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's advanced engine technology, an anti-vibration system, an ergonomic design, and a Quickstop® inertia chain brake for enhanced safety. Below are key sections for safety, features, assembly, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: Engine with STIHL engine technology, Ergonomic handle with anti-vibration system, Quickstop® inertia chain brake, Side-access chain tensioner, Tool-free fuel and oil caps, and a transparent fuel tank.

Safety Information

Read and understand all safety instructions before operation. Always wear appropriate personal protective equipment (PPE).

1. Personal Protective Equipment (PPE): Wear safety glasses, hearing protection, gloves, chainsaw chaps, and sturdy boots.
2. Pre-Operation Check: Inspect chainsaw for damage, ensure chain is sharp and properly tensioned, check fuel and oil levels.
3. Safe Operation: Maintain secure footing, keep bystanders away, be aware of kickback zone, never operate above shoulder height.
4. Fuel Safety: Use only STIHL MotoMix® or fresh fuel/oil mixture (50:1). Refuel in a well-ventilated area with engine off and cool.

WARNING! Failure to follow safety instructions can result in serious injury or death. The chain brake is a safety device but does not eliminate the risk of kickback.

Assembly & First-Time Setup

Unpack and inspect contents. Assemble guide bar and chain.

1. Attach Guide Bar & Chain: Remove bar nuts and cover. Place chain over sprocket and onto bar. Align bar with studs and replace cover. Hand-tighten nuts.
2. Tension Chain: Use side-access tensioner to adjust chain until it snaps back when pulled from the bar. Tighten bar nuts securely.
3. Fill Fluids: Add STIHL 2-stroke engine oil to fuel tank (50:1 mix). Fill bar and chain oil reservoir with STIHL chain oil.
4. Start Engine: Place on clear, flat ground. Set master control lever to cold start position. Pull starter rope until engine fires. Move lever to run position and restart.

CAUTION! Ensure chain brake is disengaged before starting. Keep hands away from the chain at all times.

Specifications

• Engine Displacement: 45.4 cc
• Power Output: 2.6 kW (3.5 bhp)
• Fuel/Oil Mixture: 50:1 (STIHL 2-stroke engine oil)
• Fuel Tank Capacity: 0.47 L (16 oz)
• Chain Oil Tank Capacity: 0.26 L (8.8 oz)
• Guide Bar Lengths: 16" or 18" (recommended)
• Chain Pitch/Gauge: 3/8" PMM / .050"
• Weight (without bar & chain): 5.2 kg (11.5 lbs)
• Sound Power Level: 112 dB(A)
• Vibration Levels (Front/Rear Handle): 4.5/2.5 m/s²

Controls and Components

Master Control Lever: Combines stop switch, choke, and run positions. Throttle Trigger & Interlock: Prevents accidental throttle activation. Chain Brake Lever: Manual activation; inertia-activated during kickback. Decompression Valve: Reduces pulling force for starting. Side-Access Chain Tensioner: For tool-free chain adjustment.

Main Components: Engine, Fuel Tank, Chain Oil Tank, Air Filter, Spark Plug, Muffler, Guide Bar, Saw Chain, Chain Sprocket, Chain Catch, Handle, and Anti-Vibration Elements.

Operation Guide

Starting, cutting techniques, and shutdown procedures.

1. Starting (Cold Engine): Engage chain brake. Set master control to cold start (choke). Press decompression valve. Pull starter rope until engine fires. Move lever to run position. Pull again to start.
2. Starting (Warm Engine): Set master control to run position. Pull starter rope.
3. Cutting: Use full throttle when cutting. Let chain do the work; do not force. Be aware of pinch points, especially when cutting logs under tension.
4. Kickback Prevention: Hold saw firmly with both hands. Do not cut with the tip of the bar. Maintain a secure stance.
5. Shutdown: Release throttle. Set master control to stop position. Engage chain brake.

Tip: Regularly check chain tension, especially during the first few minutes of operation as a new chain stretches.

Maintenance & Care

Daily and periodic maintenance schedule.

1. Daily: Clean air filter, check chain tension and sharpness, inspect guide bar for wear, clean cooling fins, check all nuts and bolts for tightness.
2. Weekly/Periodic: Clean spark arrestor screen, check spark plug, clean fuel filter, replace chain and bar oil filters as needed.
3. Fuel System: Use fresh fuel mix. Empty tank for storage over 30 days. Clean fuel cap vent.
4. Cleaning: Use a brush to remove sawdust and debris. Do not use water or high-pressure cleaners.

WARNING! Perform maintenance only when the engine is off and cool. Disconnect the spark plug boot for safety.

Chain Sharpening & Replacement

Sharpening the chain and replacing worn parts.

1. Sharpening: Use a round file and file guide matching chain pitch. Maintain correct filing angle (typically 30°). File all cutters to the same length. Check and adjust depth gauges with a flat file and gauge.
2. Replacement: Replace chain when cutters are worn down to the limit line, or if damaged. Replace guide bar if rails are unevenly worn or damaged. Replace drive sprocket when worn to prevent chain damage.

CAUTION! A dull or damaged chain increases the risk of kickback and operator fatigue. Always wear gloves when handling the chain.

Storage Guidelines

Prepare chainsaw for short-term and long-term storage.

1. Short-Term (under 30 days): Run engine until fuel tank is empty. Clean exterior. Store in a dry, secure place.
2. Long-Term (over 30 days): Drain all fuel from tank and carburetor. Run engine until it stops. Remove spark plug, add a teaspoon of engine oil into cylinder, pull starter rope slowly to coat interior, reinstall plug. Clean entire saw thoroughly. Remove chain and bar, clean and coat with oil. Store in a dry, frost-free location.

CAUTION! Storing with fuel in the system can cause carburetor damage and starting problems.

Regulatory Information

EPA Phase III and CARB Tier III compliant for emissions. Meets ANSI B175.1 safety standards. Contains parts that may be subject to trade restrictions. For genuine STIHL parts and accessories, see your authorized STIHL dealer. Copyright © STIHL Incorporated. All rights reserved. STIHL and Quickstop are registered trademarks. For support, visit www.stihlusa.com or contact your local authorized STIHL dealer.

Troubleshooting

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

Support: www.stihlusa.com | Find a Dealer: 1-800-467-8445