STIHL MS 251 Chainsaw Manual

The STIHL MS 251 is a lightweight, professional-grade chainsaw designed for demanding cutting tasks. It features a powerful engine with STIHL anti-vibration system, quick-stop chain brake, side-access chain tensioner, and an ergonomic design for comfortable operation. Below are key sections for safety, features, assembly, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: Engine with electronic ignition, Quickstop chain brake, Side-access chain tensioner, Anti-vibration system, Ergonomic handles, Fuel and oil caps, Chain catcher, Throttle trigger lockout.

Safety Information

Read all instructions before operation. Wear proper PPE.

1. Personal Protective Equipment: Always wear safety glasses, hearing protection, gloves, chainsaw chaps, and sturdy boots.
2. Work Area Safety: Clear area of obstacles, ensure secure footing, keep bystanders away.
3. Chain Brake: Test before each use by engaging manually.
4. Kickback Awareness: Be aware of kickback zone; use reduced-kickback chain.

WARNING! Chainsaws are dangerous tools. Failure to follow safety instructions can result in serious injury or death. Never operate when tired or under the influence.

Assembly & Preparation

Unpack and inspect contents. Assemble guide bar and chain.

1. Attach guide bar: Place bar on studs, align chain sprocket cover, tighten nuts hand-tight.
2. Install chain: Place chain on bar with cutting edges facing forward, engage drive links in sprocket.
3. Tension chain: Use side-access tensioner to achieve proper tension (slight sag on bottom).
4. Fill fluids: Add STIHL 2-stroke engine oil to fuel mix (50:1 ratio). Fill bar and chain oil reservoir.
5. Final check: Ensure all fasteners are secure, chain moves freely, brake disengaged.

CAUTION! Use only STIHL-approved bar, chain, and lubricants. Improper fuel mix can damage engine.

Specifications

• Engine Displacement: 45.4 cc (2.77 cu in).
• Power Output: 2.6 kW (3.5 bhp).
• Fuel Capacity: 0.5 L (16.9 fl oz).
• Oil Capacity: 0.26 L (8.8 fl oz).
• Guide Bar Lengths: 40 cm (16") to 50 cm (20") recommended.
• Chain Pitch: 3/8" PMM or .325".
• Weight (powerhead): 5.0 kg (11.0 lbs).
• Sound Pressure Level: 106 dB(A).
• Vibration Levels: Front handle: 4.2 m/s², Rear handle: 5.8 m/s².
• Ignition: Electronic, capacitor discharge.

Controls and Components

Main controls located on rear handle and body.

Left Side: Chain brake handle, Throttle trigger lockout, Master control lever (choke/run/stop). Right Side: Throttle trigger, Hand guard. Top: Decompression valve (if equipped), Spark plug. Front: Muffler, Bar mounting studs. Rear: Fuel cap, Oil cap, Starter grip.

Master Control Lever Positions: STOP (0) - engine off, RUN (I) - normal operation, START/CHOKE (II) - cold start position.

Starting & Stopping

Follow proper starting procedure to avoid injury.

1. Cold Start: Set master control to START/CHOKE (II). Press decompression valve if equipped. Pull starter cord until engine fires. Move lever to RUN (I). Pull starter cord to start.
2. Warm Start: Set master control to RUN (I). Pull starter cord to start.
3. Stopping: Release throttle, move master control to STOP (0). Engage chain brake for storage.

Tip: Always start on ground or solid surface, never drop-start. Hold rear handle with right hand, place foot through handle for stability.

Operating Instructions

Proper cutting techniques ensure safety and efficiency.

1. Basic Stance: Stand with feet shoulder-width apart, left foot slightly forward.
2. Grip: Hold front handle with left hand, rear handle with right hand. Keep thumbs wrapped.
3. Starting Cut: Apply light pressure, let chain do the work. Avoid forcing saw.
4. Kickback Prevention: Keep nose of bar out of cut. Be aware of pinch points.
5. Bucking: Support log properly. Cut from top down for compression side, bottom up for tension side.
6. Limbing: Work from trunk outward. Watch for springback.
7. Felling: Not recommended for inexperienced users. Consult professional training.

WARNING! Never operate above shoulder height. Always plan escape path when felling.

Maintenance & Cleaning

Regular maintenance ensures optimal performance and longevity.

Daily: Check chain tension, sharpness, and lubrication. Clean air filter. Inspect for loose fasteners. Weekly: Clean cooling fins, check spark plug, clean spark arrestor screen. Monthly: Replace fuel filter, clean carburetor air passage, check anti-vibration elements.

CAUTION! Always disconnect spark plug wire before performing maintenance. Allow engine to cool completely.

Chain & Bar Care

Proper chain maintenance is critical for safe operation.

1. Sharpening: Use round file matching chain pitch. Maintain correct filing angle (typically 30°). File all cutters evenly.
2. Depth Gauges: File depth gauges to proper height (0.025" / 0.65 mm below cutter) using depth gauge tool.
3. Bar Maintenance: Clean bar groove regularly. Flip bar weekly for even wear. Dress bar rails if burred.
4. Lubrication: Always use STIHL bar and chain oil. Check oil flow before each use.

Note: Dull chain increases kickback risk and operator fatigue.

Storage

Proper storage prevents fuel system issues.

1. Short-term (under 30 days): Run carburetor dry, empty fuel tank, clean exterior, store in dry place.
2. Long-term: Add fuel stabilizer to fresh fuel, run 2 minutes. Drain completely. Remove chain, clean and oil. Store in original box if possible.

CAUTION! Never store with fuel in tank for extended periods. Gasoline deteriorates and damages carburetor.

Troubleshooting

Service: For repairs beyond basic maintenance, contact authorized STIHL servicing dealer.

Warranty: Limited warranty applies. See separate warranty document for details.