Stihl MS 391 Chainsaw

Safety

The Stihl MS 391 is a mid-size saw intended for larger firewood, farm, and cleanup work. Its 64.1 cc engine produces 3.3 kW / 4.4 HP, and the extra torque makes kickback, pull-in, and pushback more severe than on small homeowner saws. Plan the cut, clear footing, and keep helpers outside the felling or bucking zone. Use wedges for bind rather than forcing the bar.

• Wear full chainsaw PPE: helmet, face and eye protection, hearing protection, chaps, gloves, and boots.
• Check chain brake function before every tank.
• Use two hands and keep the thumb wrapped around the front handle.
• Do not run a 25″ setup unless the chain is sharp and the wood supports safe bar control.
• Fuel with clean 50:1 two-stroke mix only after the saw cools.

Unboxing & first run

Inspect the powerhead, bar, chain, scrench, scabbard, and paperwork before setup. The MS 391 powerhead is about 13.7 lb before bar, chain, fluids, and chips, so use a bench or tailgate for assembly. The listed bar range is 18″ to 25″, with a common 20″ setup using 72DL. Fill bar oil before fuel so the first start never runs the chain dry.

• Confirm the chain is 3/8″ standard pitch, .063 gauge.
• Mount the bar, loop the chain over the sprocket, and seat all drive links.
• Tension cold with the bar nose lifted, then tighten both nuts evenly.
• Fill the 21.6 fl oz fuel tank with fresh 50:1 mix.
• Start on the ground or another secure surface, brake engaged, bar clear.
• Verify oil sling from the bar tip before cutting.

Controls layout

The MS 391 control layout is built around gloved operation and higher cutting loads. The front hand guard activates the chain brake manually and by inertia. The rear handle holds the throttle trigger and lockout. The master control lever manages stop, run, starting throttle, and choke positions. The side cover retains the bar, tensioner, clutch drum, rim sprocket area, and chain catcher. Fuel and oil caps are separate and should be cleaned before opening.

• Front handle: wrap grip for felling, bucking, and limbing control.
• Chain brake: push forward to lock; pull back to release before cutting.
• Rear handle: throttle lockout must move freely before the trigger can open.
• Top cover: access for air filter and spark plug service.
• Cutting mount: accepts the listed 18″ to 25″ bar range when matched to chain and sprocket.
• Starter side: keep recoil housing and cooling intake free of chips.

Routine maintenance

The MS 391 should be cleaned and checked whenever it works in chips, bark, or milling-like dust. Large bars increase heat and load, so chain sharpness and oil delivery matter more than engine power. Begin each session with a clean filter, visible bar oil, secure muffler area, and a chain that moves freely by hand. After heavy cutting, let the saw idle briefly before shutdown so heat stabilizes.

• Every tank: check chain tension, bar oil flow, brake action, throttle return, and visible fasteners.
• Daily: clean filter, clutch cover, bar groove, oil ports, sprocket, and cooling fins.
• Weekly: inspect fuel pickup, tank vents, starter rope, rim sprocket, and bar rails.
• Spark plug: inspect and replace if fouled; typical installation torque is 20 N·m unless plug guidance differs.
• Rotate chains so one loop is not run far past the bar and sprocket wear pattern.

Blade or chain replacement

Replace the chain when cutters are cracked, tie straps are damaged, rivets loosen, or the loop has stretched beyond proper adjustment. Replace the bar when rails are uneven, the groove is widened, the tip sprocket binds, or the bar shows heat discoloration from poor oiling. A common 20″ MS 391 setup uses 72DL with 3/8″ standard pitch, .063 gauge; longer bars require a different drive-link count.

• Stop the engine, engage the brake, and let the saw cool.
• Remove clutch cover and back off the chain tensioner.
• Inspect the rim sprocket; replace it if hooked, grooved, or sharp.
• Clean the oil path before fitting the new bar or chain.
• Install cutters forward on the top bar run and tension with the nose lifted.
• Recheck tension after the first heat cycle and again after hard bucking.

Sharpening or cleaning

A large saw with a dull chain wastes fuel and becomes harder to control. The MS 391 should pull chips steadily without heavy feed pressure. If it makes dust, burns the kerf, curves in the cut, or chatters, stop and sharpen. Keep the cutter angles, top-plate length, and depth gauges balanced around the loop. Clean resin and packed chips before filing so the file rides the cutter cleanly.

• Secure the saw at waist height or remove the chain for bench sharpening.
• File every cutter from inside to outside with steady, full strokes.
• Match left and right cutter length to prevent curved cuts.
• Set depth gauges conservatively with the correct tool after several filings.
• Clean bar rails, oil holes, and nose sprocket; then apply bar oil.
• Replace chains that have heat damage, missing chrome, or stiff links that cleaning will not free.

Troubleshooting

Start diagnosis with the cutting system because a sharp 20″ chain changes how the engine feels under load. If the saw starts but bogs, check chain tension, bar oil, filter condition, and fuel quality before turning carburetor screws. If the saw was stored, discard old mix and inspect lines for stiffness or cracks. Stop work immediately if vibration, chain derailment, or brake malfunction appears.

• No start: confirm stop switch position, fresh 50:1 fuel, spark plug condition, and flooded-start procedure.
• Weak in cut: sharpen, check depth gauges, clean filter, and inspect fuel pickup.
• Chain overheats: refill oil, clear oil holes, check bar groove, and reduce feed pressure.
• Throws chain: verify drive-link count, bar groove wear, sprocket wear, and correct tension.
• Excess vibration: inspect rim sprocket, chain damage, bar straightness, clutch drum, and anti-vibration mounts.

Compatible parts

Choose parts by the actual bar stamp and sprocket, not only by model name. The listed MS 391 configuration uses 3/8″ standard pitch, .063 gauge; a 20″ setup commonly uses 72DL. Larger bars may fit but need enough oil, sharp chain, and correct technique. If the saw is used for repeated large-diameter hardwood, keep at least two sharpened loops and one serviceable rim sprocket available.

• AP—20CB—STD: 20″ standard-pitch guide bar for the matching mount.
• AP—72LPX—72: 20″ chain loop, 3/8″, .063 gauge, 72DL.
• AP—AF—MS391: replacement air filter for the MS 391.
• AP—SR—08RIM: rim sprocket service part for compatible setups.

Replace a new chain and badly hooked rim together; mismatched wear shortens chain life and can cause rough engagement.