How to prevent cracked muffler on 912uls

[avidflyer]

Last summer, I got a 912 muffler that was cracked up pretty good in the same place. Interestingly all the exhaust pipes were stuck in the muffler and wouldn't come out for anything. I ended up making a cut up the length of each nipple and was able to spread/expand the nipples that way and got the exhaust pipes out. Had it all welded back together, and I think it will last a good while in the next project. I ended up with the exhaust pipes being looser in the muffler than they probably were from the factory, but that will likely help things move around more and maybe not as likely to crack. Could be something to take a look at as well. A looser connection may leak a bit of exhaust, but not as much as a cracked muffler I'm thinking. Especially if the entire pipe breaks off. JImChuk

[3kdscf]

Interesting, my four pipes came out of the muffler very easily and also came off the block easily. When I installed my muffler I used loctite LB 8150 anti-seize on the muffler end of each pipe as well as the engine block end of each pipe. It makes sense that if they became stuck that it could lead to cracking just from the thermal contraction and expansion.

[Dave S]

Interesting discussion - my experience mimics Dan's. The original installation involved a generous application of the silver/aluminum paste antisieze compound. In replacing the engine this winter (after a mere 500 hours) the muffler and pipes came apart with hand bumps. Also, the muffler has a brace going between the stinger and the muffler. Inspection after disassembly revealed no cracks.

[Delta Whisky]

If this is a contest, then I win! The shame of it all, I read this thread a couple of weeks ago but failed to look as closely as I should have before its last flight. You can see from the picture, I should have bent down and looked really close - it appears that there should have been some evidence of the impending doom. (Its there now!)

And, the last contact the vagrant exhaust pipe made as it left its welded home was one of the new fairings on the good ol Monster Shock.

Empty exhaust hole.jpgMissing exhaust pipe.jpg

Now I need two things: the ability to rotate these pictures and a new exhaust muffler.

[Shadowrider]

Oh dang that stinks! Did you get to fly before it broke and test the new fairings out?

[avidflyer]

Wonder if you would dare to weld in another elbow? Add a patch on the end of the muffler with the correct size hole in it, weld in the new elbow, and then add the braces from edge of muffler to new elbow. I wish now I had put braces in mine when I had it at the welder. JImChuk

[Delta Whisky]

Dustin - your question touched on the irony of the planned flight. That day, Monday, showed hopes of being a relative calm day aloft and was the reason for the planned flight. I had previously made two flights in hopes of obtaining decent data but the turbulence makes the data suspect. None-the-less, I've started an analysis and, as soon as a head cold clears, I'll see if any part of what I have is useful.

Jim - Your idea has merit but one detail of what a close look into the muffler reveals gives me pause. I'll try to get a better picture the next time I'm at the airport but it appears the exhaust pipe/stinger is (or was) an extension of an internal pipe that runs the length of the muffler. This pipe contains numerous holes the exhaust gases have to pass thru on their way to freedom. If that is the case, four breaks were extremely clean and normal to the center line of the pipe.

[jrevens]

I may be wrong, but it seems to me that this issue could be due to the design &/or construction of the Kitfox-specific designed/built exhaust system. There doesn’t seem to be the same frequency of these kinds of failures with the genuine Rotax exhaust systems, for instance, used on many other designs.

[AvDES LLC]

you make a good point. I haven’t seen under the cowling of any planes with 912’s installed except kitfoxes. That said,it would be interesting to see how the cowl is shaped around the exhaust of other aircraft with the 912 as this would give us an idea as to how close other designers have placed the fiberglass.

It’s a bummer to see so many of us have had the same issue.

[avidflyer]

I have a different muffler that has ball joints near the muffler, and then shorter pieces go into the muffler. 2 pipes instead of one for each run. That would probably reduce some of the vibrations our Kitfox mufflers see. JImChuk