Loosening exhaust bolts

[GearHead36]

I recently took apart my Stihl MS 192 TC chainsaw to clean it and rebuild the carb. I found that the muffler was completely loose. I found one of the two bolts needed. I reinstalled the muffler with the one bolt, and cranked it to see if it made it quieter. IMO, that saw was way too loud for such a small saw. It was quieter, but the muffler started moving around after about 5 seconds. The one bolt had already loosened. I tried the bolt in the other mount hole, but it wouldn't go. I suspect the threads are carboned up. So I need to find a tap to chase the threads, and get another bolt. But even if I do, will the bolts stay tight? Is there some sort of high temp thread locker I need to use?

 

[hlw49]

Are you sure the bolt did not break off in the block?

 

[GearHead36]

Are you sure the bolt did not break off in the block?

That's possible, but I don't think so. The bolt goes in, and slowly gets tighter. If there were a broken bolt in there, I'd expect it to go in easily, then hit a hard stop. I'll check tomorrow.

 

[GearHead36]

The bolt wasn't broken. I stuck a thin spray tube down both holes, and they went to the same depth. I bought a couple of new OEM bolts. Stihl is proud of those bolts. $6 ea. I asked the parts guy what thread it was. D5. Some Stihl specific designation, I'm guessing. That's not SAE or metric. I used a gun cleaning brush, and tried to clean out as much as I could. Then I used the old bolt, and ran it down the clogged hole, a bit at a time. I finally got it all the way in. Then I installed the new bolts. It's much quieter now than before. I also did a carb rebuild, and it also starts & runs much better than before. Previously, it was always a job to get it running, and it was LOUD. Compared to my my CS-590, it was much louder and much harder to start. I was seriously tempted to replace it with an Echo top handle saw. I'm looking forward to using it now.

 

[lowmow]

To clean dirty threads on old bolt hole, cut a notch in your old bolt at the very end with a hacksaw, but only cut on one side of the threads.
This will give the crud somewhere to go instead of jamming the threads.
It might take several passes to clean a really clogged hole.
After cleaning rinse with brake cleaner, and apply RED Loctite (88442) to the threads.
They will not loosen now.
Your bolt should look similar to this:

 

[Peva]

The bolt wasn't broken. I stuck a thin spray tube down both holes, and they went to the same depth. I bought a couple of new OEM bolts. Stihl is proud of those bolts. $6 ea. I asked the parts guy what thread it was. D5. Some Stihl specific designation, I'm guessing. That's not SAE or metric. I used a gun cleaning brush, and tried to clean out as much as I could. Then I used the old bolt, and ran it down the clogged hole, a bit at a time. I finally got it all the way in. Then I installed the new bolts. It's much quieter now than before. I also did a carb rebuild, and it also starts & runs much better than before. Previously, it was always a job to get it running, and it was LOUD. Compared to my my CS-590, it was much louder and much harder to start. I was seriously tempted to replace it with an Echo top handle saw. I'm looking forward to using it now.

The bolt wasn't broken. I stuck a thin spray tube down both holes, and they went to the same depth. I bought a couple of new OEM bolts. Stihl is proud of those bolts. $6 ea. I asked the parts guy what thread it was. D5. Some Stihl specific designation, I'm guessing. That's not SAE or metric. I used a gun cleaning brush, and tried to clean out as much as I could. Then I used the old bolt, and ran it down the clogged hole, a bit at a time. I finally got it all the way in. Then I installed the new bolts. It's much quieter now than before. I also did a carb rebuild, and it also starts & runs much better than before. Previously, it was always a job to get it running, and it was LOUD. Compared to my my CS-590, it was much louder and much harder to start. I was seriously tempted to replace it with an Echo top handle saw. I'm looking forward to using it now.

On the "D5" Stihl screw thread designation, I think you're past needing the information, but I tried to find an answer out of curiosity. I couldn't find a clear definitive explanation, but it appears Stihl bolt part numbers starting with "D" are self-tapping screws typically used for muffler/exhaust attachment.

I think the "5" means 5mm diameter (M5). I see some Stihl screw numbers starting with D5, others starting with D5.3, followed by "x" and two numbers (example: "x 24") obviously meaning length in mm.

The "D5.3" could mean a slightly larger 5.3 mm diameter, or perhaps the ".3" is an option code, such as thread pitch, head style, or finish or plating. Anybody's guess at this point. Perhaps a Stihl dealer could clear up the mystery.

Attached is one D5 Stihl screw listing that I found on line.