Start by locating the crankcase cover on the right side–marked with three Torx T27 screws. Remove these fasteners first to expose the fuel pump diaphragm and impulse line connection. Misalignment during reassembly here causes starting issues. Check the gasket seal (part #1123 020 6001) for cracks; replace if compression loss occurs.
The clutch assembly sits opposite the flywheel, secured by a left-hand thread nut (24mm socket). Before disassembly, inspect the needle bearing (part #1122 005 0300) for wear–excessive play indicates replacement. The bar oil pump connects to the clutch drum via a splined shaft; verify rotation direction when reinstalling to prevent oil starvation.
Inside the engine housing, focus on the piston pin clips (retainer rings). Use snap-ring pliers to remove, then examine the cylinder bore for scoring. The exhaust port accumulates carbon faster–clean with a brass brush to maintain airflow efficiency. For the carburetor, note the HS needle (part #1120 120 0203) position before removal; mark its depth to avoid tuning errors.
Reassembly requires torque specifications: cylinder head bolts (6.5 Nm ±0.5 Nm), flywheel nut (40 Nm), and clutch nut (50 Nm). Apply thread locker (Loctite 243) to bolts prone to vibration. The air filter box incorporates a pre-filtration membrane–damage here causes premature engine wear. Always cross-reference visual guides with the official exploded view (serial-group S-ES-391-101) to confirm part compatibility.
Benchtop Chainsaw Model Breakdown: Key Components for Maintenance
Locate the cylinder housing (part #1123-450-0600) beneath the muffler–it secures the piston assembly and requires torque specs of 18–22 Nm during reassembly. Remove the spark plug (NGK BPMR7A, gap 0.5 mm) first to avoid accidental ignition; store it in a cloth-lined tray to prevent threading damage. The air filter cartridge (felt/paper hybrid, part #1119-120-1223) demands replacement every 25 operating hours in dusty conditions–clean only twice before disposal to maintain airflow efficiency. Tension the cutting chain (3/8″ pitch, 56 drive links) by turning the lateral screw clockwise until the bottom of the links lifts 3–5 mm above the bar rail; verify with a 0.1 mm feeler gauge to prevent binding.
| Component | Tool Required | Torque/Lubrication | Inspection Frequency |
|---|---|---|---|
| Clutch drum | T27 Torx wrench | 8–10 Nm | 50 hours |
| Guide bar bolts | 13 mm socket | 12–15 Nm | After each use |
| Fuel filter | Small hook pick | N/A (replace) | Every 3 months |
| Anti-vibration mounts | Flathead screwdriver | Moly grease (NLGI 2) | Annually |
Locating the Authorized Chainsaw Model 391 Assembly Breakdown
Begin with the manufacturer’s primary service portal at www.stihl.com/technical-service. Filter by region–US, EU, or Asia–and select “Product Documentation.” The platform hosts certified schematics for all power tools, including the 391 series, under “Exploded Views.” Files download instantly as PDFs with vector-based precision, ensuring print clarity down to individual fasteners.
Regional dealers maintain physical archives. Contact an authorized service center via the locator tool on the main site. Request part number 1143 011 4303–the official technical drawing for the 3.9 kW variant. Dealers often email high-resolution scans within hours if digital copies prove elusive.
Specialized repair forums like ArboristSite.com or ChainsawBarrens.com archive user-uploaded microfiche scans. Search threads tagged “engine housing” or “clutch assembly” for the 391 series. Contributors post watermarked OEM diagrams stripped from dealer manuals, though verify source credibility against the manufacturer’s portal.
Corporate Support Channels
Direct technical support via phone (+1-800-784-4533 for North America) confirms diagram availability. Provide the model’s serial number–located on the housing near the muffler–to bypass generic responses. Agents dispatch PDFs or fax copies upon verified ownership, typically within one business day.
Commercial tool distributors like ToolPartsDirect.com embed exploded views in their catalog listings. Navigate to the “Repair Help” section for the MS saw lineup. Diagrams sync with inventory databases, letting users click components to display part numbers and pricing. Note: third-party hosts may delay updates by 3–6 months behind OEM revisions.
Printed literature persists in unexpected places. Obsolete dealer manuals surface on auction platforms (eBay, Facebook Marketplace) under keywords “service book” or “workshop binder.” Target listings specifying “edition year 2020 or newer” to ensure compatibility with post-2018 emissions adjustments. Prices range $15–$40, excluding shipping.
Documentation Workarounds
Reverse-image search engine Yandex Images uncovers diagrams hosted on lesser-known repair blogs. Upload a cropped screenshot of the saw’s housing–the algorithm detects low-visibility schematics masked in forum signatures or spare-parts listings. Refine results by excluding keywords like “clone” or “aftermarket” to avoid counterfeit reproductions.
Locating and Requesting Spare Components with Schematics
Begin by matching the malfunctioning component’s appearance to the exploded view in the manual. Note the item’s designation–typically a numeric or alphanumeric code next to the illustration. This identifier is critical for cross-referencing with dealer inventories or online catalogs. For precision, measure key dimensions like shaft length, thread pitch, or mounting hole spacing if the schematic lacks scale, ensuring compatibility before checkout.
- Filter search results by model variant if available–some engines share components but differ in tolerances.
- Prioritize suppliers listing OEM codes directly in product descriptions to avoid counterfeit risks.
- Use the schematic’s section breakdown (e.g., “Powerhead,” “Carburetor”) to narrow down categories if browsing without the exact code.
When ordering, request the schematic’s revision date from the supplier if the component interfaces with updated calibrations or emission compliance standards. Verify delivery lead times for oversized or uncommon spares (e.g., cylinders, crankshafts) as delays can extend downtime. Keep digital copies of transaction records tied to the schematic’s reference number for warranty claims or future maintenance tracking.
Critical Elements in the Chainsaw Assembly Breakdown
Identify the piston group first–key tolerances for the 64mm bore require micro-measuring clearance between piston rings and cylinder walls. Standard specs demand no more than 0.04mm deviation; deviations exceeding 0.06mm indicate wear requiring full replacement. The piston pin should rotate freely but lack lateral play–secure with circlip retention verified by tug testing.
The carburetor tuning diaphragm’s integrity determines consistent fuel delivery. Inspect for tears or warping; even minor imperfections disrupt the 1:20 oil-to-gas ratio mix. During reassembly, ensure the diaphragm sits flush against the metering lever–misalignment causes lean burns and pre-ignition.
Clean the air filter element with compressed air directed from the inside out. Replace paper filters when particulate accumulation exceeds 30% surface coverage; foam variants tolerate washing but degrade after 10 cycles. The intake manifold’s rubber seal must remain pliable–hardened seals crack under vibration, introducing unfiltered air that accelerates engine wear.
Ignition and Drive Systems
The flywheel’s magnets must maintain a minimum pull force of 1.2 kg when positioned 2mm from the ignition coil. Weaker magnets necessitate flywheel replacement–weak fields result in intermittent spark failure. The ignition coil gap requires precision: 0.3±0.05mm space prevents arcing; misalignment risks coil burnout.
Clutch shoes expand at 3,800 RPM–verify their unfettered movement by hand; seized shoes prevent chain engagement. The spring mechanism’s tension ensures smooth engagement; a stiff spring reduces power transfer efficiency. Replace springs if free length shortens below 45mm.
Examine the sprocket nose bearing for axial play–any detectable movement accelerates chain derailment. Pack with high-temperature grease during installation; insufficient lubrication causes heat buildup and seizing. The guide bar’s oil holes must align perfectly with the powerhead’s ports–misalignment starves the cutting edge, causing accelerated wear.
Structural Integrity
The anti-vibration system’s rubber dampeners degrade under UV exposure; replace every 200 operational hours regardless of visible condition. Cracked dampeners amplify handle vibrations above 5 m/s², risking operator fatigue and loss of control. The front handle’s mounting bolts torque to 25 Nm–loose bolts shear under load, compromising stability.
Inspect the chain brake band’s friction surface for grooves. Replace if uneven wear exceeds 1mm depth; shallow grooves reduce braking force by 30%. The kickback protection mechanism activates within 0.1 seconds–verify responsiveness by triggering the safety with a force of 3-5 N; slower activation risks injury during kickback events.
Precision Guide: Taking Apart and Rebuilding Your Saw Mechanism
Begin by securing the powerhead in a sturdy bench vise lined with protective jaws to prevent scratches. Remove the bar cover screws with a T27 bit, then lift the cover straight off–don’t twist. The clutch drum sits directly beneath; take note of its orientation: the sprocket teeth must align with the oil port when reassembling.
Release the chain tensioner by turning the side-plate adjuster counterclockwise until slack allows the drive link comb to slide off the guide plate groove. Store the chain flat in a shallow container of clean oil to prevent kinking. Next, detach the sprocket housing bolts–use a 10 mm socket and brace the flywheel with a strap wrench to avoid spinning the crankshaft.
Label each screw and spacer as they come out; a muffin tin with labelled sections works well. The carburettor backplate attaches with three 8 mm bolts–mark their exact positions with a permanent marker before removal. Gently pry the intake manifold gasket from the cylinder ports; residue can be scraped with a brass brush but avoid metal picks that might gouge aluminium surfaces.
Extract the piston pin retainers using circlip pliers, keeping thumb pressure against the side cover to prevent spring-loaded clips from launching. The piston itself slides out toward the exhaust port–tilt carefully to clear the crankcase opening without scratching cylinder walls. Inspect piston ring gaps: the top ring gap should measure 0.15–0.25 mm; replace if gaps exceed 0.4 mm.
Clean all disassembled components in non-aromatic degreaser at 45 °C for 20 minutes, agitating every 3 minutes. Dry parts with compressed air, directing nozzles at 45° angles into crevices to dislodge debris. Replace paper gaskets; metal gaskets can be reused if undamaged and lightly coated with high-temperature silicone sealant on reassembly.
Reinstall the piston with arrow markings toward the exhaust port, ensuring rings are seated in grooves and pinned correctly–top ring’s gap clockwise from the locating pin, second ring gap counterclockwise. Apply synthetic assembly grease to the wrist pin and crankpin journals; avoid excess that could clog carburettor jets. Slide the crankshaft assembly into the crankcase, aligning balance weights with the timing marks visible through the inspection port.
Torque all fasteners incrementally: 6 Nm initially, then 10 Nm, then final 12 Nm–cross-pattern sequences prevent warping. Check chain tension by lifting mid-span: 3 mm sag is optimal. Prime the oil pump manually before first start by squeezing the bulb until oil drips steadily from the bar nose; this prevents scoring the guide plate during initial operation.