STIHL MS 391 Parts Diagram: Component Breakdown (2026)
The STIHL MS 391 parts diagram details the 64.1 cc engine configuration, including the Walbro HD-49 carburetor system, crankcase assembly, flywheel, and oil pump layout. It maps component part numbers, housing structure, and fastener torque locations required for complete engine reassembly and accurate component identification during chainsaw servicing.
📌 Key Takeaways
- The MS 391 features a 64.1 cc 2-MIX engine structure requiring T27 Torx fasteners tightened to 3.5 Nm on cylinder base bolts.
- Key component identification relies on matching 1140-series OEM part numbers marked on the crankcase and air filter housing layout.
- Inspect the impulse line and intake manifold boot for air leaks before performing carburetor adjustment or high-RPM testing.
- The most common failure point is clutch spring fatigue and clogged spark arrestor screens affecting exhaust gas system flow.
- Replace filters, spark plugs, and sprocket rims via DIY; seek professional service for internal crankshaft and bearing press work.
Technicians, professional arborists, and small-engine mechanics relying on the STIHL MS 391 require precise, factory-level schematics to perform accurate teardowns, system overhauls, and routine repairs. The MS 391 mid-range chainsaw features STIHL’s advanced 2-MIX stratified scavenging engine technology, an active pre-separation air filtration layout, and a heavy-duty anti-vibration spring configuration designed for demanding forestry work. Navigating an official stihl ms 391 parts diagram enables you to cross-reference OEM part numbers, evaluate spatial component relationships, and analyze structural sub-assemblies. This comprehensive technical guide breaks down the MS 391 engine schematic, fuel delivery system, drive clutch assembly, and chassis layout to optimize your service and repair workflows.
Deconstructing the Stihl MS 391 Parts Diagram: Core Systems
The STIHL MS 391 structural architecture is divided into five distinct sub-assembly groups within the master exploded view. Understanding how these mechanical groups interface is critical when diagnosing internal failures or performing full engine rebuilds. Similar to the spatial configuration found on the STIHL MS 362 parts schematic, the MS 391 utilizes a split-crankcase design made from die-cast magnesium, housing a high-torque 64.1 cc powerhead capable of producing 3.3 kW (4.4 bhp).
Powerhead and Engine Cylinder Assembly
The engine diagram group details the 49mm bore cylinder, two-ring piston configuration, and double-counterweighted forged crankshaft. The cylinder base connects directly to the magnesium engine pan via four M5x20 socket head screws. The intake porting configuration incorporates a dual-channel 2-MIX system where fresh air cushions are introduced between the burnt charge and the fresh fuel-air mix in the crankcase. The crankshaft rotates on two heavy-duty deep-groove ball bearings (6202 series), sealed by spring-loaded radial shaft seals on both the flywheel and clutch sides to maintain precise primary compression levels.
Fuel, Air, and Intake Filtration Configuration
The air and fuel schematic illustrates the path from the tank housing through the carburetor flange to the cylinder intake boot. Air enters via a centrifugal pre-separation duct before passing through the high-efficiency HD2 pleated PET filter element (OEM 1141 140 4400). Fuel delivery is managed by a diaphragm-type carburetor (Walbro HD-48 or Zama equivalent) equipped with an integral fuel pump, impulse chamber, and compensator connection. The intake manifold utilizes a flexible rubber boot held by a wire clamping strap to isolate engine vibration from the carburetor housing.
Drive Clutch, Brake System, and Oil Pump Structure
The centrifugal clutch subsystem is mounted directly to the PTO (Power Take-Off) side of the crankshaft. It includes a three-shoe clutch assembly with tension springs, a needle cage bearing, and a spur or rim sprocket drum. The mechanical chain brake utilizes a wrap-around spring band anchored to the crankcase, actuated by the hand guard lever mechanism. Positioned behind the clutch spider is the automatic, gear-driven oil pump (OEM 1140 640 3200). A drive worm gear driven by the clutch drum drives the pump plunger, delivering chain lubricant proportionally to engine RPM.
STIHL MS 391 Factory Specifications: Displacement: 64.1 cc (3.9 cu. in.) | Cylinder Bore: 49 mm | Piston Stroke: 34 mm | Idle Speed: 2,800 RPM | Max Allowable Speed: 13,000 RPM | Ignition Air Gap: 0.2 mm to 0.4 mm (0.008″ – 0.016″) | Spark Plug Gap: 0.5 mm (0.020″)
| Component Group | OEM Part Number | Torque / Fit Spec | Key Technical Function |
|---|---|---|---|
| Cylinder & Piston Kit (49mm) | 1140 020 1205 | 11.5 Nm (M5x20) | Primary power generation, 2-MIX scavenging ports |
| Crankshaft Assembly | 1140 020 1200 | Press-fit / 6202 Bearings | Converts linear piston stroke into rotational force |
| Ignition Module (Microcontroller) | 1140 400 1302 | 4.5 Nm / 0.25mm gap | Generates timed high-voltage arc to spark plug |
| Carburetor Assembly (Walbro) | 1140 120 0600 | 3.5 Nm (Flange nuts) | Mixes air/fuel at stoichiometric ratio across RPM range |
| Adjustable Oil Pump | 1140 640 3200 | 4.0 Nm (Mount screws) | Metres oil to guide bar based on clutch drum rotation |
| Centrifugal Clutch Spider | 1140 160 2000 | 50.0 Nm (Left-Hand Thread) | Transmits torque to drive sprocket above engagement RPM |
How to Read the Stihl MS 391 Schematic Blueprint for Maintenance
Interpreting a technical stihl ms 391 parts diagram requires an understanding of standard STIHL engineering drafting conventions. Explosive views break down three-dimensional mechanical structures into expanded two-dimensional layouts, utilizing numerical callouts, directional lines, and bracketed sub-assemblies. Mastering these conventions ensures fast and error-free component ordering and precise reassembly sequences.
Navigating Explosive Views and Reference Callouts
Each individual component in the MS 391 schematic is assigned an item callout number that matches an entry in the OEM parts list. Dotted lines drawn through components represent an alignment axis, indicating the exact insertion vector during assembly. When multiple components are enclosed within a dashed bounding box, those parts are sold together as a complete service kit—such as the full engine seal and gasket set (OEM 1140 007 1050). If a part number is listed in italics or parentheses, it indicates a regional variant or a part specific to high-altitude/heated-handle configurations.
Tracing Fasteners, Thread Locking, and Torque Requirements
Fastener callouts on the blueprint designate thread pitch, length, and head drive types (typically Torx T27 on STIHL equipment). Pay close attention to standard versus left-hand threaded components on the layout. For instance, the drive clutch spider is explicitly marked with a reverse-thread symbol (LH), indicating that it unthreads clockwise and torques counter-clockwise. Hardware symbol codes also specify where medium-strength (blue) or high-strength (red) liquid thread locker must be applied to prevent vibration-induced loosening.
Identifying Rebuild Kit Sub-Components and Gasket Sets
When performing a complete top-end overhaul as detailed in our MS 391 cylinder assembly replacement manual, interpreting the schematic callouts prevents missing crucial small hardware. Small internal components such as piston circlips (OEM 9553 003 0400), gudgeon pin needle bearings, impulse hose connectors, and cylinder base gaskets are grouped sequentially. Always inspect the schematic line references to verify whether small sealing rings or O-rings are seated in machined grooves or recessed housings before tightening major structural bolts.
Left-Hand Thread Warning: The crankshaft PTO shaft uses a M10x1 left-hand thread for the clutch spider assembly. When removing the clutch using the diagram schematic guidelines, rotate the clutch spider clockwise to loosen. Always install a piston stop plug (OEM 0000 893 5903) into the spark plug hole prior to applying 50 Nm of break-away torque.
Diagnosing System Faults Using the Stihl MS 391 Diagram Layout
A technical parts schematic serves as a primary diagnostic map when troubleshooting engine running faults, air leaks, or mechanical failures. By tracing operational circuits—such as air/fuel flow, vacuum impulse lines, and electrical grounding routes—you can systematically pinpoint broken, worn, or incorrectly installed components.
Resolving Fuel System Delivery and Air Leakage Issues
If the engine exhibits lean running conditions, high idling speeds, or erratic throttle response, use the fuel and intake schematic to trace the vacuum path. Unmetered air entry frequently occurs along the intake boot flange, impulse hose, or crankshaft radial oil seals (OEM 9639 003 1743). Follow these diagnostic steps mapped to the schematic:
- Impulse Hose Connection: Locate the rubber impulse hose running from the rear crankcase port to the back of the carburetor air box base. Check both barb ends for cracking, hardening, or detachment.
- Intake Manifold Seal: Inspect the rubber intake elbow on the schematic. Ensure the internal plastic support sleeve is installed correctly inside the rubber boot to prevent collapse under high vacuum.
- Crankcase Pressure/Vacuum Test: Block the exhaust outlet using a sealing plate (OEM 0000 855 8106) and seal the carburetor intake flange. Apply 0.5 bar (7.25 psi) of positive pressure and negative vacuum to verify seal integrity.
Correcting Chain Oiling and Bar Lubrication Failures
When chain lubrication stops, consult the oil pump diagram section. Inspect the plastic drive worm gear (OEM 1140 640 7100) mounted on the crankshaft behind the clutch. Overheating or a locked chain brake can strip the nylon drive notch on the worm gear arm, preventing the oil pump piston from reciprocating. Verify that the oil suction hose filter pickup inside the tank is unblocked and that the rubber oil delivery connector sleeve interfaces cleanly with the guide bar pad port on the side casing.
Troubleshooting Ignition Interrupter and Electrical Faults
When experiencing no-spark conditions, refer to the electrical and ignition system layout. The ignition module (OEM 1140 400 1302) houses integrated digital timing control circuitry. Trace the black short-circuit wire and ground wire running from the module terminals to the Master Control Switch shaft. Inspect the insulation along the entire wire routing channel in the air box floor; pinched or chafed wiring will ground out the primary ignition loop, preventing spark generation across the electrode gap.
Pressure Testing Safety Limit: Do not exceed 0.5 bar (7.25 psi) during crankcase pressure and vacuum testing. Excessive pressure can dislodge the crankshaft radial shaft seals outward from their press-fit seats in the magnesium crankcase housings, leading to catastrophic engine air leaks during operation.
Stihl MS 391 Parts Diagram Frequently Asked Questions
Are MS 391 components interchangeable with the STIHL MS 311 chainsaw model?
Yes, many structural components are shared between the MS 311 and MS 391 because both saws utilize the same crankcase architecture, air filter housing, recoil starter, handle frame, and oil pump systems. However, key top-end powerhead components are not interchangeable. The MS 391 features a 49mm cylinder bore (64.1 cc), whereas the MS 311 utilizes a 47mm cylinder bore (59.0 cc). Consequently, the piston, piston rings, cylinder, and specific carburetor jet configurations differ between these two models on their respective parts diagrams.
What torque specifications apply to the MS 391 cylinder base bolts?
According to STIHL OEM service standards, the four M5x20 cylinder base mounting socket screws must be torqued to exactly 11.5 Nm (8.5 ft-lbs). During reassembly, clean the tapped threads thoroughly, apply a light coat of medium-strength thread-locking compound, and torque the bolts in a cross-pattern sequence to ensure even seating across the cylinder base gasket surface.
How do I identify left-hand threads on the MS 391 clutch schematic?
On the official parts schematic, the crankshaft PTO end and the clutch spider (OEM 1140 160 2000) are explicitly marked with “LH” or a left-pointing directional arrow. This denotes left-hand metric threading (M10x1 pitch). To remove the clutch assembly, spin the spider clockwise; to tighten it onto the crankshaft shoulder, rotate counter-clockwise to 50.0 Nm (36.8 ft-lbs).
Where is the impulse hose located in the MS 391 parts breakdown?
The impulse hose is located within the carburetor and air intake manifold diagram group. It runs directly from a barbed vacuum port molded into the upper rear quadrant of the engine crankcase to the impulse nipple on the rear housing of the carburetor box. This hose transfers the alternating pressure pulses from the crankcase directly to the carburetor’s internal fuel pump diaphragm.
What is the correct ignition module air gap on the MS 391 blueprint?
The specified air gap clearance between the poles of the electronic ignition module (OEM 1140 400 1302) and the outer magnet shoes of the flywheel is 0.2 mm to 0.4 mm (0.008 in to 0.016 in). Set this gap using a non-magnetic feeler gauge or setting gauge tool (OEM 1111 890 6400) before tightening the two module mounting screws to 4.5 Nm (3.3 ft-lbs). For complete carburetor tuning parameters following ignition servicing, refer to our comprehensive STIHL carburetor tuning guide.
Step-by-Step Guide to Understanding the Stihl Ms 391 Parts Diagram
Identify – Locate the specific sub-assembly section (engine, fuel system, or clutch) on the STIHL MS 391 parts diagram.
Locate – Find the target part number and match the numerical callout on the schematic to your chainsaw structure.
Reference – Cross-check gasket orientation, seal placement, and bolt lengths against the exploded component layout.
Connect/Route – Route fuel lines, impulse hoses, and wiring harnesses through designated handle housing channels per diagram specs.
Verify – Torque T27 fasteners to specification and inspect anti-vibration mounts for correct seating and alignment.
Troubleshoot – Check vacuum and pressure tightness across the crankcase layout if poor idle or air leaks persist.
