Loctite?

[Nebraska]

Went through the hardware store washer assortment, holes in the muffler were to small diameter to use them, too much fiddling FWIW and I haven't been anywhere to get the correct loctite product.
 I went ahead and put it together  with the new screws snugged up as much as I dared and the muffler loosened up after about ten minutes, brightside is I only sent one screw to never never land in the grass :....Have a spare, but not going to try again without  loctite. Next time I get to town, unless I get lucky with the local mechanic who might haved some on the shelf.

[Real1shepherd]

You're not gonna get those special Stihl spring washers at a hardware store. I think it's worth the wait to order and try.

I know the Loctite product I recommended earlier will work...but in desperation for not finding it, using another red Loctite product you find at a local hardware/box auto parts store won't work either for long.

Kevin

[Al_Smith]

I've found serrated spring washers on E-bay but they want a kings ransom for them .The Amazon deal for 14 bucks for an assortment of 200 in varied sizes doesn't seem too bad .I have no idea how much they would be from a Stihl dealer .I know often a dealers prices is less then the flim flam flea bay sellers .
As far as a 5 mm spring washer those are easy to find but are  not serrated .Better than nothing I suppose .
I suppose you could put a star washer over the spring washer and get close to the holding power with a large headed Torx screw .BTW not all Stihls show a spring washer,just a Torx head screw .Wow lots of options  just to hold a muffler on . 

[Nebraska]

Local mechanic had a little loctite peg board display so I've got the red stuff. I still might wait til I get to the city to see if the stihl shop has those washers might be better to use both.  On a plus I did get my 7900 out of the shop so I have another saw running  that runs a 24 inch bar. So the Husquvarna can sit a while, and maybe get the slab rack cleaned up.