912ULS Muffler Cracks

Here's one for all you long time Kitfoxers. During condition inspection, I discovered numerous cracks in my muffler at all the openings for the exhaust tubes from the cylinders. Also the end where the exhaust pipe exits the muffler housing the weld has broken almost completely around the circumference. It was installed as per instructions with no binding, anti seize used on all joints, springs installed and saftied and only has 66.0 hrs on the engine. This has been inspected routinely and just happened in last hour or so of operation. Has anyone else had something similar happen, or is this an unusual occurance? Thanks.

Dick B

Dick, I had similar trouble with the down tube or "stinger" pipe on my 80 hp. Rotax until after a couple repair episodes we welded an angle brace from the end of the muffler down to the stinger. I will assume you have that brace already in place on your 912ULS.

I've had no similar problems with my 912ULS now with 900 hours of service.


Keeping the carbs synced helps reduce vibrations. Motor mounts in good condition. With only 60 some hours I'd guess they are fine.
Proper length motor mount bushings used?
Springs should be tight not loose. Too long of springs would allow the muffler to move too much.
If you haven't already, run a bead of high temp (red) RTP down one side of each spring to reduce harmonic vibrations and that will increase the life of the springs some. Probably won't help with cracking pipes.

Maybe check prop balance. I've never balanced mine yet but many have and improved engine smoothness by a good factor. That's good for the whole airframe. There, I've run out of ideas for now.;)

Dick,

Regarding the down pipe - I recieved my muffler & exhaust from the old skystar (but never got the engine). The Muffler had a tab welded on the bottom of it for a clamp but skystar did not supply the clamp... I talked to John Mc Bean and he assured me that there was supposed to be a strap extending from the muffler tab to a clamp around the down pipe. I got the additional part from John and have not had any cracks show up anywhere on the exhaust system with 260 hours on it.

I'm thinking the brace between the muffler and down pipe is critical to protecting the joint between the muffler and down pipe as well as limiting vibration within the exhaust pipe array by reducing movement of the downpipe. If you have the clamp & brace I am not sure what would cause the problem you have observed.

Like DF4, I used a bead of red RTV on the springs as well as safety wiring through the springs. My understanding is the RTV bead minimizes wear between the spring ends and holes they fit through by reducing resonance of the spring.

Sincerely,

Dave S

The newer 912uls exhaust systems from Kitfox do not use the angle brace from stinger to muffler like the older exhaust systems. It still shows the brace in the build manual but there is no tab on the muffler, and J. McBean said the newer ones have been beefed up internally so this is no longer necessary. Wonder if Dick's muffler is the older style that needs the brace?

I have the brace on the exhaust stinger, the cracks are on the muffler inlets that come from the cylinder exhaust tubes (all four) and the end where the stinger comes out.

Dick B

Any chance of a few photos I would like to check mine!

I had the stinger crack from vibration. Welded a gusset from the body to the stinger no cracks many years later. Make sure you have this done professionally because you can cause heat build up and hardened areas that will be problematic shortly there after. (912 UL)