & Piston: Chrome-plated cylinder, single-ring piston. Inspect for scoring, carbon buildup, and ring wear.
• Crankshaft & Bearings: Sealed ball bearings. Check for axial and radial play.
• Muffler: Equipped with spark arrestor screen. Clean regularly to prevent overheating.
• Cooling System: Centrifugal fan on flywheel draws air over cylinder fins. Keep fins clean.
• Decompression Valve: Optional. Reduces pulling force for starting.
• Rewind Starter: Rope and pulley assembly. Inspect pawls and spring for wear.

Fuel System

Components: Fuel tank, filter, lines, and carburetor (typically Walbro WT or Zama C1M).

Carburetor Adjustments (Standard Settings): L (Low Speed) - 1 turn out from lightly seated; H (High Speed) - 1 turn out; LA (Idle) - 1 turn out. Fine-tune for smooth acceleration and maximum RPM of 13,500 (no load).

Fuel Line Replacement: Disassemble tank, remove old lines, install new lines with grommets, ensure proper routing to avoid kinks.

Ignition System

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

1. Spark Test: Remove spark plug, connect to wire, ground against cylinder, pull starter. Look for strong blue spark.
2. Air Gap: Check flywheel to coil gap (0.3 mm / 0.012 in). Adjust by loosening coil screws.
3. Module Testing: Resistance values: Primary coil ~0.5 Ω, Secondary coil ~8 kΩ. Refer to specific service manual for exact values.
4. Stop Switch: Test for continuity when off (closed circuit), open circuit when on.

Tip: Weak or no spark is commonly caused by a faulty ignition module, broken wire, or incorrect air gap.

Cutting Equipment

Guide bar, saw chain, drive sprocket, and chain tensioner.

1. Guide Bar: Inspect for straightness, groove wear, and sprocket nose rotation. Clean oil passages daily.
2. Saw Chain: Sharpen with round file matching chain pitch. Maintain correct filing angle (typically 30°). Check depth gauges (rakers) and lower with flat file.
3. Drive Sprocket: Replace if worn (teardrop shape instead of round). Can be spur or rim type.
4. Chain Tensioning: Loosen bar nuts, adjust tensioning screw until chain snaps back when pulled from bar, tighten bar nuts.

WARNING! Always use matched chain/bar/sprocket pitch. A dull or loose chain increases kickback risk.

Clutch & Drive System

Centrifugal clutch, drum, and chain brake assembly.

Clutch Inspection: Remove cover and drum. Check clutch shoes for wear and spring integrity. Engagement RPM is approximately 2,800 - 3,200.

Chain Brake: Manual (front hand guard) and inertial activation. Test by running saw at idle and striking hand guard forward—chain should stop immediately. Inspect brake band for wear and tension spring.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, flooded, fuel issue	Check spark, air gap, ignition wire. Dry plug if flooded. Check fuel lines, filter, carburetor diaphragms.
Engine starts then dies	Fuel starvation, air leak	Check fuel vent, filter, carburetor settings. Perform pressure/vacuum test on crankcase.
Lacks power/overheats	Clogged air filter, muffler screen, carb adjustment, piston scoring	Clean filter and spark arrestor. Adjust carburetor. Inspect cylinder through exhaust port.
Chain does not turn	Clutch worn, sprocket seized, brake engaged	Inspect clutch shoes and springs. Check sprocket rotation. Disengage chain brake.
Excessive vibration	Loose components, unbalanced cutting equipment, AV mounts damaged	Tighten all fasteners. Check chain balance and bar straightness. Replace anti-vibration buffers.
Oil not feeding to bar	Clogged oil passages, faulty pump, wrong oil viscosity	Clean bar oil holes and tank filter. Check pump drive and diaphragm. Use STIHL chain oil.

Disassembly & Assembly

Key procedures for major service.

1. Top End (Cylinder/Piston): Remove muffler, carburetor, and intake. Unbolt cylinder. Note orientation of gasket. Inspect piston rings and cylinder walls.
2. Crankcase Splitting: Requires special tool (crankshaft puller/installer). Remove all external components, separate case halves. Note bearing and seal locations.
3. Carburetor Rebuild: Disassemble, clean all jets and passages with compressed air. Replace diaphragms, gaskets, and needle valve. Set metering lever height.
4. Clutch Removal: Hold piston at TDC with rope, use clutch removal tool to unscrew (left-hand threads).

CAUTION! Mark components for reassembly. Use proper tools to avoid damage. Replace all gaskets and seals during reassembly.

Torque Specifications

Critical fastener torque values (Nm / in-lbs). Use a calibrated torque wrench.

Fastener	Torque (Nm)	Torque (in-lbs)
Cylinder Nuts	20	177
Spark Plug	25	221
Clutch Nut (Left-hand thread)	40	354
Flywheel Nut	50	443
Guide Bar Nuts	12	106
Crankcase Screws	10	89
Muffler Screws	8	71

Reset: After major repairs, reset carburetor to standard settings and retune.

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