Stihl MS250: wingnut in sprocket cover keeps coming loose

[buckthorn]

I just replaced the tensioning gear and the sprocket cover for this unit (which has a quick tensioner), which is only 3-4 years old (the first was defective; the second was worn, perhaps as a result of the first). Now when I'm cutting, I can see the darned wingnut slowly turning bit by bit and coming loose. The first couple of times I ran it with the new tensioner, the chain popped off; probably (I now think) because of the wingnut, which kind of freaked me out. After that I started watching it yesterday, I had to keep tightening it because it kept coming loose. Does this need to be replaced too? Has anyone ever had to replace one of these? I don't get it; I have a MS 270 that's more than 10 years old, and I've never had to replaced either the tensioner or the wingnut (or anything, for that matter).

[turnkey]

I just replaced the tensioning gear and the sprocket cover for this unit (which has a quick tensioner), which is only 3-4 years old (the first was defective; the second was worn, perhaps as a result of the first). Now when I'm cutting, I can see the darned wingnut slowly turning bit by bit and coming loose. The first couple of times I ran it with the new tensioner, the chain popped off; probably (I now think) because of the wingnut, which kind of freaked me out. After that I started watching it yesterday, I had to keep tightening it because it kept coming loose. Does this need to be replaced too? Has anyone ever had to replace one of these? I don't get it; I have a MS 270 that's more than 10 years old, and I've never had to replaced either the tensioner or the wingnut (or anything, for that matter).

Can't help with that problem.  I have much the same with my MS361. It likes toloosen both bar cover nuts.  I have had the cover, the chain and the bar drop off while I am cutting a few times.  One would think I'd learn to tighten them occasionally but apparently that is too much trouble.  Happened to me just the other day again.

Harry K

[beenthere]

Have never had a loose bar nut on my MS361.

A mystery to me how that could happen to yours... 

I don't overtighten the nuts, but just snug them up.

[turnkey]

Have never had a loose bar nut on my MS361.

A mystery to me how that could happen to yours... 

I don't overtighten the nuts, but just snug them up.

I don't understand it either.  I run 4 Stihls and have run a bunch of older saws in the past 35 years and none but that 361 have ever done it.

Harry K

[AKDoug]

Are the little teeth on the wing nut that engage the notches in the cover still there? That's what stops it from rotating.

[buckthorn]

Are the little teeth on the wing nut that engage the notches in the cover still there? That's what stops it from rotating.

Hmm, I don't recall seeing any teeth on that piece. When I snapped the old wingnut into the new sprocket cover, I was sort of puzzled as to how the thing is actually supposed to work. Can you please show me a pic of what you mean, or describe it in a little more detail? This may be the key to the who thing.

[AKDoug]

Does this help?
Never mind...need to figure out how to do a picture on this site.

[AKDoug]

See the three teeth, the middle one has a piece of metal on it?

[buckthorn]

Thanks for that pic. Never noticed those before, but I never had to reinstall the wing nut before, either. I take it that all of the quick tensioner wing nuts have these? I'll check when I get home tonight, and will post back.

[buckthorn]

Update: I replaced the wingut ($10) and now everything works fine. Hopefully no more part replacements for a while <knock on wood>

==================
I took a look and it all came back to me. The teeth aren't the issue. The part that you pull out to turn is loose; it should snap tight when you finish tightening the wing nut. That spring must be wornworn (why, I don't know). I now remember seeing that piece flapping around while I was cutting; I'd never seen that before. With the teeth disengaged from the notches, the entire wing nut is free to turn and come loose. Seems like I need a new wing nut now.   
I tell you, the quality of some of these parts seems really poor. This is my **third** tensioning gear; I still have the original on my 10 year-old 270. Sprocket cover. Now the wing nut. Engine runs great, though. And the saw always starts. 

Thanks again for helping me figure this out.

Does this help?
Never mind...need to figure out how to do a picture on this site.

[double clutchin weasel]

Update: I replaced the wingut ($10) and now everything works fine. Hopefully no more part replacements for a while <knock on wood>

==================
I took a look and it all came back to me. The teeth aren't the issue. The part that you pull out to turn is loose; it should snap tight when you finish tightening the wing nut. That spring must be wornworn (why, I don't know). I now remember seeing that piece flapping around while I was cutting; I'd never seen that before. With the teeth disengaged from the notches, the entire wing nut is free to turn and come loose. Seems like I need a new wing nut now.   
I tell you, the quality of some of these parts seems really poor. This is my **third** tensioning gear; I still have the original on my 10 year-old 270. Sprocket cover. Now the wing nut. Engine runs great, though. And the saw always starts. 

Thanks again for helping me figure this out.

Does this help?
Never mind...need to figure out how to do a picture on this site.

The parts to convert this to a standard side-mount adjuster are not all that expensive.  Here are the instructions, straight from Stihl...
.
.
.
1. You will need to remove the aluminum rail in the recess of the engine housing just
under the bar stud.
2. Remove the rear collar screw aft of the bar stud and replace it with 1 bar stud part
#1123 664 2400.
3. You will need the following part numbers for a side access chain tensioner
application: 1123 007 1004 (Spur gear/chain adjusting screw kit), 1123 640 1900
(Tensioner slide), 1123 664 1400 (Thrust pad), 1123 664 2200 (Cover), and 9075 478
3015 (Pan head self-tapping screw).
4. Once you have the parts in the above statement assembled, you need part # 1123
640 1704 (Sprocket cover), and 2 of part # 0000 955 0801 (Bar nuts). This should
complete the conversion process.
.
.
.
There is a kit 1123 007 1000 which contains the entire tensioner.  But, from what I can see, the individual parts may be cheaper.

Aside from the tensioner itself, there are only a few parts (the side cover, the stud, and the nuts).

You're only looking at $40-50 to eliminate the problem forever.

[deerslayer]

may want to ask around to see if someone has used parts on a dead saw if you want to convert to the non c version.

I have an 018C and don't like the tool less chain tensioner any more than I like flippy caps.