I`d bet that your chain is "stretching" rather than the tensioner backing off. I don`t know how many bar nuts the 011 has but for the chain to throw because the tensioner is failing would require the tensioner to loosen AND the bar nuts to lose tension on the bar. Are you running a dull chain, worn out sprocket, or not enough oil?

Consider this on drive sprocket wear. As the sprocket wears, it`s effective diameter decreases while the effective pitch of the chain actually increases leading to a vicious cycle of accelerated wear on both components. If wear in excess of .015" is present it`s time for a new sprocket.

Thanks for the reply.

This model has one bar nut.
The chain is still sharp---throws decent size chips
The bar oil is being consumed at an appropriate rate---when I run out of gas, I'm almost out of oil. The brand of bar oil is new to me though---Poulan---it was cheap

I always checked the tension at the start of the day and kept an eye on it during the day. I've never really had any problems with this saw---till just recently. Each time the chain became slack while cutting, I found the tensioner was loose (would take several turns to tighten) but the bar nut was still tight. I thought the tensioner or the tensioning screw might be stripped but the next time the chain fell off, I found the screw had gone missing but the bar nut was still tight.

When I replaced the screw, I was able to determine that the tensioner was not stripped and I thought that the new screw---which snugged down just as it should---was going to solve the problem. Two tanks of gas later, off comes the chain and the screw is missing again. The bar nut is still tight.

I asked about the drive sprocket thinking the chain might be moving around on there and allowing the chain to vibrate loose as the tension varies according to where it's riding on the sprocket???

All this happened right before leaving for work yesterday so I didn't have time to look into it any further. While at work, I posted the question so the night wouldn't be a total waste ;-)

Thanks for the advice on the sprocket. I read somewhere that if you replace the drive sprocket, do not use your existing chains---buy new ones. Do you agree with that?

Just a couple more questions:

Is there an accepted best place on-line to buy parts?
Should I stick with Stihl parts or is aftermarket stuff OK?
Is there a pictorial/tutorial on replacing the drive sprocket?

It is time for a new bar and the drive sprocket is original so I'm thinking of replacing bar, tensioner and sprocket anyway.

Thanks again!

1) You have to apply upward pressure to the end of the bar (especially on single bar nut saws) when you tighten the bar nuts. Chain will get loose every time if you don't.

2) The 011 has a small piece of tubing (P/N 1120 664 2600) that is `sposed to be on the adjuster screw, this keeps the screw from falling out of the case half when it gets loose.

3) If you have an 011 with the original bar and spur on it you should replace both. As far as replacing the spur goes it's pretty simple. Bring the piston up to TDC and back it off counterclockwise about an inch. Feed about 2' of recoil rope thru the plug hole to act as a piston stop. Put a 3/4 socket on the clutch spider and turn it clockwise to remove (ba$tard threaded) Replace the bearing while you're in there and be sure to grease it.

I also agree sprocket has deep worn grooves and the chain pops in the grooves and loosen up. Had this happen to me the end result was the chain tension wouldn't stay adjusted right and the chain would actually lock up like you put the brake on also (IMO the chain would jump out of the worn sprocket and now the chain would be too tight and lock up).

sometimes hitting a hard spot in the wood or going through a knot can cause the chain to Kick this action IMO the chain tooth catches and the rest of the chain is still trying to travel. the result is a knot or bunched up area and it that area is out at the end of the bar it knocks the chain off especially if the tension is not right from worn parts.

If you use this saw alot and want the problem to go away I would replace the sprocket, chain and bar. That's what I had to to on mine to fix the problem of the chain locking up and coming off the bar. I would use stihl parts cost more but they probably will work better and last longer JMO

Thanks all.

I'll try to track down the parts today

mebbe apples and oranges, but the original 3/8 pitch chains on my Stihl 041. stretched to the point that they couldnt be tensioned. I removed the worst looking tooth and contined to use the chain for a while longer. A Felling mishap pinched the original bar about 25 yrs ago and I replaced it. I have since gone through several chains but have never replaced the sprocket or the clutch.

I highly doubt that you`ve worn out the bar on your 011, does the chain rock excessively side to side? Enough to cut crooked even with a new chain? If so, you are supposed to file or grind the rails true again, several times, before you throw the bar away.

Should you stick with OEM or are aftermarket parts OK? I`d buy a Stihl bar(or Oregon Pro Lite) and either a Stihl or Oregon sprocket. Get a rim sprocket set if you can, it will make your next sprocket replacement much cheaper and a rim supports the chain better than a spur sprocket.

Now nothing personal Torowrench, but I have to disagree with some of the following advice:

"If you have an 011 with the original bar and spur on it you should replace both."
Why? Did these components exceed their shelf life? We have no idea how much wear they actually have and you NEVER, and I MEAN NEVER have to replace a bar with the first sprocket. The only people who do replace bars every time they replace a sprocket are unscrupulous OPE techs and consumers who don`t know any better.

"Replace the bearing while you're in there"......why? Did the bearing exceed it`s shelf life also? The only time the bearing is loaded is when the saw is running at idle. When the clutch is engaged the bearing is static.

".........and be sure to grease it."
OK, we agree on that statement.

Plympton,

You can use your old chains with a new sprocket as long as they don`t "click" when you pull them around by hand or go tight...lose...tight...lose when pulled, properly tensioned of course! If a chain has stretched so much that a link needs to be removed I`d can it. Chains don`t really stretch, they wear at the pins and rivet holes and if they have degraded that much they have lost a significant amount of strength.

Barbedwire Thanks for posting the corrections of some other posters, I find myself believing some of the advise given here, without really thinking/asking myself does that make sense? I'm sure the other posters were trying to be helpful.

"I'm sure the other posters were trying to be helpful."

No doubt! I believe in the general goodness of human nature...........but every once in awhile a spoiler comes along.