Chain wont stay tight on Stihl 021

Ive got a Stihl 021 that has been a great little saw. It starts right up, runs great, but Im having issues with the chain not staying tight.
You can tighten the chain correctly and tighten the nuts on the side cover as hard as you want and after 5-10 minutes of using the saw the chain is loose again.
The other day my brother and I took it apart and the tenioner looks OK, but perhaps the screw is worn and due to the vibration it just works itself loose over time.
Does this sound right? Thats really all that I think it can be. Ive tried various chains and even tried a different bar and Im getting the same results.
The 021 has been a good little saw to me. 8 years ago I traded a pizza oven for it and its never given me any trouble up to now. Its just annoying that you have to stop every 5-10 minutes and retighten the chain. I know that the chain is supposed to slacken somewhat after you use the saw for a while just due to the chain heating up a little, but this is to the point where the chain will almost come off of the bar.
Im honestly to the point where Im about ready to just go out and buy a different chainsaw, but if its something simple and inexpensive like the tensioner being bad, Id hate to go out and spend $300 on a new saw when I could fix my old one for $20.

Slap me if I'm telling you how to suck eggs...

Are you holding the bar nose up when you tighten the chain and side nuts?
If you don't, the chain will slacken off regardless how tight you do the side nuts.
Double, triple check you're getting the tensioner post into the hole in the bar...

If you've had it for 8 years, its probably going to be the tensioner assembly, check it all out, is any of the plastic around the assembly worn or broken, how does the assembly itself look?
I can't think it would be too much $$ for a new assembly, but if the plastic seating area it worn, then there will be more to do...

My 021 is just a little older than yours, still going strong!!

Your sprocket is likely worn out, and you need that, and a new chain.

Bermie said:

Slap me if I'm telling you how to suck eggs...

Are you holding the bar nose up when you tighten the chain and side nuts?
If you don't, the chain will slacken off regardless how tight you do the side nuts.

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Yup, Im making sure that I hold the bar up when I tighten the chain and the nuts.

Double, triple check you're getting the tensioner post into the hole in the bar...

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Trust me, its in there.

If you've had it for 8 years, its probably going to be the tensioner assembly, check it all out, is any of the plastic around the assembly worn or broken, how does the assembly itself look?
I can't think it would be too much $$ for a new assembly, but if the plastic seating area it worn, then there will be more to do...

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The plastic that it sits in isnt worn, its fits in there snugly. I was looking at it today and I noticed that it doesnt take much force to turn the screw. I could hold the screw between my thumb and index finger and turn it pretty easily.

My 021 is just a little older than yours, still going strong!!

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The saw itself is 10-15 years old and it still runs great. It starts in 2-4 pulls and runs strong. Thats why I hate to go out and spend $200-$300 on another saw if its a simple fix.
I'll try a new tensioner and see if that fixes it. Im not sure how to change the sprocket and Id bet that for what it would cost to have it fixed for me I could just go out and buy a new Echo CS-370, so I see little sense in investing the money to fix my 021 unless its a cheap fix.
Thanks for all the help.

A new sprocket and chain will be very reasonable.

Im not too worried about the cost of a new chain, that should be about $35. Even the cost of the sprocket shouldnt be too bad, but I dont know how to change it and the cost of having it changed for me is going to be the real killer.
Like I said, I could go out and buy a new Echo that is comparable to my old 021 for $200, so if its going to cost me more than $200 to fix my old saw, I might as well just go out and buy a new one.
No doubt I could sell my 021 on Ebay and get at least $100 for it.

Remove the E clip and the sprocket comes off.

ray benson said:

Remove the E clip and the sprocket comes off.

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Ok, thanks!

Just out of curiosity, why would the sprocket cause the chain to not stay tight?

Bakemono said:

Just out of curiosity, why would the sprocket cause the chain to not stay tight?

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That's a good question!

I'll take my best uneducated guess here ....

With a new sprocket and chain, the drive links on the chain fit nice and snug into your sprocket. When your sprocket is worn, you can seemingly get it tight, but what it is really doing is snugging up against one side of the recesses in the sprocket. When you run your chain, your drive links maneuver themselves to the area of the sprocket that provides the least resistance, thereby allowing the chain to loosen.

Now one the guys that actually know can let us both know if I am correct.

Wow, chain prices up there must be real high

Sprocket should be $15.95 from a Stihl dealer, $12 from anyone else.
Good loop of Oregon or Carlton chain @ $15-$18, 3 minutes to replace.

Chains wear grooves in the sprocket teeth. The chain shifts in and out of the worn grooves when running and causes the chain tension to change.

Would I be able to see these grooves on the sprocket?

Fish said:

Wow, chain prices up there must be real high

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I could probably get an Oregon chain for about $20 at the local Farm and Fleet. I know that you pay a premium for anything that says, "Stihl" on it...
At least now I have an excuse to get rid of that stupid "safety chain" (low kickback) that came on my saw.

here is a good sprocket, beside one with some wear, yours is likely notched
much worse.

Hmmm, ok. I'll have to take a look at it sometime today. Thanks.

Chain won't stay tight on stihl 021

You could have to do what I did and put studs in stead of the bolts. Then use a nut as a jamnut. Then I ground down the top and bottom so that the bar would slide over them taking the slop out of the bar and keeping the chain tight. Its worked for me for about 2 years now on my 021.

Fish said:

Wow, chain prices up there must be real high

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That's what I was thinkin'

Sound's like you know what you're doing, but I have to add... I've got an 021 also, and I experienced the same thing you described and it turned out my oiler was clogged not getting enough bar oil to the bar/chain causing it to heat up and expand, thus was loosing tension about the timeframe you stated. So, check to make sure you are getting plenty of bar oil and the chain is not running dry.