Stihl MS 391 Chainsaw (Firewood Cutting Test)
The STIHL MS 391 is a 64.1-cm³, 3.3-kW saw suited to controlled firewood bucking when fitted with a manufacturer-approved bar and chain. For routine rounds, an approved 18–20-inch setup is often easier to manage than the longest option. This guide explains what to check, how to assess performance safely, and what the specifications can and cannot prove.
As colder weather approaches, many owners start cutting and stacking wood before the busiest part of the heating season. If you are comparing saws now, the MS 391’s power rating and bar range may look reassuring, but numbers alone do not show how it will feel in your woodpile.
I assess a firewood saw by its fit for the work, its condition, and how it cuts with a sharp, correctly matched chain. I do not have verified timed-cut results or measured fuel-use figures for this model to report, so I will not invent a “test time.” Instead, this guide gives you the confirmed specifications and a practical way to judge the saw during a controlled cutting test.
What the MS 391’s specifications mean for firewood
The MS 391 has a 64.1-cm³ engine rated at 3.3 kW, or 4.5 bhp. Those figures describe engine size and rated output, not a guaranteed cutting rate. Wood species, log diameter, chain sharpness, bar length, and operator technique all affect how quickly the saw bucks firewood.
For this kind of work, bar length is a trade-off. A longer bar can reach farther across a log, but it adds reach and may be less convenient to handle. An approved 18- or 20-inch bar is a sensible starting point for many routine firewood tasks; it is not a promise that either length suits every user or log.
| Item | Confirmed specification or guidance | What it means in a firewood test |
|---|---|---|
| Engine displacement | 64.1 cm³ | Engine size; not a measure of cut time |
| Rated power | 3.3 kW (4.5 bhp) | Useful for comparing stated output, not a guarantee of performance |
| Approved bar range | 16–25 in. | Use only a listed bar-and-chain combination |
| Common firewood setup | Approved 18–20 in. bar | Often more manageable than choosing the longest approved option |
| Chain pitch | 3/8 in. standard pitch | Check the bar, chain, and drive sprocket markings |
| Chain gauge | Bar-dependent; often .050 in. (1.3 mm) | Do not assume all compatible bars use the same gauge |
| Idle speed | 2,800 rpm | The chain must remain stopped at idle |
| Maximum engine speed | 12,500 rpm | Do not adjust the carburetor to exceed this limit |
| Fuel mix | 50:1 gasoline to STIHL two-stroke oil | Prepare fresh mix and follow the applicable manual |
A chain gauge is the drive-link thickness that fits the bar groove. Pitch is the distance that defines the chain’s size and fit with the sprocket. Both must match the equipment. Check the markings on the bar and chain, then confirm the drive sprocket is compatible before cutting.
Equipment Setup and Safety Preparation
Before testing a saw, confirm that its safety parts work and its cutting gear matches. Wear a helmet, eye and hearing protection, chainsaw-protective legwear, gloves, and suitable boots. Clear the work area, set a stable footing, and keep bystanders outside the work zone.
Inspect the chain brake, throttle lockout, chain catcher, handles, bar, and chain. Check chain tension and make sure the chain receives oil. The chain must stop at idle; if it keeps moving, stop and have the saw checked before cutting. For workplace use, follow relevant OSHA rules and ANSI Z133 practices.
Use fresh fuel mixed at 50:1 with STIHL two-stroke engine oil, subject to the saw’s manual and fuel requirements. A wrong mix can damage the engine. Do not guess at the ratio, use straight gasoline, or store fuel in a way that conflicts with the manual.
Start on stable ground and follow the manual’s cold- or warm-start steps. Engage the chain brake for starting and transport. Before cutting, verify that oil reaches the chain, then release the brake only when ready to work. A proper setup is part of the test, not a step to skip.
Firewood Cutting and Field Operation
A useful cutting test checks control and cut quality as well as speed. Support each log so it cannot roll, and consider where it is under compression or tension before starting a cut. Keep the bar nose clear, hold the saw with both hands, and cut from a stable stance without reaching or working above shoulder height.
Rotational kickback is a sudden upward and backward movement that can happen if the upper tip of the bar hits wood or another object. Keep that tip clear, use a firm two-handed grip, and do not cut with a damaged or poorly tensioned chain. Stop if the log shifts or the saw does not cut as expected.
For a fair comparison, use similar logs and the same bar-and-chain setup. Note the wood species, approximate diameter, chain condition, and whether the logs are dry or green. Do not compare one cut in small, dry wood with another in large, wet wood and treat the time difference as a saw ranking.
| Test step | What to record | What a sound result looks like |
|---|---|---|
| Check the setup | Bar length, chain pitch and gauge, chain condition | All parts match the saw and are listed as compatible |
| Prepare the log | Species, diameter, support, and movement | Log is stable and the cut path is clear |
| Make several cuts | Cut time, control, chip quality, and any stalls | Saw cuts smoothly without forcing or unusual movement |
| Recheck the saw | Chain tension, lubrication, and visible damage | Chain remains correctly tensioned and oil reaches the bar |
| Compare results | Similar logs, same operator and setup | Differences are judged in context, not from one cut |
I treat a slow cut as a reason to inspect, not to push harder. A dull chain can make a powerful saw feel weak. Stop, inspect, and sharpen or replace the chain when needed. Forcing a dull chain raises heat and wear and can increase kickback exposure.
These are field-use scenarios, not claims of a formal timed test. In smaller, manageable rounds, an approved 18- or 20-inch setup may give a good balance of reach and handling. Larger rounds may need a different approach or equipment, but a longer bar is not an automatic upgrade. Stay within the approved combinations and the limits of your skill.
Maintenance, Chain Sharpening, and Tuning
A firewood test is only useful when the chain and bar are in good condition. Keep the chain sharp, confirm correct tension, and clean sawdust from the saw and oil passages. Stop the engine before inspection or adjustment, and allow hot parts to cool before service.
Depth gauges limit how deeply each cutter bites into the wood. Set them to the chain maker’s directions. For an applicable STIHL 3/8-inch chain, 0.65 mm (0.026 in.) is a common filing-gauge value, but chain models differ. Confirm the exact chain model before filing rather than relying on pitch alone.
The stated engine speeds are 2,800 rpm at idle and 12,500 rpm maximum. Do not change carburetor settings to exceed the manufacturer’s limit. If the saw idles poorly, the chain moves at idle, or cutting performance stays weak with a sharp chain, use the manual or qualified service support to diagnose it.
After cutting, engage the brake and shut down the saw before maintenance. Check the bar and chain for wear or damage, and retension only when it is safe to do so. A damaged chain or bar is not a test condition to work around; repair or replace it before using the saw again.
Storage and Seasonal Care
Good storage keeps the saw ready for the next firewood session. Clean it after use, inspect the bar and chain, and follow the manual’s directions for fuel and long-term storage. Allow the saw to cool before refueling or doing maintenance, and keep fuel away from heat or ignition sources.
Before the next season, review the saw’s condition rather than assuming it is ready because it ran well last year. Check the safety parts, chain tension, bar groove, and chain lubrication before starting. If you cannot confirm safe operation, do not use the saw until it has been inspected.
Is the MS 391 a good match for your firewood work?
The MS 391’s rated output and approved bar range make it a candidate for controlled firewood bucking, but the best fit depends on your wood and handling needs. I would compare a compatible 18- or 20-inch setup first for routine work, then confirm the exact bar and chain match in STIHL’s applicable documentation.
The strongest buying decision comes from more than one specification. Check the saw’s condition, verify the cutting setup, and try a controlled cut in representative wood if possible. Since no verified timed test data is available here, treat any precise claim about cords per hour, fuel use, or superiority over another model as unproven unless the test method and conditions are shown.
FAQ
What bar length should I use on an MS 391 for firewood?
Choose only a bar-and-chain combination listed for the saw. Approved lengths span 16–25 inches; an approved 18- or 20-inch setup is often more manageable for routine bucking than the longest option.
What is the MS 391’s engine size and rated power?
Its displacement is 64.1 cm³, and its rated power is 3.3 kW, or 4.5 bhp. These figures do not predict a specific cut time.
What chain pitch does the MS 391 use?
The standard pitch is 3/8 inch. Chain gauge depends on the bar, so match the markings on the bar, chain, and drive sprocket.
Is .050-inch chain gauge correct for every MS 391 bar?
No. .050 inch (1.3 mm) is common, but compatible configurations may use another gauge. Confirm the bar and chain markings before use.
What fuel mix does the saw require?
The stated mix is 50:1 gasoline to STIHL two-stroke engine oil. Prepare fresh fuel and follow the applicable manual and fuel requirements.
What should I do if the saw cuts slowly?
Stop and inspect the chain, bar, tension, and lubrication. Sharpen or replace a dull chain; do not force it through wood.
What depth-gauge setting should I use?
Follow the chain maker’s instructions. For an applicable STIHL 3/8-inch chain, 0.65 mm (0.026 in.) is common; confirm the exact chain model before filing.
Can I raise the maximum engine speed for more power?
No. The stated maximum is 12,500 rpm. Do not adjust the carburetor to exceed the manufacturer’s limit.
Why does the chain move while the saw idles?
The chain should stop at idle. Shut the saw down and have the cause checked before cutting.
Is there a verified cut-time result for this guide?
No. Without a documented test setup and measured results, a precise cut-time or fuel-use claim would not be reliable.
(This article was written by one of our staff writers, Marcus Thorne. Visit our Meet the Team page.)
