STIHL MS 251 C Service Manual

Overview and purpose

The STIHL MS 251 C chainsaw is a compact, professional-grade tool designed for reliable felling, bucking, and pruning tasks. This section presents a structured overview of the saw's core purpose, primary operating principles, and the intended outcomes of routine maintenance and servicing. It emphasizes understanding the machine’s design philosophy, including safety features, engine management, and cutting dynamics, to help technicians diagnose issues accurately and perform precise repairs. By recognizing the intended function and normal operating behavior, technicians can distinguish between normal wear and developing faults, enabling proactive upkeep and extended service life. A clear grasp of the device’s role supports efficient diagnostics, safer repairs, and consistent performance in varied work environments.

The MS 251 C is engineered to balance power, weight, and maneuverability for professional users who require dependable, on-demand cutting performance. Its design integrates a fuel-efficient two-stroke engine, ergonomic controls, and a robust filtration system to minimize downtime during demanding tasks. Servicing this model involves attention to engine timing, carburetion setup, chain synchronization, and lubrication delivery, ensuring the saw operates within specified parameters. Understanding the intended use cases—such as routine tree pruning, storm cleanup, and general timber work—guides procedures for maintenance intervals, part inspection, and replacement schedules. This knowledge helps technicians plan service visits effectively and communicate realistic expectations to end users.

Intended audience and prerequisites

The intended audience for this manual section includes certified outdoor power equipment technicians, service technicians in authorized STIHL service centers, and experienced hobbyists who perform routine maintenance and minor repairs under supervision or with proper safety precautions. Readers should possess a foundational understanding of gasoline engine operation, fuel handling, and basic mechanical diagnostics. Prior experience with two-stroke engine systems, carburetor adjustments, and chain drive assemblies is assumed, along with familiarity with safety protocols for handling cutting tools. A prerequisite to most procedures described here is access to appropriate PPE, a clean workspace, and the availability of standard hand tools and STIHL-specific service tools.

Before attempting any service, technicians should review the machine’s model-specific specifications, including engine displacement, compression ranges, recommended fuel/oil ratios, and torque values for fasteners. The MS 251 C employs a purge of cautionism: never operate or test the unit without the chain brake engaged and the bar cover installed when required. Understanding these prerequisites ensures that procedures are performed safely, correctly, and with adherence to manufacturer guidelines, which ultimately protects the technician and preserves the performance and warranty of the equipment.

The STIHL MS 251 C chainsaw is a high-performance tool designed to cut efficiently while prioritizing operator safety. Before any operation, carefully review all safety warnings and ensure that protective equipment is worn consistently. Essential gear includes cut-resistant gloves, eye and ear protection, a hard hat with face shield if necessary, and sturdy boots with good ankle support. Eye protection should be impact-rated, and earmuffs or plugs must be used to guard against prolonged exposure to loud noise. Maintain a clear, level work area free of obstructions to minimize the risk of slips, trips, or loss of balance during starting, cutting, or felling tasks. Regularly inspect protective clothing for wear and replace any damaged items to maintain full protection during use.

Operators should be trained in safe starting procedures, proper grip, and controlled cutting techniques to prevent kickback and other dangerous incidents. Always ensure a stable stance, both feet positioned firmly, and use both hands to hold the saw with a firm but not overly tense grip. The chainsaw should never be operated above shoulder height, and adjustments should be made only when the engine is stopped and the chain is stationary. In addition, keep bystanders well clear of the work zone and establish a clear communication method with any helpers. Regular breaks help maintain focus and reduce fatigue, which in turn lowers the likelihood of accidents. The tool should be stored in a secure, dry location when not in use, with the chain cover in place and the bar dry to prevent accidental contact or corrosion.

Compliance with local regulations is essential for legal and safe operation of the STIHL MS 251 C. Operators must adhere to restrictions on noise levels, safe operating distances from populated areas, and permissible hours of use in certain regions. Always verify that your maintenance activities, including idle running tests or adjustments, are conducted in accordance with local environmental and safety codes. If your jurisdiction requires periodic inspections or certifications for mechanical equipment, ensure that records are kept up to date and readily available for auditing. Be mindful of waste disposal guidelines for used fuels, oil, and chainsaw parts, following local recycling or hazardous waste programs. Proper licensing, if required, should accompany any professional or commercial use of the chainsaw to avoid penalties and ensure compliance with safety standards.

Tooling and setup

Preparing the correct tools and a clean workspace is essential for safe and effective maintenance of the STIHL MS 251 C. Begin by assembling a dedicated toolkit that includes a metric hex set, screwdrivers with Torx bits, a torque wrench, needle-nose pliers, a scraper, and a soft-faced mallet. Have a magnetic parts tray to keep small components organized and a caliper or micrometer for precise measurements when inspecting wear items. Ensure you also have fresh fuel and a spark plug gap tool, as well as a clean rags or shop towels and appropriate cleaning solvents. A vice or stand to stabilize the saw during service helps maintain safety and accuracy, especially when handling the bar, chain, and muffler components. Finally, prepare a stable, well-lit work surface with adequate ventilation and a clear space to lay out parts in the order they are disassembled, reducing the chance of misplacing items.

Set up the workspace to minimize dust, debris, and potential fuel spills. Remove any accessories from the chainsaw and place them in labeled containers to avoid cross-contamination with replacement parts. If you will be draining fuel or oil, have absorbent mats or a spill kit ready to contain any leakage. Lay out a clean, lint-free cloth or non-abrasive mat to place the saw on during disassembly, and use a soft brush to remove dirt from the cooling fins and external surfaces before disconnecting components. Verify you have a functional fire extinguisher nearby as a precaution when working with fuel and hot parts. Finally, double-check that the power source is off and that the chain brake is engaged before touching the bar and chain assembly to ensure personal safety.

When selecting tools, match the correct size to the fasteners on the MS 251 C to prevent stripping or rounding. Use the correct size Torx and metric sockets to remove the cover screws, recoil housing, and clutch components. A torque wrench is important for reassembly to spec, particularly for the ignition module screws, carburetor mounting, and muffler bolts. Keep spare screws, pins, and seals in a labeled container so you can restore the saw to its original configuration. If you encounter corrosion or stubborn fasteners, apply a penetrating oil and allow an appropriate dwell time before attempting removal to avoid breaking studs or damaging threads. Finally, organize tools in the order they will be used for each maintenance step to streamline the workflow and reduce downtime.

Before performing any maintenance, verify the model and serial number to ensure you are following the correct service procedures for the STIHL MS 251 C. Review the operator’s manual for specific torque values and tightening sequences, and cross-check component part numbers if replacing items like the bar, chain, or muffler. Wear appropriate PPE, including cut-resistant gloves, eye protection, and hearing protection when testing or starting the saw after service. Establish a clean exhaust-free area for testing the saw’s operation and perform a brief functional check after reassembly, ensuring no fuel leaks and that the chain stops when the safety features are engaged. Keep a maintenance log detailing dates, parts replaced, and observations to help track wear patterns and plan future service intervals.

Safety checks before maintenance

Before removing any covers or exposing internal components, perform a comprehensive safety check to identify potential hazards. Ensure theSaw is on a stable, non-slip surface with the chain brake engaged and the spark plug wire disconnected to prevent accidental starting. Confirm the fuel cap is securely tightened and there are no fuel leaks or vapor buildup around the fuel system. Inspect the bar and chain for cracks, wear, or looseness, and ensure the chain tension is adjusted according to the manufacturer’s specification. If the bar or chain shows excessive wear, replace them with STIHL-approved parts to maintain cutting performance and safety.

During maintenance, maintain good ventilation to avoid inhaling fuel vapors and keep the work area free of ignition sources. Avoid tools striking the cooling fins or spark plug area to prevent cracks that could lead to engine failure. If you must work near the muffler or exhaust system, allow ample cooling time to reduce the risk of burns. Confirm the recoil starter, clutch mechanism, and fuel lines are in good condition and free of cracks or signs of deterioration. Finally, ensure that all safety features, such as the chain catcher, hand guards, and chain brake, remain functional after any reassembly or adjustment, and test them as part of the final startup procedure.

When draining fluids or removing the air filter, take care to contain any residue and dispose of it following local regulations. Use a labeled waste container for used oil and a separate container for fuel if you are draining the system. After reassembly, perform a bench test using low idle and a gentle throttle engagement to verify that the carburetion is correct and there are no abnormal sounds or vibrations. If any issue is detected, halt further testing, recheck torque values, fastener seating, and component alignment, and consult STIHL service guidance before proceeding. Maintaining meticulous safety checks before, during, and after maintenance helps protect you and extends the life of your MS 251 C.

Model overview

The STIHL MS 251 C is a compact gas-powered chainsaw designed for a balance of portability and performance, suitable for both domestic users and light professional tasks. It features a 45.4 cc engine with electronic ignition and a semi-automatic carburetor, providing reliable starting and stable power output under typical cutting conditions. The saw is built with a magnesium-aluminum crankcase and a low-emission engine design that meets modern environmental standards while delivering crisp throttle response. Its ergonomic design includes a comfortable handle layout, reduced overall weight, and balanced weight distribution to minimize user fatigue during extended use. The model typically includes a decompression valve and simplified maintenance features that help technicians and owners perform routine service with fewer tools and less downtime.

The MS 251 C is commonly equipped with STIHL's standard anti-vibration system, which reduces vibration transfer to the operator’s hands and arms during operation. This feature, combined with a short bar and chain combination, makes it well-suited for pruning, limbing, and small to mid-size felling tasks. The machine uses a simple lubrication system designed to maintain chain oiling under varying workloads, with indicators and accessible components that streamline inspection and refilling. Overall, the design emphasizes user-friendly serviceability, with clear access to the air filter, spark plug, and fuel system for routine maintenance without specialized equipment. The compact configuration also aids in transportation and storage, which is a practical consideration for homeowners or rental customers who alternate use among several sites.

Common service considerations involve regular inspection of the air filter, fuel system, and chain lubrication when assessing performance. It is important to verify the carburetor settings and the spark plug gap for optimal combustion and throttle response, especially after a period of inactivity or after refueling with a different fuel grade. The ignition system should be checked for proper timing and compression, as a worn piston ring or muffler restrictions can impact overall engine efficiency. Routine maintenance should include checking the muffler screen for obstructions, ensuring the anti-vibration mounts remain intact, and confirming that the chain brake and rear-handguard function correctly for safe operation. Proper storage and fuel stabilization practices help prevent internal component corrosion and degradation of seals, which can extend the life of the saw and maintain consistent performance.

When planning service, technicians should consider common wear items such as the drive sprocket, clutch drum, and bar studs, as these components experience frequent engagement and can affect cutting performance if worn. Chain tensioning should be performed per STIHL specifications to prevent chain derailment or excessive wear on the guide bar. Users should also monitor coolant-free or low-oil indicators if present and ensure oiling is uninterrupted during full-throttle work. The MS 251 C’s service manual emphasizes routine checks every 25 hours of operation or at least quarterly, along with a more thorough inspection during seasonal storage. Adhering to these guidelines helps maintain safe operation, predictable power delivery, and reliable starting, which are all critical to efficient cutting and equipment longevity.

In terms of safety and preventative maintenance, replacing the air filter as part of a regular service cycle helps ensure clean intake air and consistent fuel-air mixtures, reducing carbon buildup and engine heat. It is essential to inspect the fuel filter and fuel lines for cracks or leaks, as even small defects can lead to air intrusion and reduced performance. The chain brake should be tested with the engine off to verify immediate engagement, and the chain lubrication system should be tested under load to confirm adequate oil flow to the bar. For professional users, keeping a log of maintenance intervals, parts replaced, and observed issues supports traceability and helps forecast future service needs. This comprehensive approach promotes safer operation, maximizes fuel efficiency, and extends the service life of the STIHL MS 251 C.

Throughout service procedures, technicians should follow all safety guidelines and use the manufacturer’s recommended lubricants, fuels, and replacement parts. Adherence to torque specifications for fasteners and correct chain compatibility with the guide bar is essential to prevent failures during operation. When in doubt, consult the official STIHL service bulletin or authorized dealer network for the most current guidance and part availability. A well-documented maintenance routine reduces unexpected downtime, enhances cutting performance, and preserves the saw’s resale value by maintaining its original specifications and workmanship. By approaching service with a methodical, safety-conscious mindset, technicians can ensure the STIHL MS 251 C remains a reliable tool for years of demanding use.

Common service considerations for this model also include environment-friendly disposal of spent fuels and old parts according to local regulations, as well as proper calibration of the carburetor after any major maintenance operation. Keeping the air and exhaust systems clean helps prevent overheating and prolongs engine life, especially under high-load conditions. The bar and chain assembly should be inspected for proper alignment and sharpness, with routine sharpening and filing performed by qualified personnel to maintain optimal cutting efficiency. Finally, maintain a clean work area during service to prevent debris from entering the carburetor or exhaust paths, which could compromise performance and safety.

The STIHL MS 251 C chainsaw requires careful pre-maintenance procedures to ensure safe operation and accurate diagnostics. Begin with a thorough visual inspection of the exterior of the saw, checking for any signs of damage, lubrication leaks, loose fasteners, or cracked components. Verify that all safety features, including the chain brake, throttle interlock, and kickback protection, move freely and return to their safe positions when released. Confirm that the bar and chain are properly mounted and that there is no excessive play in the bar mount or chain tensioner. Document any irregularities with clear notes and take photographs if possible to aid future servicing. This initial check helps prevent misdiagnosis and reduces the risk of injury during maintenance tasks.

Disassemble only what is necessary for the inspection, keeping track of fasteners and small parts. When removing covers, use the correct screwdriver and keep track of shielded components such as the spark plug boot and fuel line clamps. While accessing the carburetor area or fuel system, take care not to spill fuel or mix fuels unintentionally, as residual gasoline can damage seals and create a safety hazard. Clean the exterior thoroughly to prevent dirt and debris from entering the intake or cooling fins during subsequent disassembly. A clean workspace with adequate lighting allows for better identification of wear patterns and potential failures. Record temperatures and general operating condition observed during startup, noting any unusual smoke or odors that could indicate pre-existing issues.

Proceed to inspect the air filtration system for cleanliness and integrity. A clogged air filter reduces engine efficiency and can cause lean running conditions that lead to overheating. Remove the air cover and carefully remove the filter, inspecting for cracks, tears, or excessive dirt buildup. Clean or replace the filter as required, following the manufacturer’s recommended interval and using appropriate filtration media. Ensure the filter housing seals correctly when reinstalled to maintain proper engine vacuum. This step is essential for maintaining performance and prolonging engine life, as clean intake air improves combustion quality and reduces the likelihood of dirt-related damage.

Evaluate the fuel system for contamination and correct fuel quality. Check the fuel for ethanol content and ensure it is fresh and properly mixed if used in two-stroke configurations. Inspect the fuel filter, line, and primer bulb for cracks, swelling, or hardening, and replace any compromised components. If the saw uses a carburetor with adjustable screws, exercise caution not to adjust settings before diagnosing root causes, as improper adjustments can lead to engine instability. Always drain the tank if water or debris is suspected, and replace fuel with a fresh, clean supply following the factory specifications. Proper fuel system inspection reduces the risk of running issues and helps distinguish between fuel-related faults and mechanical faults during later testing.

Next, verify the cooling system and bar/chain lubrication pathways. Inspect cooling fins for obstruction by sawdust and ensure there is unobstructed airflow around the clutch and stator region. Confirm that the bar oil reservoir is clean and that the oil pump and pick-up tube are free of debris. Check the oil outlet line for cracks or leaks and confirm that oil delivers to the bar during operation; observe a proper oil flow at idle and under load if feasible. A well-maintained lubrication system prevents overheating, which is a common cause of premature wear on chains and bars. Proper lubrication also contributes to more consistent chain tension and reduces wear on drive components.

For discrete fault indicators and symptoms, identify common early warning signs that point to specific issues. A hard-start or no-start condition can indicate fuel delivery problems, ignition issues, or a flooded carburetor, whereas inconsistent idle or erratic revving often points to air leaks, carburetor misadjustment, or ignition timing concerns. Unusual vibrations, black smoke, or strong fuel odors may signal worn bearings, a clogged exhaust, or exhaust port restrictions. If the chain is not lubricating properly, inspect the oil pump, bar oil line, and filter; if the chain runs too hot or dulls quickly, check for improper chain tension, incorrect chain pitch, or lubrication failure. Document all fault indicators with environmental context, including engine temperature, load conditions, and recent maintenance history, to guide accurate diagnostic decisions in subsequent repair steps.

In all cases, maintain a clean and organized workspace and adhere to safety practices, including removing the spark plug wire when performing mechanical checks and using appropriate PPE. After completing these pre-maintenance checks, restore any removed components with proper torque specs and test the chainsaw under controlled conditions to verify that all subsystems operate correctly. Keeping meticulous records of inspections and symptoms supports effective troubleshooting and helps ensure reliable performance when the saw returns to service. When in doubt, consult the STIHL service manual for spec-specific procedures and torque values to avoid over-tightening or under-tightening critical fasteners.

Disassembly and inspection

When preparing to disassemble the STIHL MS 251 C, begin by ensuring all safety precautions are observed and the tool is cool. Disconnect the spark plug boot to prevent accidental ignition and remove the fuel cap to relieve any pressure in the tank. Place the saw on a clean, stable work surface with adequate lighting and keep track of small fasteners by using labeled containers or magnetic trays. Document the order of components as you remove them to facilitate accurate reassembly, and avoid forcing parts that appear tight to prevent damage. With the exterior housing, verify you have the appropriate tools, including hex keys, screwdrivers, and pliers, to avoid marring screws or covers during removal.

External housing removal steps begin with loosening the screws that secure the body covers, usually located at the rear handle and near the carburetor side of the saw. Gently pry the halves apart using a plastic pry tool to reduce the risk of scratching the plastic components. As you detach the cover, inspect the seals and gaskets for signs of wear or damage, such as cracking or compression set, and replace any compromised parts before continuing. Once the housing is separated, carefully disconnect any electrical connectors or alarm switches if present, taking note of their orientation for correct reconnection during reassembly. Finally, remove the air filter housing and intake components to gain access to the carburetor and the fuel system, keeping track of any soft seals that may require replacement.

Carburetor and fuel system inspection requires a methodical approach to identify air leaks, clogged passages, or worn metering components. Begin by removing the carburetor mounting screws and separating the carburetor from the intake boot and fuel line, ensuring you do not bend the linkage or disturb the metering needle. Inspect the carburetor body for scoring or corrosion, and verify that the diaphragm, gaskets, and needle valve seats are flexible and free of cracks. Check the fuel line for cracks, stiffness, or swelling and replace it if any degradation is found. Use a spray bottle with a small amount of carb cleaner to check for intake leaks around the intake boot and gaskets; a change in engine idle or misfire indicates a leak that must be addressed. When inspecting the fuel system, ensure the fuel is fresh and correctly mixed for the MS 251 C, and verify that the fuel filter is clean and free of debris before reinstalling the carburetor assembly.

During the inspection process, it is important to verify the condition of the ignition system, including the spark plug and boot, while ensuring the recoil starter and flywheel nut are secure. Remove the spark plug and inspect the electrode gap; replace the plug if the electrode is eroded or the insulator is cracked. Clean the boot contact surfaces and verify there is no arcing or corrosion at the connection. When reassembling, apply a light coat of engine oil to any rubber seals or O-rings to facilitate a proper seal and ease of future disassembly. After completing the basic inspections, perform a function check by reinstalling the saw components in the reverse order of disassembly and verifying proper operation with the air filter cover and safety features intact. Finally, verify that the bar and chain brake mechanism engages when testing to ensure the saw is safe to operate once reassembled.

The powerhead and cylinder mechanism of the STIHL MS 251 C is the core of the saw’s performance, demanding careful inspection and precise adjustment to maintain efficiency and safety. Regular valve clearance and timing checks ensure optimal air-fuel delivery, compression, and exhaust scavenging, which in turn affect starting reliability and cut quality. When performing these checks, use manufacturer-approved tools and follow the recommended tolerances to prevent excessive wear or mis-timing that could lead to engine damage. Always verify both intake and exhaust valve play, as imbalances can result in reduced power, kickback risk, or erratic idle. Establish a routine that includes cleaning, measurement, and adjustment as needed to keep the powerhead operating within specification across service intervals. Additionally, document all measurements for trend analysis, which helps predict wear progression and plan preventative maintenance before critical failures occur.

Valve clearance and timing checks require careful disassembly of the top-end components, while preserving gasket integrity and ensuring proper alignment of timing marks. Begin by removing the spark plug cap, air filter, and ignition module to access the cylinder and piston region safely. When measuring valve lash, use a calibrated feeler gauge and consult the service data for the correct clearance ranges at operating temperature or cold conditions, as specified by STIHL. If measurements fall outside tolerance, perform the prescribed adjustment procedure, which may involve shimming, stem seating, or adjustment screws depending on the model’s design. After any adjustment, recheck both valves to confirm stability through multiple cycles of rotation and ensure there is no binding or abnormal resistance. The timing check should confirm that the crankshaft and camshaft relationship yields correct valve opening and closing moments for the compression and exhaust strokes. Any misalignment detected during timing checks should be corrected by following the exact timing procedure outlined by STIHL, including proper reference marks and torque specifications for fasteners.

Cylinder and piston inspection and replacement procedures require a systematic assessment of wear, scoring, and ring integrity. Remove the cylinder retaining nut or screws with the appropriate torque to avoid warping the mount, and inspect the cylinder wall for vertical scratches or shiny wear that indicates lubrication issues. Examine the piston for skirt scuffing, ring land wear, and any signs of overheating such as discoloration or melted coatings. If piston rings are stuck or if the piston shows signs of excessive play, replacement is warranted; measure ring end gap and piston-to-wall clearance using the manufacturer’s limits to determine if a fresh bore or oversized rings are necessary. When replacing the cylinder or piston, ensure correct orientation, gasket seating, and proper lubrication port timing to prevent ring snagging or improper ring seating. After installation, perform a careful reassembly, ensuring all fasteners are torqued to specification and that the cooling fins are clean and unobstructed to facilitate heat dissipation. Finally, conduct a thorough leak-down and compression test to verify that the powerhead maintains appropriate compression and that there are no leaks at the cylinder seals or o-ring interfaces.

Proper chain and guide rail maintenance is essential for safe operation and optimal cutting performance on the STIHL MS 251 C. This section provides detailed procedures for assessing chain tension and sharpness, as well as inspecting and replacing the guide bar. Consistent maintenance reduces wear, minimizes the risk of kickback, and extends the life of the saw’s cutting components.

Chain tensioning and sharpness assessment requires a methodical approach to ensure the chain engages the bar correctly without binding or skipping. Begin by inspecting the drive link alignment with the drive sprocket and ensuring there is no excessive side-to-side play in the chain. When the chain is properly tensioned, you should be able to pull the chain slightly away from the guide bar by about 1/8 to 3/16 of an inch at the midpoint of the chain. Sharpness evaluation involves checking the depth gauges and cutting teeth; dull teeth produce increased engine load, excessive vibration, and poor cutting performance. Use a dedicated sharpener or file gauge to verify that each tooth is maintained at the correct height relative to the depth gauges, and address any uneven wear promptly.

To tension the chain on the MS 251 C, first ensure the chainsaw is cool and on a stable surface. Loosen the bar nuts slightly to relieve tension, then adjust the chain tension using the tensioning screw located at the front of the guide bar area. After tightening the screws to secure the bar, recheck tension by performing the same 1/8 to 3/16 inch test at the midpoint of the chain. Rotate the chain manually to confirm smooth operation without binding. It is important to avoid overtightening, as excessive tension can cause the drive link to wear prematurely and may damage the bar or sprocket. Regularly monitor tension during maintenance intervals and after a few minutes of use, especially under heavy load or high heat conditions.

Guide bar inspection and replacement involves a careful examination of wear patterns, trimming dimensions, and the bar’s overall integrity. Inspect the bar rails for burrs, scoring, or excessive vertical misalignment, which indicate uneven wear or contamination. Check the drive link slots for blue discoloration or abnormal wear, and verify that the bar’s nose hook remains intact and straight. If the bar shows significant burring, deep grooves, cracked body sections, or worn out rails beyond the manufacturer’s recommended limits, it is time to replace it. When installing a new bar, ensure the bar nose and rails are clean and free of chips or sawdust, and lubricate the bar rails with bar oil before operation to minimize friction and prolong life.

During replacement, align the sprocket hole of the bar with the drive sprocket and secure the bar with the bar nuts in a balanced manner to avoid twisting. After installation, perform a chain tensioning check as described earlier, and run the saw briefly without load to distribute lubrication evenly along the bar and chain. Regularly replacing worn bars and maintaining proper lubrication will help sustain consistent cutting performance and reduce the likelihood of dangerous kickback scenarios. Document maintenance intervals and observed wear patterns to optimize future service and ensure the MS 251 C remains within safe operating specifications.

Oil and fuel mixture guidelines are essential for optimal performance and longevity of the STIHL MS 251 C. Use only the recommended two-stroke engine oil for air-cooled saws, mixing it with unleaded gasoline of 89 octane or higher. Prepare mixtures at the correct ratio specified in the manual, and use a clearly labeled container to prevent accidental misfueling. Always mix vigorously to ensure even dispersion of oil, and never mix oil directly in the carburetor or fuel tank after the engine has started. Store mixed fuel in approved containers, away from heat sources, and discard any unused mixture if it has aged beyond the manufacturer’s recommended shelf life. Regularly inspect the fuel for signs of separation or contamination before refueling, and avoid using stale or substandard gasoline that can cause reduced lubrication and increased wear on the engine components.

Lubrication points and grease types cover the critical interfaces that keep the MS 251 C running smoothly. The bar and chain require a continuous supply of bar and chain oil, which should meet the specifications for low-ash, non-clogging oils designed for chain lubrication. Ensure the oil delivery system is clean and free of obstructions; check the oil reservoir capacity and verify that the oiler nozzle and chain brake spring are functioning correctly. For internal components that benefit from periodic lubrication, use manufacturer-approved grease at sealed bearing points and pivot joints, avoiding over-application which can attract dirt and debris. When performing maintenance, always clean joints and contact surfaces before applying grease to prevent abrasive contaminants from accelerating wear. Document lubrication intervals based on running hours and duty cycle, adjusting for operating conditions such as dust, heat, and load to maintain peak performance and extend component life.

Inspecting the spark plug is a fundamental maintenance task for the STIHL MS 251 C, as a clean, properly gapped plug ensures reliable starting and consistent engine performance. Begin by locating the spark plug boot and removing it carefully to gain access to the plug. Use an appropriate socket to lift the plug from its cylinder head without applying excessive torque that could damage the porcelain insulator or the gasket seat. Visually inspect the plug for signs of fouling, such as a black oily deposit, light tan to grayish coloration indicating proper combustion, or a white crust that may indicate overheating. If electrode wear is evident or the insulator shows cracks or chips, replace the spark plug with a manufacturer-recommended type, ensuring the new plug matches the correct heat range for optimal ignition. Always reinstall with a clean gasket surface and hand-tighten before finishing with the specified snug torque to avoid loosening during operation. After installation, reattach the ignition lead firmly and perform a functional check by cranking the engine in a safe, non-fueling environment to confirm a strong, consistent spark and smooth starting behavior.

Setting the correct spark plug gap is critical for ignite reliability and engine efficiency. The MS 251 C typically uses a specific gap recommended by STIHL; verify this value in the service data or owner's manual before adjusting. To measure the gap, remove the plug and use a calibrated feeler gauge across the electrode gap, not the spark plug threads. If the gap is too wide, gently bend the side electrode to reduce it, ensuring the bend is precise and does not create an irregular edge that could cause misfiring. If the gap is too narrow, carefully widen it by flexing the electrode, taking care not to apply lateral force that could fatigue or snap the center electrode. After achieving the correct gap, inspect the plug for any nicks or burrs and clean any residue from the threads to ensure a proper seal when reinstalled. Reinstall the plug with a fresh sealant on the gasket surface or as specified by STIHL, and torque to the manufacturer’s recommended specification to maintain a consistent ignition seal and prevent air leaks that could affect performance.

When diagnosing with the spark plug in place, look for signs that indicate ignition issues beyond the plug itself. A persistent hard-start condition, misfiring under load, or a drop in engine performance can point to a faulty ignition module rather than a worn plug. Begin by ensuring the spark plug consistently fires by laying a spark tester across the plug boot and grounding it to the engine, then cranking the engine to observe a bright, continuous spark. If the ignition is weak or intermittent, check the ignition coil for cracks in the housing, loose connections, or damaged wiring that could impede spark delivery. A degraded ignition coil often correlates with symptoms such as high-failure rates or extended cranking times, and may require testing with a dedicated diagnostic tool or substitution with a known-good module to confirm the fault. Finally, ensure the air gap in the flywheel gap safety feature is clean and free of debris, as this can influence the triggering of the ignition system and result in erratic performance. Precise inspection and timely replacement of the spark plug and ignition components will protect engine health and provide reliable starting and running characteristics for the STIHL MS 251 C.

Ignition module troubleshooting requires careful, methodical checks to isolate faults without replacing parts unnecessarily. Start by verifying the module’s mounting hardware is secure and that the module surface remains clean and free of oil or residue that could insulate and dampen the signal. Inspect the module’s connections for corrosion, frayed insulation, or loose terminals; any compromised wiring can cause intermittent ignition or complete failure. If basic electrical checks do not reveal the cause, perform a controlled resistance test across the module’s primary and secondary windings using a calibrated multimeter, comparing readings against the service manual’s specified ranges. Unearth potential timing issues by confirming the flywheel key is intact and not damaged, as a sheared key can offset the ignition timing and cause misfiring or poor performance even with a healthy module. If the ignition module proves faulty, replacement with a genuine STIHL part is advised to maintain optimal timing and fire sequence, ensuring the saw starts promptly and runs smoothly across the full operating range.

As part of routine maintenance, keep the ignition system clean and dry, especially in dusty or damp environments common to outdoor work. Regularly inspect the spark plug, wiring harness, and connector seals for signs of wear, water intrusion, or oil contamination that could degrade electrical performance. Use fresh, compatible fuel and a proper oil mix to prevent fouling and deposits that could influence ignition quality. In servicing the spark and ignition system of the MS 251 C, document all measurements, part replacements, and settings to maintain a reliable maintenance history. Adhering to these practices will extend engine life, promote easier startups, and sustain consistent power delivery under load during demanding cutting tasks.

The air system and filters of the STIHL MS 251 C are critical to engine performance and longevity. Proper maintenance begins with understanding how air intake affects fuel combustion and overall efficiency. Regular inspection of the air filter housing and related components helps prevent dust and debris from entering the carburetor and cylinder, which can cause scoring, reduced power, and increased fuel consumption. Ensure all fasteners are secure, seals are intact, and there are no cracks in the housing that could allow unfiltered air to bypass the filter. A clean, properly seated air filter protects the engine from contaminants while maintaining optimal air flow for consistent operation.

Air filter removal and cleaning require meticulous attention to safety and cleanliness. Begin by removing the spark plug cap to prevent accidental starting, then remove the air filter cover and the element. Inspect the filter for tears, deteriorated rubber seals, or excessive oil saturation, which can indicate the need for replacement rather than cleaning. If the filter is reusable, tap it gently to remove loose dust, then wash with a mild soap solution and warm water, rinse completely, and allow to dry fully before re-oiling if the manufacturer specifies oiling steps. When reinstalling, ensure the filter sits flat and the seals are correctly engaged to maintain proper suction and prevent unfiltered air entry. Never operate the saw with a damaged filter or without a filter in place, as this can rapidly accelerate engine wear and degrade performance.

Carburetor adjustments related to air intake are essential for reliable starting and steady idle, as well as for maintaining the correct air-fuel ratio under varying loads. After confirming the air filter is clean and properly installed, use the appropriate tools to access the carburetor adjustment screws, typically labeled for high and low speed settings. Begin with the manufacturer’s baseline settings and perform gradual, incremental adjustments while the engine is at operating temperature. Fine-tuning should prioritize a smooth idle, stable acceleration, and absence of black smoke or exhaust popping, which can indicate overly rich or lean conditions. Always test the engine under load after adjustment to observe real-world performance, and document any changes for future maintenance reference. It is important to avoid excessive adjustments, as small changes can have significant effects on emissions, fuel consumption, and engine longevity. If performance issues persist after adjustments, re-check the air filter seal, intake manifold connections, and boot clamps for leaks, as any intake air leak can negate carburetor tuning efforts.

Proper air system maintenance also includes timely replacement of the air filter according to service intervals or when signs of damage or heavy contamination are evident. Adhere to OEM specifications for replacement parts, including filter grade, seals, and any insulating gaskets, to ensure an airtight seal and consistent air flow. Keep the air intake path free of obstructions such as leaves, sawdust, or debris around the exterior of the intake to prevent unfiltered air from entering. Periodic inspection of the purge or crankcase breather lines, if applicable, helps prevent pressure buildup that can impact carburetor performance. By combining careful cleaning, precise adjustments, and timely part replacement, the MS 251 C can deliver reliable power and efficient operation in demanding outdoor environments.

Cooling fin cleanliness is essential to maintain optimal engine temperatures and prevent overheating in the STIHL MS 251 C. Regular inspection should focus on removing dust, sawdust, and debris that accumulate between the cooling fins on the cylinder and around the flywheel housing. Use a soft brush or compressed air at low pressure to clear the fins, taking care not to bend or damage the delicate cooling surfaces. When cleaning, ensure the saw remains cool and avoid using water or solvents near the air intake to prevent moisture ingress. After cleaning, inspect for bent fins or any obstruction that could impede airflow and address any damage promptly to preserve cooling efficiency.

Exhaust port and muffler inspection is critical for engine performance, fuel efficiency, and emission control. Begin by visually inspecting the exhaust port for carbon buildup, scoring, or obstructions that could restrict exhaust flow. Use a flashlight to illuminate the port and carefully remove light deposits with a non-metallic brush or a suitably sized pick, avoiding aggressive scraping that could damage the port edges. Examine the muffler body and sparking arrester screen for holes, cracks, or excessive clogging, which can lead to back pressure and reduced power. If carbon deposits are significant, perform a controlled decarbonization following the manufacturer’s guidance, ensuring the engine is cool and the saw is secured before any disassembly. After cleaning, reinstall all components with proper torque on fasteners and verify that the muffler seals are intact to prevent exhaust leaks and maintain proper performance.

When performing maintenance on the cooling system and exhaust, always wear approved eye protection and gloves to shield against sharp edges and hot surfaces. Keep the work area well-ventilated and free of fuel vapors, as exhaust components can retain heat and pose burn risks. Document any signs of wear or damage, such as cracked insulation, loose mounting hardware, or corrosion, and replace defective parts using OEM specifications to ensure compatibility and safety. Finally, after completing any cleaning or inspection, run the saw briefly in a controlled environment to verify that cooling airflow is unobstructed and that the exhaust system operates without unusual noise or excessive heat buildup. Regular adherence to these procedures will help extend engine life and maintain consistent cutting performance.

The STIHL MS 251 C is equipped with a compact power package and a range of electrical sensors that monitor safety, performance, and starting reliability. Proper inspection of the electrical system begins with a thorough battery and switch assessment. Verify that the battery connections are clean, dry, and seated firmly in the connector housing, and inspect for any signs of corrosion or swelling. Use a calibrated multimeter to confirm battery voltage aligns with the manufacturer's specifications, and check for consistent voltage at the wiring harness during key cycles. Test the on/off switch, choke/stop lever, and throttle lock to ensure smooth operation without intermittent contact or resistance. If any switch exhibits brittleness or improper travel, replace the component to prevent erratic engine behavior or unexpected shutdowns during operation.

Sensor and safety switch diagnostics require careful verification of all monitoring devices that influence starting, idle, and shutdown logic. Begin by inspecting the ignition module and spark plug circuit for spark quality, ensuring a strong, consistent spark under a range of finger compression and with a properly gapped spark plug. Check the kill switch circuit continuity to confirm there is no unintended grounding that could cause sudden engine stops. Examine the presence and integrity of any safety switches such as the chain brake interlock and throttle lockout, confirming that engaging and disengaging these features yields immediate, reliable responses in the control system. Use the service manual’s voltage and resistance specifications to compare readings from sensor leads, and identify any faults that suggest degraded sensors or harness damage that may trigger fault codes or limp-home mode.

When performing electrical testing, exercise caution to avoid static discharge or short circuits that could damage sensitive electronics. Disconnect power sources before performing continuity checks on wiring harnesses, and use insulated tools to reduce the risk of accidental shorts. Document all measurements with corresponding machine identifiers, including battery type, serial numbers, and connector pinouts, to support traceability and future maintenance actions. If a fault is detected, isolate the suspect component by performing a controlled swap with a known-good unit or by using a diagnostic connector to verify whether the issue lies within the harness, module, or sensor itself. Re-test the system after each replacement to ensure the air filter, fuel system, and ignition timing behave as expected under normal operating conditions.

For routine maintenance, establish a regular inspection schedule that aligns with operating hours and environmental exposure. Visual inspections should be complemented by functional tests, especially after storage, transport, or exposure to moisture. After any electrical repair, recheck all safety interlocks, including the chain brake and switch assemblies, to confirm they return the saw to a safe state when released. Maintain thorough records of all diagnostics, including observed anomalies, test methods, and remediation steps, to assist with ongoing reliability and to support future troubleshooting for the STIHL MS 251 C.

Reassembly and functional tests

Reassembly of the STIHL MS 251 C requires meticulous attention to torque specifications and clean, dry components. Begin by confirming that all gasket surfaces are free of debris and that any seals are correctly oriented to prevent air or fuel leaks. Install the crankcase halves with a new gasket, following the prescribed sequence to ensure even pressure distribution. Tighten fasteners to the manufacturer's torque values in the recommended pattern to avoid warping or misalignment. Prior to installing the exhaust system, verify that the muffler mounting points are clean and free of burrs to prevent stress concentrations that could lead to premature failure. When reinstalling the carburetor and intake assemblies, ensure that all rubber connections are fully seated and that hoses are clipped in place to maintain proper vacuum and fuel flow during operation.

With critical components reassembled, perform a controlled prime and choke test to verify that the fuel system seals are intact and that the diaphragm and metering components move freely without binding. Check that the ignition system is correctly timed and that the flywheel key has not sheared, which could cause poor starting or misfiring. Reinstall the air filter and cover, ensuring no gaps exist around the filter housing that could allow unfiltered air to bypass the filtering element. Inspect the cooling system pathways, including the flywheel housing fins and cooling shrouds, to confirm there are no obstructions that could lead to overheating during extended operation. Finally, reinstall the fuel lines with new clamps as required, and ensure there are no signs of leakage at the connections after a brief priming test.

Operational test and performance verification involves verifying proper startup behavior, idle stability, and throttle response under load. Start the engine in a well-ventilated area, observing for smooth idling without stalling or excessive vibration. Gradually advance the throttle to mid-range and then full throttle, watching for consistent acceleration and stable engine speed. Check for any abnormal sounds such as knocking, pinging, or rubbing that could indicate internal clearance issues or misalignment. Conduct a short-term cut-off test by engaging the chain brake and verifying that the chain does not spin while the throttle is engaged, and then releasing to confirm proper chain movement and braking action. Finally, perform a load test by making a modest cut in a controlled material, noting the chain speed, vibration levels, and overall cutting efficiency to ensure that the saw meets expected performance specifications before returning it to service.

Throughout reassembly and testing, keep a detailed log of torque values, part replacements, and any observed deviations from nominal operation. Document serial numbers of replaced components and note environmental conditions such as ambient temperature and humidity, as they can influence fuel mixtures and start-up behavior. Use manufacturer-approved lubricants and gaskets, and dispose of any spent consumables in accordance with local regulations. Adhere to all safety precautions, including the use of eye protection, gloves, and secure handling of the saw during startup and testing. By following these steps carefully, the MS 251 C will return to service with reliable performance and extended component life.

Fault diagnosis and troubleshooting

Fault diagnosis and troubleshooting for the STIHL MS 251 C requires a systematic approach to identify the root cause of performance problems. Start with a careful initial assessment of symptoms such as failure to start, poor acceleration, loss of power, misfiring, or unusual noises. Collect contextual information including recent maintenance, fuel quality, air filter condition, spark plug state, and the saw’s operating environment. Document any error codes if the model displays them, and compare observed symptoms with known patterns to guide your inspection sequence. A thorough visual check should follow, looking for loose connections, damaged wiring, worn anti-vibration mounts, and signs of fuel leaks or air intake restrictions. Maintaining a logical diagnostic tree helps prevent unnecessary part replacements and accelerates repair time while preserving safety margins for the operator.

Common faults by symptom provide a practical framework for narrowing down potential causes. If the saw exhibits hard starting or no starting, verify fuel quality, check the spark plug for fouling or wear, inspect the carburetor for obstruction or improper adjustment, and confirm that the air filter is clean and the chill chain brake mechanism is disengaged. When acceleration is lagging or the engine stalls under load, examine the air intake for obstructions, assess the carburetor’s idle and high-speed circuits, and inspect the muffler for scoring or restricted exhaust flow. If the chain does not engage smoothly or the saw experiences excessive vibration, inspect the drive sprocket, clutch assembly, chain tension, and bar condition, ensuring that all fasteners are secure and properly torqued. For overheating or excessive exhaust smoke, evaluate fuel mix ratios, confirm proper bar oiling, and inspect the cooling fins and muffler for deposits that could impede performance. Each symptom guides specific checks, numbers, and tolerances that reduce trial-and-error diagnostics and support timely repair planning.

Step by step corrective actions outline a practical workflow from symptom to solution. Begin by ensuring the tool is in a safe state: remove the battery or disconnect the spark source as applicable, relieve any stored energy, and place the saw on a stable work surface with the chain brake engaged. Next, address fuel and air delivery by replacing old fuel with a fresh, properly mixed mixture, cleaning or replacing the air filter, and inspecting the fuel lines for cracks or leaks. If ignition is the issue, test the spark plug and ignition coil, clean or replace the plug, and verify proper electrical contact and continuity. Carburetion concerns require carburetor cleaning, float adjustment if applicable, and replacement of any worn diaphragms or gaskets. Clutch and bar/chain problems call for inspection of the clutch shoes, springs, and springs’ wear, as well as chain tension and bar integrity; replace worn components and reassemble with correct alignment. Finally, perform a controlled test run after each corrective action to confirm that the symptom has been resolved, noting any new observations that could indicate a secondary issue. A disciplined approach with sequential checks minimizes downtime and ensures the saw returns to reliable operation promptly.

Maintenance interval and recordkeeping

Routine service intervals for the STIHL MS 251 C are designed to maximize engine performance, longevity, and safety. Establish a baseline inspection after every full day of heavy use, and perform a more thorough check after every 25 operating hours under normal conditions. For professionals operating in dusty or harsh environments, shorten these intervals to account for accelerated wear, inspecting air filtration, fuel system components, and spark plug condition more frequently. Recordkeeping should mirror the intervals chosen, ensuring that technicians can trace service history in case of performance deviations or warranty questions. Consistent adherence to these timelines helps prevent unexpected downtime and extends the life of critical components such as the clutch, bar and chain, and intake system.

During routine service, inspect the air filter housing for cracks or loose fasteners, and confirm the filter media is clean and free of oil saturation or debris. Check the fuel system for leaks, fuel line integrity, and clamps, replacing any damaged parts promptly. Evaluate the spark plug for wear, correct gap, and signs of fouling, replacing it if necessary to maintain reliable starting and consistent idle. Lubrication paths, including the bar oil system, should be verified for proper flow and reservoir level, with contaminants removed from the oil to prevent clogging. After completing each maintenance cycle, document the date, hour meter reading, and any parts replaced to build an accurate service history for future diagnostics and warranty compliance.

Maintenance log templates should be concise yet comprehensive, capturing essential details such as service date, operator name, engine hours, and a checklist of maintenance tasks performed. Use standardized fields for parts replaced, part numbers, and manufacturer recommendations to ensure uniform records across multiple technicians. It is beneficial to include photos or notes about unusual wear patterns or suspected issues to aid future troubleshooting. When feasible, implement a centralized or digital log where technicians can input updates in real time, enabling supervisors to track maintenance compliance across a fleet of saws. Regular review of the maintenance records should be part of the facility’s quality assurance process to identify recurring failures and adjust interval guidelines accordingly.

Parts and accessories

The STIHL MS 251 C is designed to operate with a specific set of parts and accessories that maintain performance, safety, and reliability. When selecting components, prioritize OEM parts to ensure compatibility with the saw’s gauges, clearances, and materials. Using approved parts helps preserve the engine timing, fuel system integrity, and bar and chain alignment, which are critical for efficient cutting and safe operation. Regular inspection of consumables such as air filters, fuel filters, and spark plugs ensures the saw maintains optimal air-fuel mixtures and reliable ignition during demanding tasks. Always verify part numbers against the manufacturer’s catalog to avoid substitutions that could affect warranty coverage or performance parameters.

Recommended parts list includes items that are commonly required for routine maintenance or repair, such as air filters, pre-filters, spark plugs, fuel filters, and carburetor gaskets. Keep a stocked set of these parts to minimize downtime and to perform timely maintenance in the field. In addition to consumables, consider having bar and chain assemblies, sprockets, and wear parts like brake bands and clutch components available for service intervals that correspond with heavy use or difficult cutting conditions. When selecting bars and chains, match the pitch, gauge, and drive link count to the bar length and chain speed recommended by STIHL for the MS 251 C to maintain efficient cutting performance and reduce operator effort. Always inspect the bar rail wear and replace the bar if there are tight spots, burrs, or uneven wear that could cause kickback or reduced saw performance.

OEM versus aftermarket considerations are important for long-term reliability and safety. OEM parts are engineered to exact tolerances and tested to meet STIHL’s performance standards, which helps preserve timing, fuel delivery, and lubrication systems. Aftermarket parts can be suitable in many cases, but they may vary in material quality, fit, and finish, potentially impacting engine efficiency and safety features. If choosing aftermarket components, select reputable brands that provide detailed specifications and ensure compatibility with the MS 251 C’s chain saw design, including air-fuel mix, carburetor calibration, and spark plug heat range. For critical systems such as the carburetor and ignition, OEM parts are generally the safest choice to maintain emissions compliance and engine longevity, especially in demanding work environments. Document any non-OEM replacements and monitor performance closely, scheduling inspection intervals to catch wear or mismatch early.

Accessories beyond the core parts can enhance safety and efficiency, such as anti-vibration handles, chainsaw chaps, bar covers, and proper filing kits for maintaining chain sharpness. Choose filing kits that match the specific chain type used on the MS 251 C, including file sizes, filing angles, and pitch recommendations. Bar oil is another essential accessory; use STIHL-recommended bar and chain oil to ensure proper lubrication and minimal bar wear under load. In cold or remote environments, consider additional protection like spark plug boot protectors and fuel stabilizers to preserve starting reliability and fuel system integrity over time. Keeping an organized inventory of parts and accessories, with clear labeling and storage, will support rapid maintenance actions and reduce downtime on job sites.

The STIHL MS 251 C is a compact chainsaw designed for reliable performance in a variety of cutting tasks. In the Appendix and References, it is essential to consolidate wiring diagrams and schematics, along with a clear glossary of terminology, to support accurate diagnostics and maintenance. This section provides practical guidance for technicians to interpret electrical connections and to understand the terminology used throughout service documentation. Maintaining a clean reference set helps ensure safe operation, correct part replacement, and consistent repair practices across different units and production runs. When accessing wiring diagrams, verify the model year and serial number to select the correct schematic, as minor revisions can affect connector pinouts and sensor placements.

Wiring diagrams and schematics are critical for identifying how sensors, ignition components, and safety interlocks are interconnected. Start by locating the main harness routing and then trace each circuit using the color codes indicated in the diagram. Pay close attention to grounds, supply voltages, and the locations of inline fuses or fusible links, as these elements protect the electrical system from overload conditions. When diagnosing an electrical fault, compare the observed wiring routes to the schematic to confirm continuity and resistance values with a calibrated multimeter. It is also prudent to check for signs of heat damage, loose connectors, or deformation in the harness that could indicate abrasion or pin oxidation, which can create intermittent faults that are difficult to trace.

Glossary and terminology in the STIHL MS 251 C service material helps technicians communicate precisely about components and procedures. Familiar terms include ignition coil, flywheel magnet, CDI unit, and spark plug, all of which are essential for understanding the starting and running conditions of the saw. Sensor terminology such as tachometer, crank position sensor, and kill switch may appear in diagnostics, and knowing the exact function of each device prevents misinterpretation of symptoms. The glossary also covers common mechanical terms like piston, ring, and crankcase, which are often referenced in service bulletins and repair procedures. A clear understanding of these terms supports accurate part identification, torque specifications, and reassembly procedures, reducing the risk of incorrect assembly.