STIHL MS 660 Service Manual

Overview and scope

The STIHL MS 660 service manual is intended for professional technicians and experienced hobbyists who perform maintenance, repair, and optimization of this high-performance chainsaw. It provides detailed guidance on disassembly, inspection, part replacement, and reassembly to ensure the saw operates safely, reliably, and at peak efficiency. The document targets readers who have a solid foundation in small engine theory, familiarity with compression systems, ignition basics, and fuel systems. It is designed to support accurate troubleshooting, correct torque specification usage, and proper adherence to service procedures that minimize downtime and maximize equipment longevity. Users should approach the manual with a methodical mindset, laying out a clear work plan and using the appropriate tools and fixtures recommended by STIHL.

The manual emphasizes workmanship standards and proper diagnostic approaches that align with professional repair practices. It covers common failure modes observed in large-displacement chainsaws, including fuel delivery anomalies, bar and chain wear, air filtration inefficiencies, and ignition problems. Throughout the document, step-by-step procedures are paired with safety notes to help technicians anticipate risks and perform repairs without causing injury or further equipment damage. The manual also discusses calibration procedures for timing, carburetion, and idle settings, ensuring the MS 660 maintains recommended performance and emissions profiles. By following the outlined methods, users can achieve consistent results and maintain resale value for serviced units.

The scope extends from routine maintenance tasks, such as cleaning and filter replacement, to complex overhauls that may require component machining or replacement of wear-prone assemblies. It also provides guidance on proper documentation, parts identification, and the use of genuine STIHL replacement components to preserve warranty eligibility and ensure compatibility with the most current service recommendations. Although the MS 660 is a robust and durable machine, the manual acknowledges that certain procedures must be performed with precision to avoid safety hazards or degraded performance. This comprehensive approach supports technicians in delivering high-quality service that keeps heavy-cutting performance consistent under demanding field conditions.

Safety considerations and PPE requirements are a central thread throughout the manual, underscoring the importance of protecting eyes, ears, hands, and respiratory systems when working with chainsaws. The document specifies appropriate personal protective equipment such as cut-resistant gloves, impact-rated eyewear or face shields, hearing protection with adequate attenuation, steel-toed boots, and flame-resistant or long-sleeve apparel where applicable. In addition to PPE, it details system-wide safety practices including securing the work area, using proper supports for the engine and drivetrain, locking the ignition when disassembling critical components, and confirming that fuel and electrical systems are isolated before performing any intrusive procedures. The manual also covers safe lifting techniques, the use of torque wrenches within prescribed ranges, and the importance of isolating the spark plug and draining fuel when removing the carburetor or crankcase to prevent accidental starting. By integrating these safety measures into every task, technicians reduce the likelihood of injury and extend the service life of the equipment.

For personal protection and procedural clarity, the manual recommends maintaining an organized work environment with clearly labeled fasteners and components, along with a parts diagram reference to confirm correct placement during reassembly. It also highlights the importance of ventilation when dealing with fuel vapors and solvents, and advises adhering to environmental regulations for disposal of used oil, filters, and contaminated materials. The safety sections provide quick-reference checklists that technicians can consult before beginning any service action, ensuring readiness and a disciplined approach to complex repairs. Overall, the manual aims to empower qualified professionals with authoritative guidance that promotes safe operation, accurate diagnostics, and durable repair outcomes for the STIHL MS 660 chainsaw.

Equipment specifications and features

The STIHL MS 660 is a high-powered professional-grade chainsaw designed for demanding forestry applications. It features a robust two-stroke engine that balances high displacement with reliable cold-start performance, making it suitable for heavy felling, bucking, and ripping tasks. The model typically employs a large-capacity carbureted fuel system and a tuned exhaust pathway to optimize power delivery while maintaining acceptable fuel efficiency for prolonged use in the field. Overall dimensions, weight, and bar length options are tailored to professional operators, with STIHL offering bar lengths that range to accommodate both productivity and maneuverability in dense timber environments. Users should verify the specific serial number range and production year when cross-referencing parts lists and service manuals to ensure compatibility with replacement components and tuning specifications.

The MS 660 is equipped with a strong, low-vibration chassis designed to reduce operator fatigue during extended cutting sessions. The anti-vibration system combines sprung isolators and symmetrical mounting to minimize transmitted vibrations without compromising engine responsiveness or chain speed. The air filtration system is engineered to resist clogging in dusty job sites, helping to extend engine life and maintain performance under harsh working conditions. A heavy-duty clutch and drive system are standard, delivering consistent chain speed under load while offering ease of maintenance and adjustment for optimal cutting performance. Safety features, including a chain brake, chain catcher, and loaded bar design, are integrated to support responsible operation in rugged environments.

The MS 660 configurable options typically include variations in bar length, chain pitch, and sprocket configuration to match different timber species and cutting techniques. In many configurations, the saw is designed for quick air filter access, easy carburetor adjustment, and straightforward fuel-and-oil routing, enabling field technicians to perform routine maintenance with minimal downtime. The equipment may also include a purge bulb for easier starting, a top-access air filter compartment, and a designed-to-be-reliable ignition system that balances power output with durability. Operators should consult the specific model year and regional variation sheets for exact component identifiers, torque values, and recommended maintenance intervals to ensure proper service continuity and warranty compliance.

The MS 660’s fuel system is built to deliver a stable mixture of high-quality 2-stroke oil and premium gasoline under varying load conditions. The standard configuration uses a carburetor with a manual choke for reliable starting in cooler temperatures and a primer bulb to assist in rapid fuel delivery during startup. The fuel tank capacity supports extended operation, but field service often emphasizes the importance of exact oil-to-fuel ratios as prescribed in the manual to prevent scoring, piston seizure, or excessive exhaust residue. Regular inspection of fuel lines, primer bulb, and carburetor diaphragms is essential, particularly after exposure to heat cycles or fuel contamination. Properly maintaining the fuel system ensures consistent power delivery, smoother idling, and longer engine life, which is critical for performance in professional settings.

Engine cooling and lubrication systems are designed to manage high-output operation without overheating, even under heavy load. The cooling fins must be kept clear of debris, and air intake paths should be checked for obstructions after long days of operation. The crankcase and muffler assembly are manufactured to withstand rigorous vibration and heat exposure, with attention paid to gasket integrity and exhaust conduit routing. Regular compression checks, spark plug inspections, and fuel quality reviews are recommended as part of a preventative maintenance program to preserve engine longevity and reliable performance. When servicing the engine and fuel system, technicians should adhere to torque specifications and service intervals published for the MS 660 to prevent leaks and ensure safe operation in the field.

The STIHL MS 660 is a powerful chainsaw that demands meticulous preparation and disciplined maintenance planning to ensure safe, reliable operation. Before performing any service work, establish a clean, well-lit workspace with ample ventilation and a stable work surface. Clear the area of any bystanders or distractions and lay out a dedicated tool board to minimize search time during procedures. Create a written plan that outlines the sequence of maintenance tasks, the expected duration, and the tools required for each step. Documenting the planned workflow helps prevent missed checks and reduces the risk of overlooking critical components. Use factory service manuals and approved replacement parts as the authoritative references to guide every maintenance decision and adjustment. A structured approach also supports consistent results across multiple service events and different operator skill levels.

Tools and workspace setup for the STIHL MS 660 should include a complete set of standard mechanic’s hand tools, a torque wrench with manufacturer-specified values, calibrated feeler gauges, a proper cleaning brush, lint-free rags, and a magnetic parts tray. When working on the saw, ensure the ignition is disabled and the spark plug boot is disconnected to prevent accidental starting. A dedicated service cart or bench with anti-slip mats helps secure the saw during disassembly and reassembly, reducing the risk of tool slip or component damage. Maintain an organized approach by labeling fasteners and small parts, using labeled containers, and photographing each disassembly step for reference during reassembly. Adequate lighting, a comfortable working height, and an ergonomic stance are essential to reduce operator fatigue and improve the precision of measurements and adjustments.

Maintenance planning for the MS 660 should begin with establishing required maintenance intervals based on usage type, operating environment, and manufacturer recommendations. Typical maintenance areas include the air filtration system, fuel and lubrication supply, chain and bar condition, clutch and sprocket inspection, ignition system, and fuel system components. Develop a checklist that covers daily, weekly, and monthly tasks, ensuring critical items such as air filter integrity, chain tension, and bar wear are checked consistently. Incorporate scheduled professional inspections at defined hours of operation to catch wear patterns that may not be evident during routine home maintenance. Consider implementing a logging system to track service dates, observed issues, and parts replaced, which aids in diagnosing recurring problems and planning longer-term repairs. A proactive maintenance culture reduces downtime, extends equipment life, and enhances operator safety by keeping performance within specified tolerances.

Required maintenance intervals and checklists for the MS 660 should align with both the operator’s manual and the OEM’s service schedule, while accommodating the specific demands of the work environment. Daily checks should include a visual inspection of the bar, chain, and drive components for nicks, cracks, or excessive wear, plus verification that the chain brake functions correctly. Weekly routines should expand to cleaning intake screens, inspecting the air filter and fuel filter, and confirming proper carburetor idle and high-speed adjustments if needed. Monthly maintenance typically encompasses deeper inspections of the clutch drum, sprocket ring gear, and fuel line routing, along with a thorough inspection of the spark plug and ignition timing as applicable. Always perform a functional test after any repair or adjustment, confirming that chain tension, braking, throttle response, and safety features operate without binding or hesitation. Adhering to these intervals and checklists ensures predictable performance, minimizes unexpected outages, and supports safe operation during high-demand tasks.

Disassembly and inspection procedures

Chain brake and bar removal procedures require careful attention to safety and proper tool use. Begin by ensuring the saw is completely cooled and the spark plug is disconnected to prevent accidental starting. Remove the bar cover and tensioner assembly, then carefully release chain tension before lifting the bar and chain away from the saw. Inspect the bar for wear, bending, and mismatched groove depths, and check the chain for stretching or cracked tie straps. When reinstalling, apply a light film of bar groove lubricant on the bar to reduce friction, and re-torque every fastener to the manufacturer’s specified values to avoid loosening during operation. Document any abnormal wear patterns and set aside components that show signs of damage for replacement rather than reuse, ensuring the saw remains compliant with safety standards after reassembly.

During chain brake inspection and assembly, verify that the chain brake lever operates smoothly and returns to the engage position without sticking. Remove the brake band from its drum, inspect the band for glazing, cracks, or wear, and replace if contention is present. Reassemble the brake band with the correct orientation and ensure the spring tension is within specification. Fit the chain brake cover, confirming that all screws seat evenly and that no gaps are present between the cover and body. A functional chain brake is critical for operator safety, so perform a functional test before next use by simulating brake deployment and confirming the bar and chain stop promptly. Keep a detailed log of brake component wear and replacement intervals to optimize maintenance planning.

Piston and cylinder inspection and replacement guidelines require meticulous measurement and careful handling to preserve engine performance. Remove the cylinder and piston assembly following the manufacturer’s torque specifications for all fasteners, and place components on a clean, lint-free work surface. Inspect the piston crown for scoring, pitting, or burned areas, and verify that rings move freely within their grooves with appropriate end-gap clearance. Check the cylinder bore for out-of-round wear, scoring, or excessive vertical scratches, and measure the bore diameter at multiple points to determine concentricity. If any scoring or oversized wear is detected, replace the piston and ring set and, if necessary, the cylinder sleeve or liner following the exact OEM procedures. After assembly, verify proper ring gap, piston clearance, and timing marks, then conduct a dry run to confirm smooth compression and absence of abnormal noise or vibration. Maintain precise torque and sequence during reassembly to preserve seal integrity and lubrication paths essential for long-term engine durability.

Fuel system and lubrication

The fuel system on the STIHL MS 660 combines precise carburetion with a reliable lubrication plan to ensure optimal performance under a variety of working conditions. Start with a clean, well-ventilated workspace and gather the proper diagnostic tools, including a calibrated carburetor adjusting tool, a small flashlight, and a clean container for fuel testing. Inspect all hoses, clamps, and grommets for cracks, stiffness, or signs of wear, and replace any components that show deterioration. Ensure the fuel tank is free of debris and that the fuel filter screen is intact and free of obstructions before proceeding with any adjustments or testing. Proper safety practices, including avoiding open flames and wearing eye protection, should be observed during all fueling and inspection steps.

Carburetor adjustment and cleaning requires a methodical approach to restore correct air-fuel mixture across engine speeds. Begin by removing the air filter to gain unobstructed access to the carburetor body. Check the initial baseline setting and verify it against the manufacturer’s specifications, using the appropriate screwdriver or hex key for adjustments. When cleaning, disassemble the carburetor only after documenting or photographing the assembly to ensure precise reassembly. Use an approved carburetor cleaner and apply gentle, non-penetrating solvent to remove varnish and gum deposits from jets, passages, and metering systems. Do not soak critical gaskets or diaphragms for extended periods; instead, rinse with clean solvent and allow components to dry completely before reassembly. After reassembly, reconnect linkages and verify that the throttle and choke operate smoothly through full range without sticking. Finally, perform a careful idle speed and mixture adjustment, checking for stable operation without surging, and verify that the engine transitions cleanly across the low- and high-speed ranges when tested under load.

Fueling procedures and oiling system checks focus on delivering clean, correct-spec fuel to the carburetor while ensuring the lubrication system receives a steady supply of bar and chain oil. Use only fresh, ethanol-free fuelor, if ethanol-blended fuel is used, confirm compatibility with the saw and drain regularly to prevent moisture accumulation. Mix oil and fuel according to the manufacturer’s recommended ratio, precisely measuring to avoid lean or rich conditions that could affect performance or cause excessive wear. Inspect the fuel line for cracks or swelling and replace as needed, ensuring the line seating at both the carburetor and the fuel tank is secure. The oiling system should be checked by confirming adequate flow to the bar tip under operation, which may involve manual activation of the oiling mechanism and observation of oil delivery during a controlled idle and under load. If oil flow is inconsistent, inspect the oil pump, linkage, and oiling passages for obstructions, leaks, or misalignment, and repair or replace components as required to restore proper lubrication during cutting operations.

Additional lubrication considerations include maintaining clean air and fuel pathways, as contaminants can compromise both fuel delivery and combustion efficiency. Routinely inspect and replace the fuel filter, ensuring the screen remains free of debris that could restrict flow. Keep the fuel tank and cap area free of dirt, and seal the tank to prevent moisture ingress. When storing the saw for extended periods, stabilize the fuel with a compatible stabilizer and drain the fuel system if recommended by STIHL guidelines. Proper lubrication not only reduces wear on the piston, rings, and crankcase components but also extends bar and chain life by maintaining consistent oil delivery to the chain-sprocket assembly. Regular maintenance of the lubrication system helps sustain peak performance, fuel economy, and long-term reliability of the MS 660 engine.

The air system on the STIHL MS 660 is critical to engine performance, durability, and overall reliability. Regular inspection of the air filter assembly and the intake pathway helps prevent dirt and debris from entering the carburetor and combustion chamber, which can cause scoring, power loss, and premature wear. Begin by removing the air filter cover and carefully lifting the filter element. Inspect the foam or paper media for tears, hardening, or excessive debris accumulation. A properly functioning filter should feel firm and retain its shape; replace any filter that shows signs of degradation or contamination. After inspection, seat the new or cleaned filter securely, ensuring there are no gaps around the housing that could allow unfiltered air to bypass the filter. When reinstalling, confirm that all clamps, screws, and seals are tight and that the air intake ducts are free of obstructions. A clean air path supports efficient combustion and helps maintain optimum engine performance across operating conditions.

Proper air filtration is essential for maintaining the correct air-to-fuel ratio and preventing carburetor icing or fuel dilution issues. In addition to the primary filter, inspect the intake boot and seals for cracks or loose connections, as any leak can lean or richen the mixture unexpectedly. If dirt or oil residue is present on the air boot, clean it with a non-abrasive cleaner and inspect the boot for pliability; hardened or cracked boots should be replaced. For machines operating in dusty environments, consider using a temporary pre-filter or upgrading to a higher-quality filter media, but ensure compatibility with the carburetor and air box. After reassembly, perform a brief engine run to verify stable idle and smooth acceleration, listening for any unusual intake noises that could indicate a leak or obstruction. Consistent air system maintenance enhances throttle response and fuel efficiency while reducing wear on the piston and rings over time.

Air filter inspection and replacement

Periodic air filter inspection is a simple yet vital maintenance task that protects the MS 660’s carburetion system. Begin by removing the filter cover and gently lifting the filter element without dislodging any adjacent components. Examine the filter media for holes, tears, compression, or caking of dust and oily residues, which indicate a compromised barrier. Paper filters should appear light and pliable; if they become brittle or darkened, replace them. Foam filters require washing and conditioning with a compatible filter oil; ensure the oil is evenly distributed and not overloaded, which can cause startup or performance issues. When installing a new or cleaned filter, ensure a snug fit and that the sealing surfaces are free from nicks or damage that could create air leaks. Regularly replacing the air filter according to operating conditions and weather helps maintain peak power output and prevents contaminants from entering the combustion chamber.

Maintenance intervals may vary based on usage and environment, but a baseline practice is to inspect the filter at every 5 to 10 hours of operation in moderate conditions and more frequently in dusty or dirty jobs. After installation, verify that the air box is properly sealed and that there are no loose fasteners around the filter housing. Store spare filters in a clean environment to prevent contamination before installation. Document each service with the date and hours of operation to establish a reliable maintenance history. By staying diligent with air filter inspection and replacement, operators can sustain consistent performance, extend engine life, and reduce the likelihood of costly repairs stemming from unfiltered intake air.

Carburetor and intake manifold inspection and service

The carburetor and intake manifold are central to the MS 660’s fueling system and require careful inspection for leaks, gaskets, and proper sealing. Begin by disconnecting the air intake and removing the carburetor to access the intake manifold and gasket areas. Inspect the intake manifold for cracks, splits, or looseness at the mounting points; replace any compromised components to prevent vacuum leaks that can cause erratic idle and rich or lean fuel mixtures. Check all gaskets and seals around the carburetor flange for signs of drying, cracking, or degradation, and replace them with OEM equivalents to ensure precise metering. When inspecting the carburetor body, look for varnish buildup, blocked passages, or sticky needle and seat components that can impede fuel flow. If clogging is suspected, remove the carburetor, disassemble only as far as necessary to clean jets, passages, and the metering system, and reassemble with care to avoid cross-threading or misalignment. After reassembly, perform a basic fuel and air system check, ensuring the engine starts smoothly and maintains a stable idle while responding reliably to throttle input.

Proper service of the carburetor requires attention to fuel quality, lubricants, and adjustment settings as specified by STIHL’s service guidelines. Do not substitute components or modify calibration beyond the manufacturer’s recommendations. When reassembling, verify that all throttle linkages move freely without binding, and confirm that the primer and choke mechanisms operate correctly. Finally, conduct a test run under load to confirm that acceleration, peak power, and throttle response meet expected performance benchmarks while observing exhaust emissions and smoke levels. Through meticulous inspection and service of the carburetor and intake manifold, operators can restore original performance characteristics, improve fuel efficiency, and reduce the risk of malfunctions that could impact safety and reliability in the field.

Starting, warm up, and operation procedures

The STIHL MS 660 requires careful preparation to ensure reliable starts and safe operation. Begin with a thorough inspection of the machine’s fuel system, air intake, and exhaust path before attempting to start, especially after any maintenance or storage. For cold starts, ensure the choke is in the correct position and the throttle is in a neutral state to prevent unintended engagement. Verify that the fuel mix is fresh and correct for two-stroke operation, and check that the spark plug is clean and properly gapped to promote dependable ignition. When starting, use a firm, controlled pull on the starter rope and avoid yanking or rapid jerks that could cause kickback or damage to the starter assembly. After a successful start, gradually move the choke to the warm-up position and allow the engine to reach its normal operating temperature before engaging heavy workloads.

Hot starts follow a similar discipline but require attention to current engine temperature and fuel vaporization. If the engine is still warm from recent operation, ensure the throttle is in the half-throttle position and the choke remains disengaged or in the recommended warm-up setting as outlined in the operator’s manual. Avoid flooding the carburetor by attempting multiple rapid starts; instead, give the engine a moment to cool slightly if it fails to start on the first pull. During warm-up, perform light idle runs to stabilize the carburetion and clutch engagement, listening for any abnormal noises or vibrations that could indicate wear or misadjustment. Always maintain a clean internal air path, as restricted intake can cause hard starting and poor performance.

Normal operation checks and troubleshooting tips are essential for reliable performance. Before each use, verify that all safety features are functioning, the chain brake engages properly, and the chain is correctly tensioned according to the manufacturer’s specifications. Inspect the bar and chain for signs of wear, damaged drive links, or loose sprocket nuts, and ensure the chain oiling system is delivering adequate lubrication to minimize heat and wear. If you notice sluggish acceleration, excessive exhaust smoke, unusual noises, or a drop in power, perform a quick diagnostic check of the spark plug, fuel lines, and air filter; replace components that appear degraded or contaminated. In the event of starting resistance or stall conditions, confirm that the fuel mixture is correct, the carburetor settings are within the recommended range, and that there are no air leaks in the intake boot or crankcase gaskets.

Operational safety and efficiency depend on a disciplined routine of maintenance and observation. Keep the cooling fins clear of debris to prevent overheating during extended use, and monitor the chain brake and anti-vibration system for proper function. If the saw’s idle is erratic or the chain remains stationary despite throttle input, recheck the clutch, sprocket, and brake band for wear or misalignment, as these issues can influence starting reliability and steadiness of operation. After completing work, run the engine at a reduced speed to cool gradually, then store the saw in a dry, protected area away from moisture or direct heat sources. By following these procedures, you can achieve consistent starts, safe operation, and prolonged tool life for your STIHL MS 660.

The STIHL MS 660 uses a robust, high-torque clutch and a protective chain brake system designed to manage the stresses of heavy cutting and continuous operation. Proper service of the clutch and chain brake is essential to maintaining safe operation, predictable performance, and long service life for the chainsaw. Begin by ensuring the saw is completely powered down and cooling, with the spark plug disconnected for safety. Visual inspection should look for glazing on the clutch shoes, glazing on the drum surface, or any uneven wear that could indicate misalignment or contamination. When removing the clutch assembly, always support the bar and chain to prevent a surprise movement, and use the correct tools to avoid damaging the clutch drum or main crankshaft bearings. After removal, inspect the centrifugal clutch springs, shoes, and drum for wear patterns, missing springs, or heat damage, and replace any components that show signs of excessive wear. Reassembly should follow the manufacturer’s torque specifications and alignment procedures, ensuring the clutch shoes engage smoothly with the drum to prevent slippage during aggressive cutting.

The chain brake system on the STIHL MS 660 is a critical safety feature designed to stop the chain quickly in the event of kickback or operator control loss. Begin with a thorough inspection of the chain brake handle, activation lever, and chain brake band for any signs of wear, deformation, or cracks. Check the brake band tension against the sprocket and chain for proper engagement and movement, ensuring there is no binding or excessive free play when the lever is operated. Verify that the chain brake engages fully with the clutch drum when triggered and that the mechanism resets cleanly without sticking. Clean all moving parts using a non-residue solvent, taking care to dry thoroughly before reassembly to prevent contamination of the brake surfaces. When replacing the chain brake components, use OEM parts as specified, and follow the exact sequencing required for correct alignment, so the brake surfaces contact uniformly and stop the chain effectively within the required distance. After reassembly, perform a functional test without the bar and chain, ensuring that the brake engages firmly and returns to a ready state without interference from the clutch or engine accessories.

Clutch inspection and replacement should consider several practical indicators of performance. Look for delayed engagement, sluggish acceleration, or unusual noise when the engine reaches higher rpm, which can point to worn shoes, a stretched spring, or a glazed drum surface. If the clutch shoes show deep grooves, flattening, or loss of spring tension, replacement is recommended to restore correct engagement characteristics and prevent false starts or engine overload. When replacing a clutch drum, confirm that the internal splines mate cleanly with the crankshaft and that there is no burr or dirt accumulation that could cause misalignment. The use of proper lubrication on the clutch assembly is critical; however, excessive lubricant can attract sawdust and cause clutch slippage. After completing any clutch work, rotate the crankshaft by hand to verify smooth engagement and ensure there is no binding or abnormal resistance. Conduct a controlled test cut with proper safety gear to confirm that the clutch engages cleanly and the engine reaches rated speed without surging.

Chain brake system service and safety checks must be performed with a focus on consistent performance and operator safety. Begin with a calibration check to ensure the brake responds to the lever with a firm, positive action and that return movement is prompt and complete. Inspect the brake pawl and spring assemblies for signs of fatigue or corrosion, replacing any components that show deformation or loss of tension. The brake band’s contact surfaces must be clean and free of oil or sawdust buildup; clean with an appropriate solvent and lightly lubricate only the specified contact surfaces as directed by STIHL service guidelines. Replacing the brake drum or band should maintain precise tolerances so the brake can fully arrest chain rotation within legal stopping distances under load. Finally, verify that the chain brake achieves full arrest within the specified travel distance and that normal operation of the engine and throttle is unaffected by the brake mechanism. Documentation of inspection results and parts replacement can help track wear trends and schedule preventative maintenance before failures occur.

Bar wear inspection and replacement

The bar on the STIHL MS 660 is a critical component that guides the chain and transfers power from the engine to the cutting teeth. Regular inspection should focus on both the face and the groove for signs of uneven wear, burrs, or excessive grooving. Examine the bar rails for flatness; any warp or deep scoring can cause chain derailment or binding, compromising safety and performance. Check the bar nose for signs of tip wear or mushrooming, which indicates improper alignment or chain tensioning. If the drive-link groove shows deep scratches, or if the bar is visibly bent, replace the bar to maintain accurate chain alignment and efficient cutting. When removing the bar for inspection, ensure the chain brake is engaged, the engine is cool, and the bar nuts are loosened in a controlled manner to prevent accidental injuries. Always align the bar with the sprocket drive and verify that the oiling holes are unobstructed, as inadequate lubrication accelerates wear and shortens bar life.

Replacement criteria depend on measured bar wear limits and visible damage. A bar with a worn drive link groove or a face that is out of parallel with the chain will produce poor cutting performance and increased chain wear. The STIHL MS 660 bar life can vary with usage, feed rate, and material density; however, any bar showing excessive groove wear, cracking, or a chipped rail should be replaced promptly. When reinstalling a new or used bar, ensure proper bar alignment and verify that the chain grooves match the drive links of the chain. Tighten bar nuts to the factory specification using a torque wrench if available, and recheck when the bar warms up after initial operation. Record the date of replacement and bar type in the maintenance log to track wear patterns over time and schedule proactive replacements before failures occur.

Chain installation and initial tensioning

Proper chain installation starts with the correct chain type for the bar length and the saw model, ensuring compatibility with the drive sprocket. Inspect the drive links and tie straps for cracks, missing drive lugs, or bent or twisted links, and replace any compromised links before installation. Mount the chain by placing the drive links around the sprocket and aligning the drive links with the bar groove, avoiding any pinching or misalignment during seating. Once the chain is seated, tensioning should be performed with the guide bar parallel to the ground and the chain still on the bar, ensuring that the drive links have minimal side-to-side play. A properly tensioned chain should exhibit a slight sag when lifted from the top of the bar; if the chain is too tight, it will cause excessive wear and overheating, while a loose chain risks derailment and poor cutting performance. Always check tension after the bar is warmed by a short test run, then readjust if necessary according to the saw’s service manual specifications.

Sharpening procedure and frequency

Sharpening the chain requires maintaining the correct file type, depth gauge setting, and filing angle to preserve cutting efficiency and minimize stress on the bar and engine. Use the correct gauge file for your chain pitch and guide the file at the same angle as the existing tooth geometry, keeping consistent strokes and applying even pressure. After filing each tooth, verify that the depth gauges remain within specification and adjust as needed, since uneven depth can cause kickback or aggressive cutting. Regularly inspect the cutters for uniform wear; uneven wear indicates improper tensioning, chain rotation direction, or incorrect filing technique. Record sharpening intervals and results in the maintenance log to optimize chain life and bar integrity, and replace the chain when the cutters show excessive loss of height or visible cracking near the root. For best results, ensure the bar and chain are kept clean and free of resin buildup, which can distort sharpening accuracy and hinder proper seating on the bar groove.

Electrical and ignition system

The electrical and ignition system of the STIHL MS 660 combines reliability with a robust design that can withstand demanding outdoor use. Start by inspecting all accessible wiring for signs of wear, cracks, or insulation damage. Ensure that connector pins are clean and free of corrosion, and that mating surfaces seat fully without excessive play. A visual check can often reveal loose harnesses or frayed wires that could lead to intermittent starting issues or erratic operation. When performing any inspection, disconnect the spark plug lead to prevent accidental arcing and ensure safe working conditions. Keep the area dry and free of oil or fuel that could degrade wiring insulation over time.

Regular maintenance of the ignition system begins with routine spark plug inspection. Remove the spark plug carefully using the appropriate tool, and examine the electrode gap, ceramic insulator, and composite boot for signs of wear or damage. Replace spark plugs that exhibit a fouled appearance, excessive wear, or a gap outside the manufacturer’s specified range. When reinstalling a spark plug, torque should be applied to the manufacturer’s specification to avoid either a loose fit or thread damage. Always use a plug that matches the engine’s heat range and heat rating to ensure reliable starting and efficient combustion. After installation, recheck that the spark plug boot is seated firmly to prevent accidental arcing during operation.

Ignition coil checks are essential for consistent engine performance. Inspect the coil for physical damage, overheating coloration, or cracking, which could indicate internal degradation. Use a multimeter to measure primary and secondary resistance values against the manufacturer’s tolerances; deviations can point to coil failure. If the ignition timing or spark consistency appears compromised, verify the kill switch wiring and grounding to ensure the circuit is interrupting spark as designed. In the event of suspected coil failure, replace the coil with a compatible OEM part and re-test the ignition system under safe operating conditions. After any replacement, perform a functional test by starting the saw and confirming a strong, consistent spark while ensuring the chain brake is engaged during initial attempts.

The kill switch and related safety interlocks are critical for operator protection and proper engine shutdown. Inspect the kill switch mechanism for smooth operation, ensuring that it reliably grounds or interrupts the ignition when engaged. Check all related wiring harnesses for continuity and resistance values that indicate a complete circuit. Confirm that the kill switch returns to its resting position and that any emergency stop procedures function without sticking or delaying. If any ambiguity exists regarding kill switch operation, perform a controlled test with the engine off to verify that pulling the switch promptly terminates spark delivery. Regular testing of the kill switch helps prevent accidental engine starts and enhances overall safety during maintenance and operation.

Comprehensive electrical system health depends on securing clean connections and protecting components from environmental exposure. Use dielectric grease on spark plug threads and connector seals to reduce corrosion and improve electrical contact over time. Ensure that all connectors are fully seated and that moisture does not accumulate in wiring junctions, which can lead to intermittent failures. When working in damp or dirty conditions, shield vulnerable connections and Route wires away from hot exhaust areas to prevent insulation damage. Document any replacements or measurements in a maintenance log to track the condition of the electrical components and anticipate future service needs.

For long-term reliability, adopt a proactive maintenance routine that includes periodic spark plug replacement, coil testing, and kill switch verification aligned with engine hours or seasonal use. Establish a checklist that covers visual inspection, resistance measurements, functional tests, and safety interlock checks. This disciplined approach minimizes unexpected downtime and ensures consistent starting performance. Always consult the OEM service manual for exact specifications and tolerances, and use genuine STIHL parts to preserve warranty and reliability. When in doubt, seek professional service to avoid damage to the ignition system or other critical engine components.

In the event of a suspected electrical fault that cannot be resolved through standard inspections, isolate the engine from fuel and air sources and proceed with a controlled diagnostic procedure. Verify that the ignition system grounds properly and that there are no short circuits or pinched wires. Replace any compromised components with OEM parts and verify that all safety interlocks are restored to full functionality before returning the saw to active service. Following these guidelines helps maintain robust ignition performance, ensuring reliable starts, smooth idling, and predictable power delivery in demanding cutting conditions.

The cooling system on the STIHL MS 660 is essential for maintaining engine performance and longevity, especially during demanding cutting tasks. Begin by inspecting the cooling fins for any signs of damage, bending, or excessive clogging. Use a soft brush and compressed air to remove debris such as wood chips, dust, and resin buildup, being careful not to bend the fins. After cleaning, check that the flywheel housing ventilation passages are clear and that there is no obstruction restricting airflow around the cooling fins. If you notice any cracked or severely bent fins, replace the affected cylinder or cylinder cover to prevent heat build-up and potential engine failure. Regular cleaning after extended use in dusty or dirty environments will help sustain optimal cooling efficiency and engine reliability over time.

Water and foreign material can infiltrate the cooling system during operation, which can compromise performance and promote corrosion. Inspect the entire cooling path, including the base of the cylinder, surrounding shrouding, and any ducting for signs of moisture ingress or rust. When cleaning, use a damp cloth to wipe down surfaces and avoid using high-pressure water directly into the cooling passages, as this can push contaminants deeper into the engine. Ensure that seals and gaskets around the cooling shroud remain intact and free from cracks. After cleaning, perform a quick airflow check by manually rotating the flywheel (with the spark plug removed and the ignition disabled) to feel for consistent resistance and to confirm that debris has not created hotspots or restricted flow.

Exhaust port and muffler service is critical for maintaining proper exhaust flow, engine power, and compliance with emissions standards. Start by inspecting the exhaust port for carbon buildup, scoring, or obstructions that could restrict exhaust gas escape. Use a soft scraper or appropriate tool to carefully remove carbon deposits from the exhaust port, taking care not to scratch the metal surfaces. Check the muffler body for cracks, corrosion, or broken baffles, and ensure that the exhaust outlet is not obstructed by soot or foreign material. If the muffler shows significant wear or damage, replace the muffler or internal components per STIHL service guidelines to restore optimal backpressure and engine performance. After servicing, re-torque all fasteners to the manufacturer’s recommended specifications and recheck gasket integrity to prevent exhaust leaks that could degrade performance or present a safety risk.

Further, verify that the exhaust system components are properly sealed and that there are no exhaust leaks around the gasket interfaces. A leak can reduce engine efficiency and increase noise, while also allowing hot gases to affect nearby components. When reassembling, apply the correct gasket material and ensure a snug fit without over-tightening, which can damage threads and create later leaks. Periodic inspection of the exhaust system should accompany routine maintenance, especially after segments of heavy duty use or operation in harsh environments. Keeping the cooling fins and exhaust components clean and well-sealed will help preserve engine power, reduce heat-related wear, and extend the service life of the STIHL MS 660.

Troubleshooting and diagnostic guide

When diagnosing issues with the STIHL MS 660, start with a systematic assessment of the most common startup and running problems. Begin by inspecting the air filtration system for loose or clogged components, as a restricted air intake can cause loss of power, hard starting, or erratic idle. Check the spark plug for proper gap, fouling, or fouling deposits, and verify that the ignition system produces a strong, consistent spark. Ensure that the fuel system is delivering fuel cleanly and without air leaks, and confirm that the carburetor is clean and correctly tuned for altitude and temperature conditions. Finally, examine the compression level with a calibrated gauge to rule out worn piston rings or a leaking cylinder, which would manifest as poor starting, low power, or rapid engine deterioration. A careful combination of visual inspection, functional testing, and targeted measurements will often reveal root causes without unnecessary disassembly.

For startup and running problems, perform a step-by-step fuel and air pathway check. Start with a fresh, properly mixed fuel if the saw uses a two-stroke formulation, and inspect the fuel line for cracks or leaks that could introduce air into the system. Verify that the carburetor needle adjustments are correct according to the service data for your model and generation, and look for bent or damaged throttle linkage that could prevent smooth operation. Check the fuel filter and signal valve in the fuel tank, replacing any clogged or degraded components to restore steady flow. Finally, inspect the muffler and spark arrestor for deposits, as obstruction here can cause heat buildup and reduced performance during operation. Each of these steps should be performed with the engine in a safe state and the saw properly supported to prevent accidental starts.

When diagnosing errors and diagnostic codes, rely on the operator’s manual crossover to interpret code indicators correctly. If an error code appears, record the exact sequence and environmental conditions, then reference the service literature to identify whether the fault originates in ignition, fuel delivery, sensor inputs (if applicable), or exhaust systems. Conduct a controlled test by removing nonessential loads and cycling the saw through a controlled startup to confirm whether the issue persists under known, repeatable conditions. If required, perform a controlled teardown to inspect critical interfaces such as the clutch engagement mechanism, crankcase seals, and reed valves for signs of wear or leakage. Document all findings with measurements and photographs to support future maintenance decisions and to guide any necessary component replacements.

In addressing erroneous behavior or symptom clusters, it is often helpful to implement a process of elimination. Begin with the simplest, most common faults—fuel, air, and spark—before moving to more complex mechanical failures. Use a clean, labeled work environment and keep replacement parts matched to the exact model and serial number to avoid compatibility issues. After completing repairs, perform a thorough post-repair test that includes a cold start, a warm restart, and a controlled idle to verify that the engine runs smoothly across its operating range. Finally, reset any diagnostic indicators and ensure all safety devices and guards are correctly reinstalled before returning the saw to service. A disciplined approach minimizes downtime and extends the reliability of the MS 660 in demanding field use.

Common startup and running problems

Symptoms such as hard starting, extended cranking times, or inconsistent idle frequently point to air or fuel system faults. Inspect the air filter housing for cracks and ensure the filter itself is clean and properly seated. Examine the spark plug for proper gap and cleanliness, replacing it if the electrode shows wear or fouling. Check for fuel leaks along the fuel line and ensure the primer bulb, if present, operates smoothly to prime the carburetor. Finally, verify that the carburetor adjustment screws are set within the manufacturer’s specified range for your operating conditions and that the choke mechanism engages and disengages correctly. Addressing these aspects often resolves initial startup and running concerns without invasive repairs.

Error codes and corrective actions

When error codes are displayed, interpret them using the model’s diagnostic diagram and the corresponding fault tree. Common corrective actions include replacing worn or damaged ignition components, cleaning or rebuilding the carburetor, and replacing degraded fuel lines or gaskets that contribute to air leaks. If a fault involves piston or ring wear, perform a compression test and compare results to the manufacturer’s specification; excessive clearance or low compression warrants engine rebuild or component replacement. Exhaust system obstructions should also be considered, as a blocked muffler or spark arrestor can trigger performance faults that resemble fuel or ignition issues. After implementing corrective actions, re-check system pressures and perform multiple trial runs to confirm stable operation across cold and warm conditions. Persisting codes or unresolved faults should prompt escalation to authorized service centers with access to OEM parts and repair procedures.

Reassembly and testing

The reassembly sequence for the STIHL MS 660 requires meticulous attention to alignment, torque, and clean mating surfaces to restore optimal performance. Begin by inspecting all gasket surfaces for damage and replace any worn components before reinstalling the crankcase, bar, and muffler assembly. Ensure that bearing housings are free of debris and lubricants are applied according to the manufacturer's specifications. When resealing the crankcase, verify that the o-rings and gasket material seats evenly without gaps, and torque fasteners to the precise values specified for each joint. Use clean, dry fasteners and avoid overtightening, which can distort threaded holes or warp mating surfaces. After partial assembly, rotate the crankshaft by hand to confirm smooth operation and verify that no binding occurs within the chain brake or flywheel area. Finally, reinsert the ignition module and check that the spark timing aligns with the STIHL service data, ensuring consistent ignition across the full RPM range.

The reassembly sequence must be followed with strict adherence to torque specifications to prevent leaks and ensure reliability. Prepare a calibrated torque wrench and use the correct drivenness for each fastener, disposing of any used or stretched bolts. For example, crankcase cover bolts require a specific low-to-medium torque range, while main bearing retaining bolts demand a higher setting. Apply a light coat of fresh engine oil to rotating surfaces before final assembly to reduce startup wear. After tightening, perform a post-assembly inspection to confirm that all electrical connectors are fully seated and that the anti-vibration mounts are correctly positioned to minimize engine chatter during operation. The final step in reassembly is to reinstall the air filter, carburetor linkage, and fuel lines with careful routing to avoid kinks or interference with moving components. In all cases, double-check that the chain brake and bar nuts are secure and that the anti-kickback features are functioning as designed before proceeding to testing.

Operational testing and safety verification requires a structured approach to confirm that the saw performs within specification while maintaining operator safety. Begin with a no-load idle test to verify that the engine starts reliably, idles smoothly, and returns to idle without stalling when the throttle is released. Gradually increase to a light-load operation and observe for consistent acceleration, stable RPM, and proper carburetor response. Listen for abnormal exhaust tones or rattles that could indicate loose components or misalignment. Conduct a controlled cut in a test board to evaluate chain speed, bar lubrication, and top-end cooling under load, stopping immediately if unusual vibration or heat is detected. Verify that the chain brake engages fully and holds the chain stationary when activated, and confirm that anti-kickback mechanisms are functioning through deliberate throttle and stop actions. Finally, perform a complete safety check of the overall assembly, ensuring that the spark arrestor, muffler, and intake paths are free of obstructions and that fuel lines, clamps, and filters remain securely fastened and free from leaks. A concluding run with a full-throttle test should demonstrate stable performance, adequate lubrication, and no excessive emissions, indicating a successful service cycle.

Maintenance records and documentation

Maintaining thorough service logs and an accurate parts inventory is essential for the reliable operation of the STIHL MS 660. A well-kept maintenance record provides a historical account of all inspections, repairs, replacements, and adjustments performed on the chainsaw. This documentation helps technicians quickly identify recurring issues, plan preventive maintenance, and schedule timely interventions before faults develop into costly failures. In addition, accurate records support warranty claims and facilitate resale by demonstrating proper care and service history. Ensure entries are legible, dated, and signed by the technician who performed the work to establish accountability and traceability.

Begin by tracking core information for each service event, including hours of operation, observed symptoms, parts replaced, lubricants used, and any calibration or tensioning performed. A standardized format accelerates data entry and retrieval, making it easier to compare current conditions with previous service cycles. For the MS 660, this should include chain oil type and chain tension readings, bar wear measurements, air filter condition, spark plug state, and fuel system notes if applicable. Maintaining a consistent template reduces the risk of overlooking critical maintenance steps during inspections.

Recommended service intervals for optimal performance

Adhering to recommended service intervals is key to preserving performance, safety, and longevity of the MS 660. Establish a maintenance calendar that aligns with your typical workload, operating environment, and local conditions. At minimum, perform pre-use checks, daily quick inspections, and more thorough weekly or monthly maintenance as dictated by usage. Document any deviations from standard intervals and adjust your schedule accordingly to account for heavy usage, dusty environments, or frequent cutting in abrasive materials.

Core service tasks should include regular inspection of the air filter, spark plug, fuel system, and cooling components, with specific attention to signs of wear, contamination, or improper fit. The air filter should be cleaned or replaced as needed, and the spark plug gap verified to ensure reliable starting and smooth operation. The cooling fins and screen must be kept clear to prevent overheating, particularly in hot climates or prolonged cutting sessions. By maintaining a disciplined interval regimen, you minimize unexpected downtime and extend the service life of the carburetion and ignition systems.