Stihl MS 391 Guide Bar (Sizing Guide)
The MS 391 is a 64.1 cm³ chainsaw, but engine size alone does not determine the right guide bar. Choose a bar approved for your saw and market, then match its mount, chain pitch, gauge, and drive-link count. Commonly available lengths run from 16 to 25 inches; check your manual before fitting any length.
Could a bar that fits the saw still be the wrong choice? Yes. A bar and chain can look close enough to install yet mismatch at the drive sprocket, groove, or mounting point. Before heading out, check the bar markings and your MS 391 manual. Those details matter more than choosing the longest bar the saw might accept.
Choosing the right MS 391 guide bar
A guide bar is the steel rail that supports and guides the cutting chain. For the MS 391, choose a bar length listed for your saw in the STIHL documentation for your market, then verify that its mount and chain specifications match the rest of the setup.
The MS 391 has a 64.1 cm³ engine. Bars from 16 to 25 inches are commonly available, but availability does not mean every length is approved for every saw configuration. STIHL specifications can vary by country, bar, and drive setup, so use the current manual or a STIHL dealer’s parts information to confirm the specific combination.
For ordinary work, choose the shortest approved bar that safely reaches the cut. A shorter bar is not a rating for softer wood, and a longer one does not make the saw suitable for harder or larger timber. Wood condition and diameter affect cutting load; keep the chain sharp and avoid forcing it.
| Bar length | Practical consideration | Before buying |
|---|---|---|
| 16–18 in | Shorter reach can suit smaller cuts and may be easier to handle | Confirm length is approved and chain specifications match |
| 20 in | A commonly available middle option | Verify the exact bar mount, pitch, gauge, and link count |
| 25 in | Provides more reach, but adds bar length and can increase cutting demands | Confirm explicit approval for your saw and intended setup |
Treat this as a comparison of available lengths, not a recommendation or approval chart. Your saw’s manual and the markings on the bar and chain take priority.
Matching the bar, chain, and sprocket
Pitch is the size of the chain’s drive links and their spacing. Gauge is the thickness of the drive links where they sit in the bar groove. Both must match across the bar, chain, and drive sprocket; drive-link count must also fit the particular bar.
MS 391 setups commonly use 3/8-inch pitch. Available chain-and-bar configurations may use either .050-inch (1.3 mm) or .063-inch (1.6 mm) gauge. Do not assume which gauge your saw has. Read the bar marking, inspect the chain packaging, and check the sprocket information or STIHL parts data.
| Specification | What to match | Where to confirm |
|---|---|---|
| Bar mount | Bar’s mounting slot and oil openings to the saw | Saw manual and bar part information |
| Pitch | Bar, chain, and drive sprocket | Markings on parts and chain packaging |
| Gauge | Chain drive-link thickness and bar groove | Bar marking and chain specifications |
| Drive-link count | Chain length for that specific bar | Bar marking, chain package, or STIHL parts information |
Do not calculate the drive-link count from bar length alone. Two bars with the same nominal length can require different chains because their design or mount differs. If a marking is worn or missing, pause and identify the part by its product or part number rather than guessing.
I use the same sequence when checking a setup: identify the bar, confirm the pitch and gauge, then verify the chain length and sprocket. It is a simple check, but it prevents a near-match from being treated as a proper fit.
Equipment Setup and Safety Preparation
Before fitting or using a bar, confirm that the parts are compatible and in sound condition. Then inspect the saw’s safety features, chain tension, and lubrication. A correct bar setup does not replace protective equipment, a safe work area, or the operator’s manual.
With the engine off and cool, check the bar mount and oil openings for debris or damage. Fit only a bar specified for the MS 391 configuration you have. Install a chain whose pitch, gauge, and drive-link count match the bar and sprocket. Set tension as the manual directs; the chain should track in the groove without being loose or binding.
Before starting, inspect the chain brake, throttle lockout, chain catcher, fasteners, bar, and chain. Wear a helmet with face and eye protection, hearing protection, chainsaw-rated leg protection, gloves, and boots. Assess footing, overhead hazards, nearby people, tree lean, and a clear escape route. Felling and overhead work require task-specific training and suitable protective measures; follow applicable rules, including ANSI Z133 and OSHA 29 CFR 1910.266 where they apply.
Start the saw using the manual’s procedure, with the chain brake engaged and the saw secured. Let it warm briefly; avoid prolonged idling or high-speed revving. If the chain moves at idle, the brake does not work properly, or the saw behaves abnormally, stop and have it checked.
Field Cutting and Landscaping Operation
Bar length affects reach and handling, not the wood’s hardness rating. A suitable, sharp chain and controlled feed matter in hard, large-diameter, dirty, or frozen wood. If the chain slows or stops cutting cleanly, do not lean harder on the saw; stop and check for dullness, pinching, or a setup problem.
Hold the saw with both hands and stable footing. Keep the upper tip area, often called the kickback zone, clear of wood and other objects. Contact there can throw the bar upward and back toward you. Keep the bar nose clear, avoid tip-first cuts, and use the lower part of the bar where practical. Never cut above shoulder height.
If a cut begins to bind, the chain is pinched, or the tree’s movement becomes unpredictable, stop and reassess. A chain brake reduces risk but cannot prevent every injury. Keep bystanders clear, follow trained felling procedures, and maintain sound hinge wood when felling.
A longer bar can help reach farther into a cut, but it also changes handling and may increase the demands on the saw. It is not a fix for a dull chain or a cut that exceeds the saw’s safe working limits. Choose a bar that reaches the intended work without requiring you to force the saw.
Maintenance, Sharpening, and Tuning
A bar that is correctly sized still needs clean oil passages, a serviceable groove, and a sharp chain. Check lubrication before cutting and inspect the bar and chain after work. Use the chain maker’s sharpening instructions for the specific chain; file size and filing angles depend on chain type, not simply on the saw model.
Use fresh gasoline mixed 50:1 with STIHL two-stroke engine oil, as specified for the MS 391. Never use straight gasoline. For lubrication, use purpose-made, tacky chainsaw bar-and-chain oil. The oiling system is automatic, but delivery still needs checking. Follow the manual’s test method and choose oil suited to the ambient temperature; there is no single field-set flow rate or viscosity that replaces the manual.
After shutting down and allowing the saw to cool, clean the bar groove and oil hole, clear debris from the sprocket cover, and check chain condition and tension. Inspect for damage or wear rather than trying to correct a damaged bar by sharpening the chain. Replace parts that do not meet the manufacturer’s service guidance.
STIHL publishes approximately 2,800 rpm idle and 12,500 rpm maximum engine speed for the MS 391. These figures are specifications, not targets for owner adjustment. Do not tune the carburetor by guesswork; have speed and carburetor settings checked with a tachometer by a qualified service provider.
Storage and Seasonal Care
Storage care protects the bar and chain from dirt, corrosion, and damage, and helps keep the saw ready for its next use. Before storing or transporting the saw, shut it off, let it cool, and follow the manual’s instructions for fuel, chain, and bar protection.
Clean the bar groove and oil opening, inspect the chain and bar, and fit the guide-bar cover for transport or storage. Keep the saw in a clean, dry place, away from children and unauthorized users. Follow the STIHL manual for fuel storage and longer periods out of service; do not improvise a storage procedure based on a different model.
When seasonal conditions change, use bar oil suited to the temperature and check that oil reaches the chain. Before the next job, repeat the compatibility and condition checks rather than assuming a stored setup is ready.
Frequently asked questions
What guide-bar lengths are available for the MS 391?
Bars from 16 to 25 inches are commonly available. Confirm the length is approved for your specific saw and market in current STIHL documentation.
Can I put a 25-inch bar on any MS 391?
Do not assume so. Check the approved configuration, bar mount, chain pitch and gauge, drive-link count, and sprocket compatibility before fitting it.
What chain pitch does the MS 391 use?
MS 391 setups commonly use 3/8-inch pitch. Verify the bar, chain, and drive sprocket all specify the same pitch.
Does the MS 391 use .050- or .063-inch chain gauge?
Both gauges may be available in compatible setups. Match the chain gauge exactly to the bar groove and confirm the correct configuration for your saw.
How do I find the right drive-link count?
Read the bar marking or check the chain package or STIHL parts information. Do not determine link count from bar length alone.
Is a longer bar better for cutting large or hard wood?
Not by itself. Choose an approved bar that safely reaches the cut, keep the chain sharp, and use controlled feed. Wood condition affects cutting load.
What fuel mix does the MS 391 require?
Use a 50:1 gasoline-to-STIHL two-stroke engine oil mixture. Never run the saw on straight gasoline.
What bar oil should I use?
Use purpose-made chainsaw bar-and-chain oil. Follow the manual’s lubrication checks and choose oil suited to the temperature.
Can I adjust the engine speed myself?
The published speed figures are not adjustment targets. Have speed and carburetor settings checked with a tachometer by a qualified service provider.
What should I check if the new chain does not fit?
Stop. Recheck pitch, gauge, drive-link count, bar mount, and sprocket compatibility. Never force a mismatched chain onto the bar or sprocket.
(This article was written by one of our staff writers, David Hensley. Visit our Meet the Team page.)
