When a chainsaw chain keeps coming off, it is almost always tension, the wrong chain for the bar, or a worn bar or sprocket. When the chain smokes, it is usually a dull chain or not enough bar oil. Check them in that order, with the saw off and cool.
I run a Stihl MS 251 C and an older MS 230 on our 10 acres. Below is what Husqvarna and Stihl say causes each one, and the order I check them. If you’re still sizing up a saw, my gas chainsaw buying guide starts there.
Shut the engine off first. Stihl says to let the saw cool down before you touch it, and to wear protective gloves when you handle the bar.
Why the chain keeps coming off
1. The tension is off
Husqvarna’s rule of thumb: a cold chain should be “tight enough to not sag below the guide bar, but loose enough that you can move it by hand.” A new chain stretches, so Husqvarna says to expect frequent adjustment at first. If you set tension on a hot chain, Husqvarna says to leave it a little slack, sagging “about the width of a medium screwdriver tip” under the bar, because it tightens as it cools.
On my MS 251 C it’s a wing nut and an adjusting wheel on the side cover, no tools. The steps for that and for the regular nuts-and-screw tensioner are in my dull chainsaw chain guide.
2. The chain doesn’t match the bar
Husqvarna says to check that “you are using the proper chain pitch, gauge and number of drive links to match the guide bar.” A chain one link off, or one gauge off, will never sit right. Read the numbers stamped on your bar. My Stihl MS 251 chain size guide shows how.
3. Brush is knocking it off
Husqvarna warns that small branches can lodge “between the chain and sprocket, kicking the chain off track,” and says to use a hedge trimmer or brush cutter for brush instead.
4. The bar or sprocket is worn
Stihl’s UK blog says the rails of a guide bar wear away over time, which reduces the groove depth and leaves less room for the drive links, and that “a seriously worn chain sprocket can cause the chainsaw chain and guide bar to wear at an increased rate.” Stihl says to replace the sprocket when its wear marks reach about 0.5 mm, and that a guide bar can last the lifespan of about 4 chains. If your chain keeps coming off after you’ve fixed tension and matched the chain, have your dealer look at the bar and sprocket.
Why the chain smokes
1. The chain is dull
Husqvarna says a smoking bar and chain is most commonly a dull chain, and that too much pressure on the bar makes it worse. A sharp chain cuts with little pushing. Stihl says “it is time to sharpen if you see smoke coming from the cut despite chain lubrication and correct chain tensioning.” The other signs, from Stihl: fine sawdust instead of chips, a saw that pulls to one side, and a chain that chatters and bounces.
Dirt. Cut a log laying in the dirt and graze the saw tip in the dirt. BINGO, dull chain, just that quick. I keep a sharp spare for each saw and let my dealer sharpen the dull one, about $15 a chain last year.
2. Not enough bar oil, or the wrong oil
Husqvarna lists an empty oil tank or a blocked oil hole as a cause, and says low-quality oil gets flung off the chain instead of lubricating it. Its fix: refill, clear the oil hole, and use an oil made for saw chains. I run Stihl Platinum bar oil all year in every Stihl I own and fill it every time I fill the gas. More in my Stihl bar oil guide.
3. The smoke is from the engine, not the chain
Light gray smoke from the muffler is a different problem. Husqvarna lists too much oil in the fuel mix, old fuel, and a dirty air filter. Mix fresh at the right ratio; mine is 50:1, in my Stihl chainsaw fuel mix guide. If it still smokes, Husqvarna says to contact a dealer.
The order I check it
| What you see | Check first | Then |
|---|---|---|
| Chain comes off | Tension, cold | Chain matches the bar; then bar rails and sprocket at the dealer |
| Smoke from the cut | Is the chain dull? | Bar oil level, oil hole, right oil |
| Smoke from the muffler | Fresh fuel at the right mix | Air filter; then the dealer |
When to take it to the dealer
Bar and sprocket wear, an oiler that won’t pump, and smoke that won’t quit after fresh fuel and a clean filter all go to the dealer. On a Stihl still under warranty, that’s the safer call anyway. Find yours on the Stihl dealer locator.
The bottom line
A chain that keeps coming off is usually loose, mismatched to the bar, or running on a worn bar or sprocket. A chain that smokes is usually dull or short on oil. Set tension cold, match the chain to the numbers on your bar, keep a sharp spare, and fill the bar oil with every tank of gas.
Chainsaw chain coming off FAQ
Why does my chainsaw chain keep coming off the bar?
Husqvarna lists three causes: the chain is too loose, the chain’s pitch, gauge or drive-link count doesn’t match the bar, or brush is getting caught between the chain and sprocket. A worn bar or sprocket can add to it.
How tight should a chainsaw chain be?
Husqvarna says a cold chain should be tight enough not to sag below the bar but loose enough to move by hand. Check it often on a new chain, because new chains stretch.
Why is my chainsaw chain smoking?
Usually a dull chain or not enough bar oil. Stihl says smoke from the cut, even with good oil and tension, means it’s time to sharpen.
What happens if a chainsaw chain is too loose?
It can come off the bar. Husqvarna says never to let the drive links come completely out of the guide bar channel, so tighten it before you cut again.
When should I replace the chainsaw sprocket?
Stihl says when the wear marks on it reach about 0.5 mm, the wear limit. A badly worn sprocket wears out the chain and bar faster.
Sources
Every source below was opened and checked on 1 November 2026.
- How to stop the chain from coming off the bar: tension, matching chain, brush (Husqvarna US support)
- Why do the bar and chain smoke and cut poorly? (Husqvarna support)
- Why is my petrol chainsaw smoking excessively? (Husqvarna US support)
- How to sharpen a chainsaw: signs of a dull chain (Stihl USA)
- When to replace a chainsaw chain, sprocket or guide bar (Stihl GB)
- Preparing for Proper Chainsaw Storage: let it cool, gloves for the bar (Stihl USA)
- Find a Stihl dealer (Stihl USA)
