Muffler bolts will not stay tight and often fall out

[ChiefH]

I have a Stihl SH-86 (blower/shredder/vacuum) unit and I have having a problem with the muffler bolts (two) coming loose while operating the unit. I have done the following to try to correct: bought new bolt (lost one), both came loose; used red locktite on both bolts and both came loose; I bought some HIGH TEMP loctite, and use d it, and still the bolts came loose. For some reason I can not seem to keep the muffler bolts to stay tight. So des anyone have any suggestions. BTW I emailed Stihl for help and they said see my Stihl repair shop. I has already been there and they said try the High Temp loctite.
I am tempted to fold some aluminum foil into an 1/8" wide strip about 3" long and put it in each hole, use the High Temp loctite and screw the bolt into the cylinder with the thought that the aluminum strip would take up the space and the locktite would help also.

 

[StarTech]

You probably will need the following lock washers. You need #10 / M5 versions

 

[ChiefH]

I ordered 5 of them. I also heard from Stihl and they are saying once the bolts are torqued once then then will not hold anymore.(?). Which is strange since it two occasions I used one new bolt and one old bolt and both cam loose. Not too sure of the torquing story with my experience with new bolts.
Also Sthil did not provide the torque specification. I doubt it can be too high since it is a T-27 sockethead bolt. I twisted a T15 driver in two pieces tying to torque it on my automobile engine intake manifold bolts.

 

[StarTech]

Per SM 9 Nm (approx 80 in/lbs, 6-2/3 ft/lbs)

 

[Fish]

If it won't hold, you may have to switch to a helicoil with the correct fasteners.

 

[7394]

If all else fails drill tiny hole thru bolts heads, & run a wire thru them, like race cars do on some things.

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[StarTech]

If all else fails drill tiny hole thru bolts heads, & run a wire thru them, like race cars do on some things.

​

Just like to see you do that in those deeply recess muffler holes.

As Fish said if torquing them specs strips them out then Helicoiling is the next option. One problem these screws are they are self tappers and when reintalled they cut new threads; unless they are started in the old threads. This takes inserting the screw while turn backwards until you feel them drop into the threads and then tightening up normally. If you don't you just cross thread and cut new threads further weakening the connection.

 

[7394]

Point taken Sir.

 

[john walsh]

Lotsa good info in this thread. If it were me I would take muff off and thread bolts in about 4-5 threads and see if I could wiggle bolts, indicating amount of thread damage in cylinder. If much at all, I would have machine shop Keen-sert (rather than Helicoil)the damaged aluminum threads. I'm guessing that fretting of aluminum in cylinder is causing bolts to take off to points unknown

 

[woodstover]

I ordered 5 of them. I also heard from Stihl and they are saying once the bolts are torqued once then then will not hold anymore.(?). Which is strange since it two occasions I used one new bolt and one old bolt and both cam loose. Not too sure of the torquing story with my experience with new bolts.
Also Sthil did not provide the torque specification. I doubt it can be too high since it is a T-27 sockethead bolt. I twisted a T15 driver in two pieces tying to torque it on my automobile engine intake manifold bolts.

Sounds like BS to me. Mufflers should be able to be removed and re-installed without issues unless of course the internal threads are hooped.