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Old 11-08-2018, 06:04 PM   #1
Davidn
 
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Default Muffler Crack

Quick story - Kitfox 7SS 912 ULS. When in Georgia about 6 hours from home, I discovered a 1/4 crack in the muffler where the number 3 cylinders tube enters the muffler. I decided to fly it home. On the way back in very marginal VFR weather the charging system started acting up and it would be near dark on arrival home. I decided to land at an airport near my Daughters family. Two week later I show up to charge the battery and fly it home for repairs. The muffler crack had grown to a 1/2 inch or so. I'm thinking it'll probably make it home. ("probably", shouldn't be in my vocabulary when flying) The scheduled day home the winds are 20K and gusting on the route home. It won't be fun but it's doable. I'm in no hurry, and I spend another day with my 10 year old grandson. When I go to disconnect the battery charger and the crack is actually almost completely around the number 3 exhaust tube. I'm sure it wouldn't have made it home. I guess you can all make your conclusions. I just wanted to share the experience and hope it helps someone down the line.
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Old 11-08-2018, 06:48 PM   #2
efwd
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Default Re: Muffler Crack

What is the deal with the mufflers? Are these issues taking place on Rotax mufflers? I know some that I have heard of were custom built. Is this something I should be carrying in the luggage compartment as a spare?
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Old 11-08-2018, 07:44 PM   #3
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Default Re: Muffler Crack

Might be a good idea EFWD.

Mine also cracked where the #3 tube goes into the main muffler

I bought a spool of 308 stainless wire for my mig welder. So far it has held

This may be where a really good prop balance helps to keep the vibration as low as possible.
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Old 11-08-2018, 08:00 PM   #4
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Default Re: Muffler Crack

My muffler, supplied for the 912 ULS, is evidently custom made for Kitfox (not sure by who). It is not a genuine Rotax muffler. My understanding is that there is a history of cracking with these mufflers... someone can correct me if that's untrue. I'm hoping that mine is more reliable and improved somehow than the earlier examples. I think the brace between the muffler body and the tailpipe was added, or maybe modified, to help eliminate one of the problem areas. Does anyone know of cracking problems with the design, as it existed since maybe about 5 or 6 years ago? Are the latest ones different from those?
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Old 11-08-2018, 08:03 PM   #5
Jerrytex
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Default Re: Muffler Crack

This does seem to be a problem. Mine cracked too. I found that the slip joints were seized up. I make an effort to keep them freed up and plenty of Nickel Anti-sieze on the joints. Seems to help. I remember reading somewhere about retrofitting a better connection set up?
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Old 11-08-2018, 08:27 PM   #6
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Default Re: Muffler Crack

My muffler, supplied by Kitfox, for my 912uls about 5 1/2 years and 700 hours ago is still without cracks. I keep good anti-sieze on all the joints. I do believe vibration is the main issue and try to always have well synced carbs, good prop balance, and avoid idle below 2000 rpm. Also use a shutdown technique that shuts off the engine fairly quickly and smoothly without shaking and clunking. In spite of all this I may just be lucky so far.
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Old 11-08-2018, 09:32 PM   #7
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Default Re: Muffler Crack

Awe man. Jim, now that you say that, I should of cut my stinger pipe somewhat shorter. Swinging around a longer pipe puts more stress on it Im sure
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Old 11-08-2018, 10:06 PM   #8
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Default Re: Muffler Crack

Quote:
Originally Posted by jiott View Post
My muffler, supplied by Kitfox, for my 912uls about 5 1/2 years and 700 hours ago is still without cracks. I keep good anti-sieze on all the joints. I do believe vibration is the main issue and try to always have well synced carbs, good prop balance, and avoid idle below 2000 rpm. Also use a shutdown technique that shuts off the engine fairly quickly and smoothly without shaking and clunking. In spite of all this I may just be lucky so far.

That's encouraging, Jim. Thanks!
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Old 11-09-2018, 12:58 PM   #9
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Default Re: Muffler Crack

I've been through several Kitfox muffler systems on my plane(s). At this time I am running a new old stock system I found at a swap meet (super lucky find!). I have no hope it will last, though it does have a brace from the muffler body to the stinger (tailpipe).


When it goes, my plan is to use a Rotax muffler system and modify the cowl to make it fit, if necessary. The genuine Rotax systems use ball joints from the header pipes to the muffler can, unlike the Kitfox sleeve joint design. I believe these ball joints allow for more flex in the system which will negate the work hardening and cracking that 421 stainless in known for.
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Old 11-09-2018, 06:30 PM   #10
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Default Re: Muffler Crack

Sounds like a plan. If you think of it keep me in the loop for the progressions of your project.. Dave
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