STIHL MS 661 C Service Manual

Overview and Safety

The STIHL MS 661 C is a high-powered chainsaw designed for demanding cutting tasks in professional forestry and large-scale timber operations. It combines robust materials, precise engineering, and a powerful emission-compliant engine to deliver reliable performance under tough conditions. Proper understanding of its components, operating limits, and maintenance requirements is essential to maximize efficiency while minimizing wear and safety risks. Before engaging with the tool, operators should have a clear understanding of how the model’s features interact with real-world workload, including bar length, chain pitch, and fuel stabilization practices that align with environmental conditions and regulatory guidelines.

Safety is the cornerstone of effective operation and long-term equipment reliability. The MS 661 C is equipped with multiple protective devices, including a chain brake, anti-vibration system, and a sturdy handguard. These features are designed to minimize the risk of kickback, control loss, or operator fatigue during extended use. A thorough safety mindset extends beyond personal gear to include an inspection routine that verifies chain tension, bar integrity, fuel and oil delivery, and carburetor balance. By performing routine checks prior to each use, you reduce the likelihood of unexpected failures that could lead to injury or costly downtime.

Personal protective equipment and work area setup

Proper personal protective equipment (PPE) is non-negotiable when operating the MS 661 C due to its high power and potential debris projection. Operators should wear an approved cut-resistant chaps or chainsaw pants, a sturdy cut-resistant jacket, a certified safety helmet with a face shield, and hearing protection such as earmuffs or plugs. Foot protection should include steel-toed boots with non-slip soles to provide stability on uneven terrain. Gloves designed for impact resistance and grip are recommended to enhance control while reducing vibration-related fatigue. The combination of PPE and correct operating posture helps maintain safety margins during dynamic cuts and when handling the chain under tension.

Work area setup is equally critical to safe operation. Clear the workspace of obstacles, bystanders, and pets, and establish a stable, level stance for each cutting task. Set up proper lighting and establish a clear escape route in case of chain bind or kickback. Check the ground for hidden hazards such as buried roots, loose soil, or slick surfaces that could compromise footing during a kickback event. When refueling or performing maintenance, perform these tasks in a well-ventilated area away from ignition sources, and ensure the engine is cool before opening the fuel cap. A well-planned setup reduces the likelihood of accidental contact with hot surfaces or fuel spills and contributes to a smoother, more controlled cutting workflow.

Tool and Equipment Specifications

The STIHL MS 661 C is a professional-grade chainsaw designed for high-demand logging, forestry work, and other heavy-duty cutting tasks. It features a robust 112.0 cc engine displacement and a tuned air intake system that delivers strong power output while maintaining efficiency under load. The carburetion setup is calibrated for reliable starting and steady performance across a range of operating temperatures, making it suitable for both seasonal and year-round use.Operators should verify the availability of essential fueling equipment, including clean, uncontaminated bar and chain oil, proper fuel within the recommended octane range, and an appropriate bar size matched to the cutting task to optimize performance and reduce transmission loads on the engine.

Key mechanical specifications include the engine’s maximum RPM range, torque characteristics, and the chain pitch and gauge compatible with standard STIHL bars. The guide bar length typically ranges from 20 to 36 inches, with longer bars requiring careful attention to chain tension, drive sprocket condition, and overall machine balance. The chain is designed for consistent lubrication through the integrated oiling system, which should be checked for oil flow rate and resistance to ensure that the chain remains properly lubricated during operation. It is essential to confirm the presence and condition of the anti-vibration system to minimize operator fatigue and improve control during extended cutting sessions.

Safety-related specifications include the presence of a chain brake mechanism, engine kill switch, and a secure throttle trigger interlock system. The unit requires a sturdy, vibration-damped harness or strap when operated for extended periods to reduce the risk of musculoskeletal strain. Proper storage and handling specifications dictate that fuel and oil mixtures are stored in approved containers, away from heat sources, with the machine allowed to cool before refueling to prevent fire hazards. Routine maintenance intervals are tied to operating hours and usage type, with critical checks including spark plug condition, air filter cleanliness, and muffler integrity to ensure efficient combustion and exhaust management.

Electrical and sensor-related specifications for the STIHL MS 661 C emphasize robust ignition components, including the ignition coil and spark plug, which should be tested for consistent spark. Diagnostic routines may involve checking for fuel delivery consistency, choke operation, and the absence of air leaks in the intake manifold. The exhaust system should be inspected for signs of carbon buildup, and any restrictions in the muffler should be cleared only by qualified personnel to avoid compromising emissions compliance and noise levels. Operators should familiarize themselves with recommended lubricants, chain oil types, and maintenance fluids that meet STIHL specifications to preserve performance and prolong service life.

In terms of maintenance tools and equipment, a standard service kit typically includes a chain file or sharpening tool, scrench (a combination wrench and screwdriver), hex wrenches, screwdrivers, a torque wrench or torque driver for critical fasteners, a carburetor adjustment tool, and cleaning brushes. It is important to have a calibrated torque setting chart for fasteners that secure the bar, sprocket cover, and exhaust components, as improper torque can lead to component failure or unsafe operation. A portable, clean work area with a stable surface and proper lighting will facilitate accurate inspection, cleaning, and reassembly after maintenance tasks. Finally, maintain a detailed log of service dates, parts replaced, and any observed performance changes to track the unit’s condition over time and schedule proactive maintenance before failures occur.

Model details for the Stihl MS 661 C include the designation of C as the “C” model line’s enhanced performance variant, featuring improvements such as optimized torque curve, refined air and fuel delivery, and a design focus on reduced operator fatigue and heightened reliability in demanding environments. The engine is designed to maintain compressive strength under heavy loads, with components such as intake manifolds, carburetor housings, and cooling passages aligned for efficient heat dissipation. System diagnostics for this model prioritize consistent ignition timing, stable fuel delivery under varying loads, and chain lubrication that maintains consistent pressure and flow across operating temperatures. Operators should consult the service manual for torque specs and replacement part numbers, and always use genuine STIHL parts to ensure compatibility, performance, and warranty adherence.

Preparation and Inspection

Before attempting any maintenance or repair on the STIHL MS 661 C, ensure you have a clean, well-lit workspace with ample room to maneuver. Gather the necessary tools, including a service manual, a calibrated torque wrench, screwdrivers, pliers, a clean container for small fasteners, and protective equipment such as gloves and safety glasses. Begin by reviewing any troubleshooting notes or service bulletins pertinent to your model to identify common failure points and noted wear items. Establish a safe workflow by disconnecting the spark plug boot and ensuring the saw is cool before handling any components. Take photographs or make notes of original configurations to assist with reassembly, particularly for carburetor settings and decompression mechanisms. A clean environment minimizes the risk of contaminating fuel systems, air pathways, or bearings during disassembly, inspection, and reassembly. Inspect the exterior for any signs of damage, wear, or fuel/oil leaks that could indicate larger issues. Check the chain brake operation and chain tensioner for smooth engagement and proper calibration, as these are critical for safe operation. Verify the anti-vibration system surfaces and mounts for cracks or looseness, which can degrade handling and increase operator fatigue. Examine the muffler and spark arrestor screen for carbon buildup, holes, or deterioration that could affect exhaust flow and engine performance. Look over the fuel and oil lines for hardening, cracking, or signs of leaks, and ensure clamps are secure. Remove the bar and chain only after ensuring the chain brake is engaged and the chain is immobilized, then inspect the bar rails for wear and replace if uneven or excessively worn. After exterior checks, proceed to a more thorough internal inspection following the manufacturer-recommended procedures. Pre start checks and safety devices: Before starting the engine, confirm the correct fuel mix, typically two-stroke oil mixed with gasoline at the recommended ratio, and verify there is adequate fuel within the tank without contamination. Inspect the air filter for cleanliness; a clogged filter restricts air intake and reduces engine efficiency, so clean or replace as necessary and reseal housing to prevent unfiltered air from entering the carburetor. Ensure the air intake system is free of debris, and verify that the carburetor idle and high-speed adjustment screws have the correct settings as specified by the service documentation. Check the choke and throttle linkage for smooth operation and full travel without binding, adjusting as needed to prevent lean or rich running conditions. Confirm the spark plug condition; a fouled or worn plug can indicate combustion issues or fueling irregularities, and replace it with the correct heat range and gap. Finally, perform a functional test by starting the engine in a controlled environment, listening for abnormal noises, checking for exhaust smoke that indicates lubrication or combustion problems, and verifying that the chain brake, stop switch, and throttle lockout function correctly during operation.

Disassembly and Maintenance Procedures

The STIHL MS 661 C chainsaw is a high-performance tool designed for demanding cutting tasks, and proper disassembly and maintenance procedures are essential to preserve safety and performance. Begin with a thorough preparation that includes removing the spark plug boot, ensuring the engine is cool, and draining any residual fuel according to local environmental regulations. When removing covers and components, work on a clean, well-lit surface and use the manufacturer-recommended tools to avoid damaging fasteners or gaskets. Keep a clear layout of parts as you detach them, labeling or photographing each step to facilitate accurate reassembly. Regularly inspect the chainsaw’s body for cracks, loose fittings, or signs of wear, and replace worn or damaged components before they affect performance or safety.

During disassembly, take care to separate the chain brake assembly, anti-vibration mounts, and muffler with attention to bolt torque specifications. The service manual recommends checking the recoil starter, carburetor linkage, and ignition system for smooth operation and proper tension. When removing the bar and chain, secure the saw to prevent movement and carefully unwind the chain to avoid kinematic injury. Clean all ducting and cooling passages to prevent overheating during tests, and inspect seals, o-rings, and the air intake screen for signs of damage or clogging. After reassembly, verify that all fasteners are torqued to the recommended specifications and that a proper seal is achieved at joints and covers to prevent air leaks that can affect engine performance.

Maintenance procedures should follow disassembly to extend the service life of the chainsaw. Replace the air filter with a genuine or OEM-equivalent part and inspect the carburetor for varnish, residue, and fuel deposits. A clean air filter improves combustion efficiency and reduces engine wear. Use appropriate solvents and brushes to remove residue from the carburetor housing, and ensure that vacuum ports and choke mechanisms operate without sticking. Proper lubrication of the bar and chain system is essential; check the oiler for consistent oil flow and adjust as necessary according to the operating conditions and user manual guidelines. Finally, conduct a functional test in a safe environment to verify throttle response, chain brake engagement, and overall starting reliability before returning the tool to service.

Air Filter and Carburetor Inspection and Service

The air filtration system is a critical line of defense against dirt and debris, which can cause accelerated wear and degraded performance in the STIHL MS 661 C. Start by removing the air filter cover and inspecting the foam or pleated filter element for tears, clogging, or saturation with oil. A lightly oiled foam filter can be cleaned with warm soapy water, thoroughly rinsed, and allowed to dry completely before re-oiling with a compatible lubricant. If the filter material shows signs of deterioration or cannot be adequately cleaned, replace it with an OEM-compatible filter to maintain optimal air flow and engine protection. When reinstalling, ensure the rubber gaskets seal properly around the filter housing to prevent unmetered air from entering the intake.

The carburetor itself requires careful inspection for varnish buildup, fuel residue, or stuck diaphragms. Remove the carburetor carefully from the intake manifold, noting the position of all gaskets and springs. Clean the carburetor exterior with a non-corrosive cleaner and use a proper carburetor spray to remove internal deposits while holding the throttle plate in a closed position to avoid accidental spring disassembly. Inspect the fuel lines for cracks or hardening, and replace any lines that show aging or stiffness. Reassemble the carburetor with new gaskets as needed, ensuring proper alignment with the intake manifold and that all connections are secure. After reassembly, synchronize the throttle and choke linkage according to the service manual to guarantee smooth acceleration and reliable starting.

With the air system and carburetor serviced, perform a bench test by priming the carburetor and attempting a controlled start in a well-ventilated area. Listen for unusual noises, verify that the primer bulb (if present) functions correctly, and observe engine response across a range of RPMs. If the engine exhibits weak starting, stalls, or poor idle, recheck the linkage, gasket seals, and air intake path for leaks or misalignment. Finally, document all maintenance actions, including parts replaced, measurements taken, and test results, to support future service and warranty considerations.

The STIHL MS 661 C chainsaw relies on a precise fuel system and lubrication setup to deliver reliable performance under demanding cutting conditions. Proper maintenance of the fuel system helps prevent lean conditions, misfires, and loss of power that can occur from degraded fuels or contaminated passages. Begin by inspecting the fuel lines, fuel filter, and carburetor for signs of wear, cracking, or buildup. Replace any damaged parts and clean the filter element in a clean, solvent-free environment to avoid introducing debris into the system. When reinstalling, ensure all connections are snug but not over-tightened to maintain an airtight seal and consistent fuel delivery.

The carburetor on the MS 661 C must draw a stable fuel-air mixture for optimal combustion. Periodic inspection of the fuel tank venting and cap seals helps maintain correct pressure and prevents vapor lock during operation. Use fresh, clean gasoline mixed with the correct oil ratio specified by STIHL for two-stroke engines, and store fuel in approved containers to minimize contamination. If the engine experiences hard starting or coughing at high RPM, inspect the carburetor diaphragms, spring assemblies, and needle valves for wear, and replace as needed following the manufacturer’s service limits. After any carburetor service, synchronize the high and low speed circuits and perform a test run to verify smooth idle, steady acceleration, and stable torque at cutting speeds.

Lubrication is crucial for guiding bars and extending chain life. The MS 661 C uses an oil delivery system that should maintain a constant flow to the bar and chain during operation. Check the bar oil reservoir for correct oil level before each use and top up with STIHL bar and chain oil that matches your bar length and climate conditions. Inspect the oil nozzle for obstructions and ensure the pump is delivering oil evenly along the bar; a clogged nozzle or worn drive sprocket can cause insufficient lubrication and increased wear. Regularly inspect the bar rails for scoring or heat damage, and rotate or replace bars as needed to preserve even oil distribution and cutting performance.

For fuel mix and lubrication guidelines, always follow the specific STIHL recommendations found in the user manual for your model and serial number range. The correct two-stroke oil to gasoline ratio is essential for reliable lubrication and emission control, and using the wrong mixture can lead to engine damage or excessive smoke. Do not mix fuels with additives that are not approved for STIHL engines, and avoid ethanol-heavy fuels if not specified, as they can degrade rubber seals and fuel lines over time. When storing the unit for extended periods, drain the carburetor and fuel system, run the engine to dry any residual fuel, and store with the air filter clean and dry. Regular maintenance, including cleaning the air filter and spark plug, complements the fuel and lubrication system to maintain peak performance when engaging in heavy-cut tasks.

Chain and Bar Care

Maintaining the chain and bar of your STIHL MS 661 C is essential for optimal cutting performance and equipment longevity. Regular inspection should become part of your routine after every use. Check the guide bar for signs of wear, including unusual grooves, burrs, or dull or chipped edges along the guide rail. Inspect the drive sprocket area for any buildup of sawdust, resin, or metal shavings that could indicate excessive wear. Keep the bar rails clean and free from embedded debris to ensure smooth bar movement and consistent cutting depth. Proper cleaning after each use helps prevent seizing and reduces the likelihood of unpredictable bar behavior during operation. Additionally, apply a light film of bar oil to the bar's upper surface to minimize friction and extend bar life, taking care not to over-lubricate, which can attract dust and sap. Bar maintenance also includes checking the bar oil holes and oil passages for obstructions. Clear any blockages with a small wire or tool recommended by STIHL, ensuring that oil reaches the bar tip during operation. When removing the bar for inspection, take note of any bending or distortion; a bent bar can cause uneven cutting and increased wear on both the chain and bar. Alignment is crucial, so confirm that the bar sits square in the chassis and remains parallel to the chain after reinstallation. Regularly inspect the bar’s edge for heat or flat spots, which indicate overheating or improper tension. If you notice excessive wear, replace the bar promptly to maintain cutting efficiency and reduce the risk of kickback or chain derailment. Chain care begins with evaluating sharpness, tension, and lubrication before each cutting session. A sharp chain produces cleaner cuts with less resistance and reduces engine load, which in turn minimizes fuel consumption and heat generation. Use the manufacturer-recommended chain pitch, gauge, and drive link count to ensure proper seating in the bar groove and on the drive sprocket. Regular lubrication is essential; verify that the bar and chain receive adequate oil during operation, as insufficient lubrication accelerates wear and can cause chain overheating. Inspect the drive links for cracks or signs of stretch, and replace any compromised links promptly to prevent chain derailment or sudden breaks. Store the chain with appropriate tension to prevent kinks during idle periods, but avoid leaving it under high tension when not in use, as this can strengthen fatigue points over time. Chain sharpening and tensioning procedures provide a reliable method to maintain optimal performance without sacrificing chain integrity. Follow the correct sharpening angle specified for your chain type, using a file and guide to maintain consistent edge geometry. After sharpening, verify that the drive links engage smoothly with the sprocket teeth and bar groove, ensuring uniform tooth shape around the entire chain. Test tension by pulling the chain away from the bar; it should have a slight sag at the midpoint, usually equivalent to about a 3-5 mm deflection depending on the chain type and bar length. Re-tension if the chain is too loose, but avoid overtightening, which can cause excessive friction and accelerated wear during operation. When replacing or performing heavy maintenance, always recheck the chain’s tension and guide bar alignment to guarantee safe and efficient cutting performance. Finally, document the maintenance performed, including dates, chain type, and tension readings, to establish a reliable service history that supports predictable operation and facilitates early detection of wear patterns.

Cooling and Exhaust Management

The STIHL MS 661 C is designed with a robust cooling system to maintain optimal operating temperatures under demanding cutting conditions. Proper cooling prevents excessive engine wear, reduces the risk of pre-ignition, and extends service life by maintaining consistent performance. Regular inspection of cooling fins, shrouds, and air passages helps ensure unobstructed airflow, which is critical when operating in high-dust environments or during prolonged use. When debris accumulates, it can form heat pockets that compromise power output and lead to overheating, so a systematic cleaning routine should be integrated into every maintenance cycle. Understanding the relationship between fuel-air mixture and cooling efficiency also aids in diagnosing performance drops that may be temperature-related rather than fuel-related.

Exhaust management in the MS 661 C is essential for safe operation and emissions compliance. The exhaust system is designed to channel combustion byproducts away from the operator while maintaining optimal backpressure for engine performance. Inspecting the exhaust port, muffler, and associated gaskets for cracks, carbon buildup, and leakage helps preserve engine efficiency and reduces the risk of overheating due to restricted flow. It is important to verify that mounting hardware remains tight, as loose connections can cause exhaust leaks, noise increases, and vibration transmission to the chassis. When performing maintenance, use manufacturer-approved replacement components and follow torque specifications to ensure correct sealing and performance.

Engine cooling and exhaust performance are interdependent. A well-maintained cooling system supports consistent exhaust temperatures, which in turn helps maintain stable fuel-air ratios and efficient combustion. Operators should monitor for abnormal engine fade under load, unusual engine temps, or exhaust discoloration, all of which can indicate cooling or exhaust issues. Regular preventive checks, including visual inspection and targeted cleaning, contribute to reliable operation and reduced downtime. In the field, documenting inspection results and any corrective actions helps build a traceable maintenance history that supports warranty compliance and resale value.

Cooling System Maintenance and Exhaust Component Inspection

Effective cooling system maintenance begins with routine cleaning of cooling fins, shrouds, and air intakes. A clogged cooling path restricts air flow, causing localized overheating that can degrade fuel economy and engine longevity. Use low-pressure air, soft brushes, and a microfiber cloth to remove dust and debris without bending fins or damaging delicate fins near the flywheel. Inspect the base gasket and shroud seals for signs of wear or damage, and replace any compromised components to prevent air leaks that diminish cooling efficiency. For best results, perform cooling system maintenance at regular service intervals and after exposure to dusty or woody environments where debris accumulation is more pronounced.

Exhaust component inspection should cover the muffler, exhaust manifold, and connection points for signs of cracking, corrosion, or carbon buildup. Carbon deposits can restrict flow and alter backpressure, impacting engine performance and exhaust temperature. Clean or replace exhaust components according to the manufacturer’s guidelines, and always verify that the exhaust outlet is free of obstructions. Check gasket surfaces for flatness and integrity, and ensure that clamps and fasteners provide even seating to avoid leaks and vibration-induced wear. Document each inspection with notes and, when necessary, schedule a more thorough service to prevent compounding issues.

During combined cooling and exhaust maintenance, pay attention to the interaction between cooling ducts and exhaust heat shields. Damaged heat shields can expose cooling components to excessive heat, accelerating wear. Replace damaged shields promptly and verify that all mounting hardware remains secure after maintenance. After reassembly, run the engine briefly to confirm smooth operation, listening for unusual exhaust tones or changes in cooling fan activity. Adopting a proactive maintenance routine that includes both cooling and exhaust checks will help maintain peak performance, extend component life, and improve operator safety by ensuring reliable engine temperature management.

Troubleshooting and Diagnostics

Troubleshooting and diagnostics for the STIHL MS 661 C require a systematic approach to identify root causes and implement effective repairs. Begin by documenting all user-reported symptoms, operating conditions, and recent maintenance history. A structured diagnostic flow helps isolate issues related to fuel delivery, ignition, compression, and intake systems. When evaluating symptoms, consider not only obvious failures but also intermittent or context-specific problems, such as performance drops under load or temperature-related stalling. Maintaining clean, accurate records facilitates future maintenance and aids in determining whether a component is nearing the end of its service life.

Start with the basics: verify spark, fuel quality, air filtration, and compression. A strong, consistent spark is essential for reliable starting and smooth operation; use a proper spark tester and inspect the ignition coil, plug boot, and wiring for wear or damage. Fuel quality and delivery have a direct impact on engine performance, so inspect the fuel for correct octane, absence of water, and proper mixing ratios if applicable. Check the fuel filter, carburetor passages, and fuel lines for blockages or leaks, ensuring the vacuum and fuel systems respond correctly under the throttle range. Air filtration should be uncompromised, with a clean, undamaged filter and sealing gaskets to prevent unmetered air from entering the intake.

For engine performance issues, assess compression using a reliable gauge and follow STIHL-recommended specifications for the MS 661 C. Low compression often indicates worn piston rings, cylinder damage, or valve seating problems, which may require a rebuild or replacement of critical components. Diagnose carburetion problems by inspecting idle and high-speed jets for varnish or debris, and verify that the diaphragms and gaskets are intact and providing correct metering under all operating conditions. If the saw exhibits hard starting or surging, ensure that the choke and starting mechanism operate smoothly and that fuel is not flooding the carburetor or starving the engine at vacuum events.

Electronics and sensor-related diagnostics should be performed with the appropriate tools and manufacturer guidance. Check for warning codes or diagnostic indicators if the model features any electronic control or smart diagnostics, and verify wiring continuity and insulation integrity. Inspect the muffler and exhaust paths for obstructions or overheating, which can mimic fuel or ignition faults by creating excessive back pressure or restricted flow. Finally, perform a complete functional test after any maintenance, running the saw through its full throttle range on a safe test bench, observing for smooth acceleration, stable idle, and consistent cutting performance.

Common fault symptoms often map to predictable corrective actions, enabling efficient repair workflows. For example, persistent starting difficulty with adequate fuel and spark may indicate carburetor contamination or incorrect jet sizing, which can be resolved by cleaning or rebuilding the carburetor and replacing worn diaphragms. A lack of power under load commonly points to air leaks, such as a cracked intake boot or loose clamps, which should be resealed and tested with a smoke or pressure test. If the engine misfires or runs lean, inspect the air filter housing for leaks, verify the fuel metering is correct, and ensure the intake manifold seals are intact. Constant overheating can result from restricted exhaust or cooling fins not dissipating heat effectively, requiring cleaning of cooling paths and ensuring proper exhaust flow.

Reassembly and Testing

The reassembly process for the STIHL MS 661 C requires careful attention to sequence, torque, and compatibility of all components. Begin by verifying that all fasteners, gaskets, and seals are clean, undamaged, and correctly sized for their corresponding mating surfaces. Reinstall the crankcase halfs with the new or appropriately indicated gaskets, ensuring that the crankshaft rotates freely without binding. When aligning the drive sprocket, confirm that the woodruff key seating is precise and that any shims or spacers are in their original positions as documented during disassembly. Use a calibrated torque wrench to tighten fasteners to the recommended values, gradually and in the prescribed pattern to avoid warping or misalignment of the chassis components.

During reassembly, it is essential to reinstall the carburetor, fuel line, and primer system with meticulous routing to prevent leaks or unintended siphoning. Check that the ignition coil, spark plug boot, and kill switch wiring are properly insulated and secured away from hot surfaces or moving parts. Reconnect the recoil starter and flywheel, making sure the arrestor or inertia mechanisms engage smoothly and that the recoil rope retracts without obstruction. Once assembly is complete, rotate the engine by hand to verify that there is no contact between the piston, cylinder, and internal components before applying fuel or attempting a starting procedure.

Operational testing begins with a controlled safety setup. Confirm that the saw is mounted in a stable stand or held firmly in a workbench vise to prevent movement during testing. Before starting, double-check that the fuel system is sealed, the air filter is clean or correctly installed, and the spark plug is correctly gapped. Start the engine within the manufacturer’s recommended starting procedure and observe for smooth idle, without excessive smoke, misfiring, or unusual noises. Accelerate gradually to ensure the clutch engages correctly and the chain brake aligns with the bar, then verify that the chain spins freely with the teeth properly tracking along the guide bar. If any abnormal sounds or vibrations are detected, halt testing and recheck the associated assemblies for proper torque, alignment, and clearance.

Following initial run testing, perform a diagnostic sweep to confirm compliance with emissions, fuel air mixture, and timing specifications. Use a calibrated tachometer to measure scrubbing stability at idle and mid-range throttle, ensuring that the engine maintains stable RPM without surging. Inspect the muffler and exhaust path for leaks or obstructions, and confirm that all gaskets remain seated during operation. Revisit the cooling system components to verify there is no block or overheating tendency, ensuring cooling fins remain clear of debris. After successful testing, record all measurements, part numbers, and torque values in the service log so future maintenance can reference the exact reassembly conditions and test outcomes.

Safety remains a priority throughout reassembly and testing. Always wear appropriate PPE, including eye protection, gloves, and hearing protection when starting and running the engine. Keep the work area free of flammable materials and ensure proper ventilation during testing to minimize exposure to exhaust and fuel vapors. If any part shows signs of excessive wear or damage that cannot be safely addressed with standard procedures, replace it with OEM components and consult the manufacturer’s specifications. Proper documentation of reassembly steps and testing results helps ensure long-term reliability and safe operation of the STIHL MS 661 C.

Operational checks should include verifying chain tension and lubrication delivery to ensure consistent cutting performance. Confirm that the bar oil system maintains an adequate supply during simulated operation and that the oiler passages are free of obstructions. Re-check the alignment of the sprocket and chain with the bar to prevent uneven wear or kickback risks. Finally, perform a controlled test cut using appropriate wood species and thickness to validate cutting efficiency, chain speed, and overall machine responsiveness. Document all findings and confirm that all safety features such as the chain brake, throttle lock, and stop switch function correctly before returning the tool to service.

In summary, reassembly and testing of the STIHL MS 661 C demand a methodical approach that prioritizes correct assembly sequencing, precise torque, and comprehensive functional checks. Adhering to documented specifications for fuel, ignition timing, and lubrication ensures reliable performance and safety. Thorough testing under varied load conditions helps verify that the machine operates within manufacturer tolerances and provides a dependable baseline for future maintenance or repairs. Keeping detailed records supports traceability and accountability for all service work performed on the unit.

Operational test and safety checks are the final safeguards after reassembly. Run through a standardized checklist that includes compression observations, fuel system integrity, ignition timing, and exhaust integrity. Observe the machine under simulated load, listening for abnormal noises, detecting any chain derailment, and confirming that the chain brake engages promptly. Ensure that all protective covers and guards are reinstalled and secured before any real-world use. If the unit fails any of the checks, halt operation, perform corrective actions, and re-test to confirm that issues have been resolved before declaring the service complete.