STIHL MS 251 Service Manual

Overview and safety information for STIHL ms 251

The STIHL MS 251 is a mid-range chainsaw designed for versatile pruning, limbing, and felling tasks. It features a balanced chassis, power-to-weight ratio, and an efficient two-stroke engine that delivers reliable performance for professional and DIY users alike. When operating this saw, it is essential to understand engine specifications and compatibility to ensure you select the correct bar length, chain pitch, and accessories for the job. Always reference the manufacturer’s guidance for fuel mix ratios, oil types, and recommended spark plug gaps to maintain optimal performance and reduce wear on the bar and chain components. Regular maintenance, including air filter cleaning, chain tension checks, and bar lubrication, helps extend service life and maintain safe operation in the field.

Engine specifications and compatibility encompass several critical factors that influence how you use the STIHL MS 251. The saw typically employs a lightweight, high-performance two-stroke engine tuned for efficiency and torque delivery suitable for wood cutting tasks. Bar lengths commonly range to accommodate different workloads, with compatibility dependent on the chain pitch and drive sprocket size. Correct fuel mixture, which usually involves gasoline combined with STIHL two-stroke engine oil at a specific ratio, is essential for stable engine operation and emissions compliance. Always confirm the correct octane rating, fuel stability considerations, and whether ethanol-blended fuels are permissible for your model, as improper fuel can lead to varnish buildup or engine damage over time. When installing a new bar or chain, ensure the drive links match the sprocket and bar groove dimensions, and verify that the chain is properly sharpened to maintain efficient cutting and reduce kickback risk.

Safety precautions and personal protective equipment (PPE) are foundational to working safely with the STIHL MS 251. Wear impact-resistant eye protection, hearing protection, cut-resistant gloves, and sturdy boots with good grip whenever you operate or service the saw. A full-face shield is advisable in situations with high debris exposure. Before starting, inspect the area for bystanders, trip hazards, and loose objects, and always secure the workpiece to prevent movement during cutting. Never operate the saw when fatigued, under the influence of medications that impair alertness, or in poor weather conditions that could compromise footing. Follow proper starting and stopping procedures, keep both hands on the saw during operation, and maintain a safe kickback zone by avoiding contact between the chain and any solid object or the user’s body. Regularly check the anti-vibration system and ensure the chain brake, throttle lock, and stop switch are functioning correctly to prevent unintended movement or injury.

Warning and hazard symbols interpretation help users quickly identify risk factors and respond appropriately. A loud warning triangle with an exclamation point indicates that the operator must read the manual or follow specific safety instructions before proceeding. The chain brake icon signals the need to engage the brake when starting or when control is uncertain, providing immediate protection against kickback. A PPE icon highlights the mandatory use of eye and hearing protection, gloves, and appropriate footwear in the work area. The fuel and oil symbols indicate separate storage and mixing requirements, reminding users to maintain proper mixtures and avoid fuel/oil contamination. Finally, a maintenance wrench symbol directs operators to perform routine service tasks such as air filter cleaning, spark plug inspection, and chain tension checks. Proper interpretation of these symbols ensures that tasks are completed with attention to safety, tool integrity, and compliance with recommended service intervals.

Technical specifications and tool identifiers

The STIHL MS 251 is designed as a mid-range chainsaw that balances power, efficiency, and reliability for a variety of cutting tasks. Its bar and chain dimensions are optimized for versatility, enabling users to tackle both small and moderately demanding jobs with precision. The bar length commonly paired with this model ranges from 16 to 18 inches, providing a practical balance between cutting depth and maneuverability. Chain pitch and drive link count are carefully matched to ensure smooth operation, consistent chain speed, and predictable kickback characteristics under typical loading conditions. When selecting accessories, always verify that the bar and chain combination is compatible with the chainsaw’s model designation to maintain warranty coverage and performance expectations.

The fuel system and oiling specifications aboard the MS 251 are engineered to deliver consistent performance in varying work environments. The carbureted two-stroke engine relies on a premixed fuel-oil blend, with the recommended ratio specified in the owner’s manual to optimize lubrication and emissions. A properly tuned carburetor ensures stable idle and responsive throttle control, which is critical for safe operation. The chain lubrication system employs a pump-driven mechanism that delivers bar oil through the oiling port to maintain bar and chain cooling and reduce wear during operation. It is essential to keep the oil reservoir sight glass clean and to use the manufacturer-approved bar and chain oil to prevent residue buildup and ensure reliable lubrication at all RPMs.

Air filtration and exhaust systems are integral to maintaining engine longevity and consistent power output. The MS 251 uses a multi-stage air filter with a breathable housing that captures dust and debris before it reaches the carburetor and combustion chamber. Regular inspection and cleaning of the air filter prevent performance drops and potential engine damage in dusty environments. Exhaust components are designed to minimize backpressure while complying with emissions standards, and periodic inspection for any signs of leakage or blockage can prevent soot buildup that could affect engine efficiency. When servicing the air intake, always reseat the filter correctly and reseal the housing to avoid unfiltered air bypassing the filtration system.

The electrical and starting system details for the MS 251 emphasize reliability and simplicity. The saw uses a traditional pull-start mechanism backed by a properly timed ignition system, with spark plugs and ignition coils accessible for routine maintenance. Keep the spark plug clean and properly gapped according to the manufacturer's specifications to ensure starting ease and consistent combustion. The kill switch, on/off throttle lock, and wiring harness are integrated to provide safe shutdown in the event of a jam or stall, and regular checks of these components help prevent accidental startup hazards. When performing electrical inspections, disconnect the spark plug or battery (where applicable) before work and inspect wiring insulation for wear, replacing any damaged parts to maintain safe operation and engine performance.

For service and maintenance intervals, refer to the recommended schedule that aligns with your typical operating conditions. Routine tasks such as air filter cleaning, bar and chain tensioning, and fuel/oil mixture verification play a critical role in extending the life of the saw and maintaining cutting efficiency. Use only STIHL-approved parts and accessories to preserve performance characteristics and warranty protections. In all maintenance procedures, adhere to proper safety practices, including securing the saw, wearing protective gear, and following lockout/tagout procedures to prevent accidental starts during service. The combination of correctly identified tools, accurate fuel/oil mixtures, and diligent filtration and exhaust care ensures dependable operation across a broad range of cutting tasks.

Bar and chain identifiers, fuel system components, air filtration elements, exhaust features, and electrical starting hardware should be cross-checked against the model’s service data to confirm correct part numbers and compatibility. Documenting serial numbers, production dates, and any observed wear helps streamline future maintenance and parts replacement. When ordering replacements, consult the OEM catalog and verify that any updated revisions do not conflict with the existing system configuration. Maintaining a detailed service log improves reliability and safety, ensuring the STIHL MS 251 delivers consistent performance under demanding workloads while preserving value over time.

Initial inspection and setup procedures

The initial inspection begins with a careful unpacking of the STIHL MS 251 and a thorough external examination. Before any disassembly or operation, verify that all packaging materials are removed and that the bar and chain are properly stored away from the engine. Inspect the carburetor linkage, throttle trigger, and oiler system for any signs of binding or damage during transit. Check the engine housing for cracks or dents, and confirm that all decals and model identifiers are intact for accurate parts reference. Document any missing or damaged components and set aside the items for replacement or warranty consideration. This initial step ensures safety, accuracy, and a solid baseline for subsequent maintenance procedures.

Unpacking and inspection checklist should include verification of the spark plug cap, fuel cap, air filter, and starter rope for wear or deformation. Confirm the presence of the tool kit and user manual, and ensure that the chain brake engages and releases smoothly. A visual inspection of the chain tensioner and drive sprocket area is essential to detect abnormal wear patterns. If possible, rotate the flywheel by hand to confirm that it rotates freely without grinding or seizure. Record any discrepancies and address them before attempting startup to prevent damage to the engine and to ensure safe operation.

Torque sequences and fastener tightening are critical for safe and reliable operation. Use a calibrated torque wrench and the manufacturer’s specified values for all critical fasteners, including the cylinder base nuts, bar nuts, and carburetor mounting screws. Tighten sequences should follow a logical pattern that maintains uniform clamping force and prevents warped components. After initial startup, recheck torque values as necessary, since heat and vibration can alter fastener tension. Maintain a clean, dry surface during tightening to avoid contamination and ensure thread integrity. Document the final torque checks for service records and future reference.

Calibration and timing basics involve ensuring the fuel system, ignition timing, and carburetion are properly aligned for optimal performance. Verify the carburetor is calibrated to the correct ambient conditions, and inspect the idle speed and high-speed adjustments for smooth operation without surging. Check the flywheel timing marks and ensure the ignition timing is within OEM specifications, using a feeler gauge or timing light as appropriate for this model. If the saw exhibits power loss or erratic idle, perform a methodical check of the air filter, fuel quality, and spark plug gap, returning to baseline timing once these items are verified. Document any adjustments and validate engine response through a controlled test run in a safe environment.

Maintenance schedule and routine servicing

The STIHL MS 251 chainsaw requires a structured maintenance approach to ensure optimal performance and safety. Begin each use with a quick inspection of the outer housing for cracks, loose fasteners, and any signs of damage. Check the bar and chain for proper tension and alignment, ensuring there are no missing or damaged depth gauges. Verify that the chain brake, spark plug, air filter cover, and fuel cap are secure and in good condition. A brief test run on a stable surface can help reveal unusual noises or vibrations that warrant immediate attention. Keeping a clean machine is essential; wipe away sawdust and debris after each use to prevent buildup that could compromise moving parts.

Daily maintenance tasks focus on safety, reliability, and performance. Start by emptying excess fuel and ensuring the cooling fins are free of obstructions. Inspect the bar and chain for sharpness and proper lubrication, adjusting tension as needed to prevent derailment. Clean or replace the air filter if it shows signs of heavy soiling, and check the muffler area for any buildup that could impede exhaust flow. Lubricate the bar tip roller if the saw design calls for it, and verify that the chain catcher is intact. After a test run, listen for abnormal engine sounds, and address any issues before resuming work to avoid more extensive damage.

Periodic maintenance intervals vary with usage, environment, and operating conditions. At the end of each month or after a defined number of hours, perform a more thorough inspection of the spark plug, replacing it if worn or carbon-coated to maintain efficient ignition. Check the fuel system for leaks and inspect the carburetor connections for any looseness. Remove and clean the air filter housing, replacing the cartridge if it shows excessive wear or damage. Inspect the cooling fins and flywheel area for debris, and clean with a non-abrasive tool to prevent overheating. Ensure the chain brake, kickback guard, and anti-vibration mounts function correctly, replacing any worn components to maintain control and safety during operation.

Lubrication guidelines and replacement parts are critical for extending service life and ensuring smooth operation. Always use the recommended bar and chain lubricant, applying it through the appropriate reservoir or outlet while the engine is running at a light idle to observe proper supply. Replace the bar oil seal if you notice oil leaks around the chain tensioning mechanism. Keep the chain sharp and well-tensioned; dull or loose chains can increase fuel consumption and reduce cutting efficiency. Replace the air filter at intervals specified by STIHL based on usage, and check the fuel filter and lines for signs of wear. When replacing parts, use genuine STIHL components to preserve performance and safety, and record each maintenance entry with date, hours of operation, and performed tasks for traceability and future service planning.

The STIHL MS 251 chain saw requires proper starting procedure and fuel quality to achieve reliable starts. If the engine fails to start or is hard to start, verify that the user has followed the correct choke, priming, and ignition procedures for cold and warm conditions. Inspect the spark plug for fouling, damage, or incorrect gap, and replace it if necessary. Check the air filter for clogging and ensure the fuel is fresh and correctly mixed if using a two-stroke configuration. Confirm that the recoil rope retracts smoothly and that the starter mechanism engages properly; binding components can imitate a no-start condition. Finally, inspect the fuel shutoff valve, carburetor linkage, and fuel lines for leaks or restrictions that could impede fuel delivery to the engine.

Loss of power and abnormal vibrations are indicators of potential fuel, ignition, or mechanical issues. Begin by confirming that the chain brake is not engaged and that the chain tension is within specification, as an overly tight chain can cause excess load and vibration. Inspect the spark plug for proper timing and resistance, and replace if deposits or wear are evident. Check the air intake for obstructions and ensure the muffler screen is clean and free of carbon buildup, as restrictions can reduce engine power. Examine the clutch, sprocket, and drive system for wear or damage that may manifest as unusual vibrations or slipping. If fuel delivery is compromised, inspect the fuel filter and fuel lines for kinks or cracks and verify the carburetor is not leaking air at its boot or gaskets. Finally, perform a compression check to determine if internal wear is contributing to loss of power.

Fuel system and carburetor troubleshooting focuses on ensuring clean, uninterrupted fuel delivery and proper air-fuel mixture. Start by confirming the correct fuel mix ratio and using fresh gasoline to prevent starting and performance issues. Inspect the carburetor for varnish, gum, or debris that can affect metering and idle quality; remove and clean the carburetor body, jets, and passages if contamination is suspected. Check the carburetor diaphragm for tears or stiffening, as a compromised diaphragm can cause weak or inconsistent performance. Inspect the fuel filter and lines for cracks, leaks, or collapse that can restrict flow, and replace components as needed. Verify the fuel pump (if equipped) is delivering consistent pressure, and inspect the fuel’s venting to ensure there is no vapor lock. Finally, assess the ignition timing and idle speed settings and adjust according to the manufacturer’s specifications, ensuring the engine idles smoothly without hesitating or dying when under load.

Disassembly and reassembly procedures

Disassembling the STIHL MS 251 requires careful attention to safety and cleanliness to prevent damage and ensure reliable operation. Begin by removing the spark plug boot and disconnecting the battery or power source if applicable, though gas-powered saws typically rely on fuel rather than electricity. Drain the fuel tank and run the engine until it is dry to minimize spills. Label and organize fasteners as you remove them, using a magnetic tray or labeled resealable bags to maintain correct reassembly order. Keep the work area free of dirt and debris, and wear eye protection and gloves to guard against sharp components and hot surfaces.

Access to the internal components starts with removing the air filter cover and the intake air filter to expose the carburetor and cylinder area. Remove the fuel line from the carburetor, noting its routing for correct reattachment. For cylinder and piston access, remove the muffler to reveal the top of the cylinder, being mindful of any gaskets that may need replacement. Use a proper torque wrench and the appropriate socket to loosen and remove the cylinder screws in a crisscross pattern to prevent warping of the cylinder base. As you lift the cylinder, inspect the piston for scoring, breakage, or excessive wear, and inspect the ring gap and wrist pin for signs of distress that would indicate the need for replacement rather than reuse.

The clutch and drive system removal involves identifying the flywheel, clutch cover, and any spacers or springs. Remove the clutch cover screws in a diagonal pattern to relieve tension evenly, then slide the clutch off the crankshaft, taking care not to drop small parts into the works. Inspect the clutch drum and shoes for wear, glazing, or damage, and replace if necessary to maintain proper engagement. When removing the drive sprocket and associated chain, verify the chain tension and the condition of the drive link, and replace worn components to prevent slippage. Reassembly requires reversing these steps with attention to the correct orientation of the clutch spring and the proper seating of the clutch drum so that it engages smoothly without rubbing on the crankcase.

Carburetor and fuel lines disassembly begins with detaching the fuel tank and securely securing any remaining fuel inside the tank or lines to avoid spills. Remove the clamps and connectors of the fuel line, noting the route to ensure accurate reconnection during reassembly. Detach the carburetor mounting screws and carefully lift the carburetor away from the intake boots, inspecting the gasket surfaces for damage. If you remove the throttle linkage or idle screws, document their position or take photos to aid accurate reassembly. When inspecting the carburetor, check for a clogged jets, warped diaphragms, or damaged seals, and replace any worn parts with genuine STIHL replacements. After reassembly, verify that all fuel lines are tightened to the correct specifications and check for leaks by briefly priming the system and observing for seepage around fittings and gaskets.

Post service inspection checks are a critical step in ensuring the STIHL MS 251 operates safely and with optimal performance after any repair or maintenance. Begin by visually inspecting all fasteners, hoses, and wiring for proper seating and signs of wear or damage. Confirm that the recoil starter assembly, gas tank cap seal, and spark plug boot are securely attached and free from cracks or deformation. Verify that the muffler, air intake, and cooling fins are unobstructed and free of debris. A thorough check of the fuel system, including the fuel lines and primer, helps prevent leaks and air ingress that can compromise engine performance. Finally, ensure the chain brake, bar adjuster, and clutch works smoothly without binding or unnecessary play.

Compression testing procedure is essential to assess the engine’s health and identify mechanical issues before putting the tool back into service. Use a proper compression gauge compatible with two-stroke engines and fully warm the engine to ensure accurate readings. Remove the spark plug, install the gauge adaptor, and crank the engine with the throttle in a stable position while observing the compression value. Compare the reading against the manufacturer's specified range for the MS 251; a significant deviation may indicate worn rings, scored cylinder walls, or a faulty piston. Repeat the test on a cold start as a baseline for comparison, and perform a leak-down test if the compression is suspect to pinpoint the source of pressure loss. Document all readings and ensure there are no substantial variations between different tests, which would suggest a recurring problem requiring further disassembly.

Operational test run and leak check confirm that the saw performs under real-world conditions and that there are no external leaks that could affect safety or performance. Start with a safe, open area free from bystanders, and ensure the chain is properly tensioned and the bar is guarded during startup. Run the engine at idle to check for smooth idling, then advance to mid and high RPMs to observe throttle response, acceleration, and exhaust sound for any abnormal behavior. Monitor for exhaust leaks at the muffler and gaskets, as well as fuel leaks around connections and seals. Listen for unusual knocking, rattling, or misfiring, which can indicate underlying mechanical or ignition issues. Conduct a light-load cut to verify smooth engagement of the clutch and chain brake, followed by a brief full-load cut to assess sustained performance, temperature, and shutdown behavior. Conclude with a post-test shutdown check to ensure all safety features reset correctly and that no residual fuel or debris is left in the system.

Parts identification and replacement guidelines

The STIHL MS 251 chainsaw contains a variety of wear parts that influence performance, reliability, and safety. Accurate identification of components is essential for correct replacement and for maintaining proper timing and chain alignment. Begin by consulting the parts diagram for your specific model year to confirm part numbers, shapes, and orientations. When removing any component, document its position and order, as some assemblies rely on precise sequencing for seals and gaskets. Regular inspection should focus on the bar studs, drive sprocket, clutch drum, air filter housing, and fuel system components, as these areas are most susceptible to wear during normal operation. Maintaining a clean work environment and using the correct tools will minimize the risk of damaging adjacent parts during service.

Common wear parts and expected service life vary with usage, maintenance, and operating conditions. For example, the air filter and fuel filter typically require more frequent attention in dusty environments, while the bar and chain experience wear based on cutting frequency and chain tensioning practices. The fuel line, primer bulb, and carburetor inlet screen may degrade over time due to ethanol blends or aging elastomer materials. Chains, bars, and drive sprockets should be inspected for glazing, cracks, or excessive looseness, and replaced when measurement limits are exceeded. Predictive maintenance—checking clearances, measuring wear, and replacing filters on a scheduled basis—helps prevent sudden failures and extends overall machine lifespan.

Where to source genuine STIHL parts is best guided by authorised dealers, official STIHL parts distributors, or STIHL customer service portals. To ensure compatibility, verify part numbers against the official STIHL parts catalog for the MS 251, including any regional variations. When ordering, provide model number, serial number, and production year to avoid substitutions that could affect performance or safety. It is prudent to request dealer confirmation of item condition, such as new OEM parts or factory-reconditioned equivalents, and to obtain a warranty statement where applicable. If a part is unavailable, consult the dealer about recommended OEM-compatible alternatives that preserve the intended operating tolerances.

Cross reference and compatibility notes are critical for maintaining the MS 251’s engineered performance. Use only STIHL-recommended chain types, bar lengths, and sprockets appropriate to your engine displacement and bar size. Be mindful of chain pitch and gauge compatibility with the drive mechanism to avoid skewing, excessive wear, or chain derailment. When replacing a carburetor or fuel system components, ensure the kit includes compatible seals, diaphragms, and gaskets to avoid vacuum leaks or fuel leaks. Always recalibrate the carburetor after component replacement, following the manufacturer’s specified procedures, and perform a test run with the chain brake engaged to verify safe operation before resuming regular use.

Common fault codes and interpretations: The STIHL MS 251 employs a fault code system to indicate abnormal operating conditions detected by the engine management and safety systems. When a fault code is stored, it is essential to retrieve and interpret it using the manufacturer's diagnostic tool or service software to determine the root cause. Typical interpretations include issues related to ignition, fuel delivery, exhaust, and sensor input. Before replacing components, verify wiring continuity, connectors for oxidation or damage, and ensure that connectors are firmly seated. After addressing the suspected cause, clear the fault code and perform a functional test to confirm that the issue is resolved and that no new codes are triggered during normal operation or under load. If a code reappears, a deeper inspection of the related subsystem is required, and consulting the service bulletin from STIHL may provide updated guidance or revised fault interpretations.

Environmental considerations for service: Performing maintenance in a clean, well-ventilated area helps prevent contamination of the fuel system and air intake components. When working outdoors, avoid exposure to extreme temperatures, moisture, and dust that can affect seal integrity and sensor readings. Use clean, designated work surfaces and containerized waste for used oil, filters, and old parts to minimize environmental impact. Store flammable liquids in approved containers and dispose of them through authorized recycling or hazardous waste programs per local regulations. During service, minimize the introduction of debris into the intake, exhaust, and cooling pathways, and follow proper static discharge precautions to protect electronic components. Finally, document any environmental considerations that could influence warranty or service eligibility, such as operating conditions outside the recommended temperature or humidity ranges.

Preventive measures to extend saw life: Establish a routine maintenance schedule that includes regular inspection of the air filter, fuel filter, and spark plug, along with checking the carburetor adjustments and idle speed. Keeping the air intake clean reduces the likelihood of leaning the fuel mixture and overheating the engine, which directly contributes to longer service intervals and better performance consistency. Monitor chain tension, bar wear, and chain lubrication to prevent excessive friction that can lead to clutch or drive sprocket wear. Use the recommended oil-to-fuel ratio for lubricating the saw chain and ensure the oiler system is functioning properly to avoid overheating and accelerated bar wear. Periodically inspect the cooling fins and cooling system for obstructions, as adequate cooling is essential for sustained performance and component longevity. Lastly, maintain a meticulous maintenance log, including parts replaced, service intervals, and observed anomalies, to identify patterns and optimize future service actions.