Stihl Chainsaw Chain Coming Off (Causes & Fixes)
If a STIHL chainsaw chain comes off, stop the engine and do not restart until you find the cause. Check that the chain, guide bar, and drive sprocket match; inspect for wear or damage; then seat every drive link and set tension by the model’s manual. A loose chain, worn parts, or poor lubrication can cause another derailment.
I know how quickly a thrown chain can stall a job and raise the worry that you’ll buy the wrong part. “Find the cause before fitting a replacement,” is the rule I use when diagnosing a derailment. A chain coming off is a symptom, not a diagnosis. The fix may be as simple as correct tension, or it may require replacing a damaged chain, bar, or sprocket.
A chain derailment is when the chain leaves the bar groove or drive sprocket. Stop the saw, keep people clear, and don’t try to guide a moving chain back into place. The checks below help identify the fault before you spend money or cut again.
Equipment Setup and Safety Preparation
A safe inspection starts with the saw stopped and the chain brake engaged. Before touching the chain, clear the work area and put on suitable protective gear. Then inspect the chain, bar, sprocket, and chain catcher; these parts must fit and work together for the chain to stay on.
Wear eye and face protection, hearing protection, chainsaw-protective legwear, and sturdy protective footwear. Keep bystanders away and make sure you have a clear escape path. Stop the engine, engage the chain brake, and wait for moving parts to stop. Never inspect or refit a chain while the engine is running.
Check the fit before adjusting tension
The chain must match the bar and drive sprocket in three ways: pitch, gauge, and drive-link count.
- Pitch is the spacing used to size the chain and sprocket. Match the chain’s pitch to both.
- Gauge is the thickness of the drive links that sit in the bar groove. It must match the groove width.
- Drive-link count is the number of links that travel around the bar. It must match the bar’s listed chain size.
A marking such as 3/8″ P, .050″ tells you the pitch and gauge, but not the drive-link count. It is an example, not a universal STIHL size. Check the bar markings and the correct model manual or parts information. Never compensate for a mismatch by tightening the chain harder.
| What you find | Likely cause | Next step |
|---|---|---|
| Chain lifts easily from the bar’s underside | Chain is too loose | Check fit, then tension by the manual |
| Chain pulls tight in one spot | Damaged chain, bar, or sprocket | Stop and inspect each part |
| Drive links won’t sit in the groove | Wrong gauge or damaged bar groove | Confirm size; replace damaged parts |
| Chain keeps loosening after adjustment | Wear, stretching, or a loose bar | Inspect chain, sprocket, adjuster, and bar nuts |
| Chain runs dry or the bar heats abnormally | Oil delivery problem | Stop and check the oil system |
Refit and tension the chain correctly
With the engine off and chain brake released, remove the bar cover as directed by the manual. Seat the chain around the drive sprocket and in the bar groove. Make sure every drive link sits in the groove, and face the cutters in the correct direction. If you’re unsure, follow the chain diagram in the saw manual.
Set tension using the model’s procedure. In general, the chain should fit snugly against the underside of the bar, with its drive links still engaged in the groove. It should pull around by hand with a gloved hand. Don’t overtighten it; excessive tension does not correct a wrong chain size, a worn sprocket, or a damaged bar.
Tighten the bar nuts to the specification in the manual, then recheck tension. Bar-nut torque is model-specific, so don’t rely on a generic number. After use, let the chain cool before checking again. A hot chain can contract as it cools, so don’t tighten it cold-tight while it’s hot.
Field Cutting and Landscaping Operation
A chain that comes off during cutting can injure you, damage the saw, or cause loss of control. Stop immediately and inspect the chain path before restarting. Cutting load, a pinched bar, or a loose chain can expose a fault, but the chain should not derail during normal use.
Identify what happened before restarting
Note whether the chain came off the bar nose, the underside, or near the clutch cover. This can help guide the inspection, but it does not prove the cause. Check the chain, bar groove, sprocket, bar nuts, adjuster, and chain catcher for damage or unusual wear.
Do not reinstall a chain that is visibly damaged, stretched, or kinked and continue working. Replace damaged parts with parts that match the saw and bar. If the chain catcher or chain brake is broken, do not operate the saw until the safety fault is repaired.
Use suitable chainsaw bar-and-chain oil and confirm that oil reaches the bar and chain according to the manual. If lubrication is poor, friction and heat can increase wear and affect chain movement. Don’t guess at oil-pump output; it varies by model.
Before cutting again, start the saw as the manual directs, with the bar clear of the ground and people. At idle, the chain must remain still. Verify that the chain brake works and that the bar receives oil. Stop and correct any fault before cutting. Use a sharp chain, controlled cuts, and avoid pinching the bar or forcing the saw through wood.
Two common repair patterns
Loose chain after adjustment: A chain that repeatedly sags may be worn or stretched, or the bar nuts or adjuster may not be holding the setting. I check the chain fit and condition, then inspect the adjuster and bar hardware. If the chain is damaged or can no longer be tensioned correctly, replace it with the correct size.
Derailment with visible bar or sprocket wear: A damaged groove or worn drive sprocket can prevent the chain from tracking properly. I inspect the bar rails and groove, then check the sprocket and chain for matching wear. Replace damaged components rather than trying to make them work by over-tightening.
These are diagnostic patterns, not proof of a particular fault. A chain that comes off more than once needs a full inspection before another cut.
Maintenance, Sharpening, and Tuning
Routine checks help catch wear before it leads to another derailment. After stopping the saw and letting the chain cool, inspect its condition and tension. Clean the bar groove and oil ports as directed, and check that the chain, bar, and sprocket remain compatible.
A worn sprocket can fail to guide the chain smoothly. A damaged bar groove, bent bar, or worn rails can also interfere with tracking. Look for damaged or uneven drive links, cracked cutters, and signs that the chain has stretched. Replace parts that are damaged; don’t file or modify them in a way the manufacturer does not specify.
Keep the chain sharp and set its depth gauges with the correct tool and instructions. Depth gauges, also called rakers, control how much wood a cutter can take. .025″ (0.63 mm) is common for many chains, but it is not a universal STIHL setting. Use the chain maker’s specification for the chain you have.
If the chain is dull, don’t force it through wood. A dull chain can make cutting harder and may increase the chance of pinching or poor control. Sharpen it to the specified angles and depth, or have it serviced if the cutters are uneven or damaged.
Fuel is not a direct fix for a chain that comes off. If you need to run a gasoline STIHL saw for a post-repair test, most current models specify a 50:1 fuel-to-two-stroke-oil mix, but check your model’s manual. Never use straight gasoline in a two-stroke engine.
Storage and Seasonal Care
Good storage prevents dirt, moisture, and damage from making chain problems harder to spot. After the saw is stopped and cool, clean the bar groove and oil ports, inspect the chain and bar, and store the saw as its manual directs. Recheck fit and tension before the next use.
Do not store a saw with a damaged chain and assume it will be ready later. Keep the chain protected from impact and corrosion, and follow the manual’s guidance for lubrication and storage. When you bring the saw back into use, confirm the chain size, tension, and oil delivery before cutting.
If the chain came off during the last use, write down what you found and what you replaced. That simple record can help distinguish a one-time adjustment issue from recurring wear or a fault in the tensioning parts.
Frequently Asked Questions
These answers cover the most common reasons a STIHL chain comes off and what to check next. They do not replace the model’s manual: chain sizes, adjustment steps, and service limits vary. If the chain derails again after correct assembly, stop using the saw until the cause is found.
Why does my STIHL chainsaw chain keep coming off?
Common causes include incorrect chain tension, a chain that does not match the bar or sprocket, worn parts, or a damaged bar groove. Check the chain’s pitch, gauge, and drive-link count, then inspect the sprocket and bar. Don’t keep tightening a chain that repeatedly derails.
Can I put the chain back on and keep cutting?
No. Stop the engine and inspect the chain, bar, sprocket, adjuster, and chain catcher first. If the chain is kinked, stretched, cracked, or otherwise damaged, replace it. Restart only after the fault is corrected and the chain brake and oil delivery are confirmed.
How tight should a STIHL chainsaw chain be?
Follow the saw’s manual. As a general check, the chain should sit snugly against the bar’s underside, its drive links should remain engaged in the groove, and it should pull around by hand with a gloved hand. Check again after the chain cools.
Why does my chain become loose after I use the saw?
The chain may have worn or stretched, or the bar nuts, tensioner, or related parts may not be holding the adjustment. A new chain can also need tension rechecks during use. Stop, let it cool, and inspect it; don’t work with a loose chain.
Can the wrong chain size make the chain come off?
Yes. The chain’s pitch, gauge, and drive-link count must match the bar and drive sprocket. A chain that is too narrow, too wide, or the wrong length may not track correctly. Check the bar markings and model-specific parts information before buying a replacement.
Can a worn sprocket throw a chain?
A worn sprocket can interfere with smooth chain travel and contribute to derailment, especially alongside a loose or mismatched chain. Inspect the sprocket teeth and chain for wear, and compare them with the service guidance. Replace worn parts; don’t compensate by overtightening.
Does poor bar oil cause a chain to come off?
Poor lubrication is not the only cause, but it can increase friction, heat, and wear. Use suitable bar-and-chain oil and check delivery by the method in the manual. If oil is not reaching the bar, stop and fix that issue before cutting.
Should I tighten a hot chain?
No. Let the chain cool before checking tension. A hot chain can contract as it cools, so tightening it to cold tension while hot may leave it too tight later. Follow the manual’s procedure and recheck after the saw has cooled.
Bottom line: Don’t treat a thrown chain as a routine adjustment. Match the chain to the bar and sprocket, inspect for damage, set tension correctly, and confirm lubrication and safety systems before cutting again.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
