Help with chain commig off.

[Don_N6CRV]

Hello, a few days ago i got my saw stuck in a cut was able to get it out with the help of a back hoe. It was sucked good but seemed fine when we got it out. The next cut as soon as it hit the wood the chain came off. It was time to stop anyway. When i got it home clreaned the saw and checked the bar by laying it on a table saw and it was flat. The sides looked good and the nose sprocket turned fine. The chain was almost new but thought it might be bent some so i went and put on a new chain. Today used it and on about the 3rd cut off comes the chain. Could the chain saw sprocket be causing the trouble? It is on the 3 chain now. It does show a little ware but not much. The paint is still on the bar so kind of like to keep it a little longer. The saw is a Stihl 021.
Thanks for the help.
Don

[StihlDoc]

If you have worn out three chains you should have the sprocket looked at and replaced if necessary. It probably no longer meshes properly to a new chain. You may also have a spot in the bar rails that has been pinched together slightly which will allow the chain to rise up out of the bar at the tight spot and cause the chain to derail. Check that the chain is tensioned properly and make sure the chain moves around the bar freely with no binding. It should not make any pronounced "clicking" sound.

[Kevin]

Also make sure the bar nose sprocket, chain and drive sprocket are all the same pitch.

[ridgerunner]

i had the same problem with a 056 Stihl a few years ago after free it from a tree top. I used the loader on tractor to unhang it. the next day the chain kept jumping off. What i found was that i had striped two or three theards on chain tension screws. as soon as the chain was put in the cut the bad threards would not hold and throw enough slack in the chain so it would jump off.

[etat]

Before you completely tighten the chain slide it around the bar a few times watching the sproket and the bar and the chain to make sure everything is free and not binding.  If you're still using the chain that was on it when it got in a bind the chain itself may be warped.

[etat]

Ups, noticed you already switched chains.  Scratch that idea about a warped chain. Sorry.

[Don_N6CRV]

Hello, update: Well i put on an old bar and it worked fine. I looked over the new bar real good and it might be pinched at the tip as the sproket is a little tight. Will try it again but kind of think that i messed it up. We just got into some bad trees. Was were cutting yesterday and the trees that looked good were rotted on the inside. We live and learn all the time. Thanks for your help.
Don

[rebocardo]

If you have a bar groove cleaner run it around the whole bar to find the tight spot. When looking at the sprocket on the nose make sure it is straight in the bar and not angled to the side. The other thing to check is that a piece of wood is not jammed in the bar groove. I had that happen doing some deep cutting and it put enough drag on the chain to stall it.