Leaking exhaust knuckles on 912..

by Andy Gustafsson in 912 / 914 Technical Questions 11 years ago

Hi all.
I have a 912 in my Merin EZ, with 750+ hrs on it. It runs like a dream. I have noticed that the exhaust knuckles are leaking somewhat even though I lube the knuckles with high temp lube. What is recommended as a lubrication agent or sealer, or should I wrap the knuckles under the springs as I have seen been done. The exhaust system is stock Rotax. And is wrapping the whole pipes recommended?

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Re: Leaking exhaust knuckles on 912..

by Roger Lee » 11 years ago

Roger Lee

126 5.13k 4.56k

This is my personal opinion and others my differ.

I see this a lot. Many times I can fix it by loosening the exhaust nuts and pushing the muffler over an inch or so to help align the knuckles better. Just a little bit better alignment can help quite a bit. Keeping the joints lubed with some copper anti seize can help, but it isn't a cure.
The last thing I have done is exactly like you mentioned is using a little header wrap cloth around the knuckle, but under the springs of course and secure it with a hose clamp. This can keep any exhaust blow by in one place. I have wrapped exhaust too. I use 2" header wrap cloth. It needs to be damp, not soaked to apply. Use a hose clamp at the top and bottom of the wrap. Do not over wrap more than 3/8" on the edges because it will retain too much heat. You can go from the engine exhaust port right to the muffler and under the springs. Don't wrap the muffler. The stainless exhaust pipes are not affected like standard steel under the heat. I have seen many wrapped including mine for 10 plus years without any issues when done correctly. 2" header wrap cloth is about $50 for a 50 ft. Roll. If you have the stock mufflers and pipes you will use about 64" per pipe and have a lot left over. Once applied start the engine and let it bake in. It may smoke on the initial start when damp. When it is done and run another couple of times after flying it will be fragile so don't scrape it with heavy tools. Some people spray their wrap with a high temp black paint to help toughen up the wrap, but I haven't had or seen any issues without the paint. Keep the wrap tight as you wrap.

Do not over wrap or come in contact with EGT. probes. It will cause them to fluctuate as much as 100-120F. Something in the cloth and no one that I have found really knows why, but it isn't a big deal. Just wrap up to the probe then start back again on the other side.

The wrap really helps keep the radiated and convicted heat off hoses and wires and should help prolong their life or help stem some failure rates from constant and close proximity to that high heat source.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Andy Gustafsson

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Re: Leaking exhaust knuckles on 912..

by Ken kitzmann » 11 years ago

Ken kitzmann

0 24 16

I have a Chinook Plus 2, this spring I switched my exhaust from flex pipe to stainless after the flex pipe broke near the prop yikes! (the only thing keeping the short piece from hitting the pusher prop was the 2 springs !!

Anyhow on my new stainless system I had a small visible leak @ one knuckle, just the other day I put some High Temp Red Silicone on it, have not flown yet...but I have used that stuff on leaky wood stove pipe in a hunting cabin last fall and once cure worked Excellent ! There is no engine shroud on this pusher, so the leak was really a non issue...just cosmetic.

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Re: Leaking exhaust knuckles on 912..

by Ken kitzmann » 11 years ago

Ken kitzmann

0 24 16

...The high temp silicone trick did not work...I let it dry completely and the Exhaust pressure just blew it off.

Thank you said by: Guilherme Tomazini

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Re: Leaking exhaust knuckles on 912..

by Andy Gustafsson » 11 years ago

Andy Gustafsson

7 17 0

Thanks for the info. The very high heat and pressure will probably blow most sealers out. From the stories I have seen so far I think that the header wrap is the very best solution. Since there is probably no movement in the knuckles, due to the free hanging, spring secured muffler the wrap should do the job.

Thank you all.

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Re: Leaking exhaust knuckles on 912..

by Roger Lee » 11 years ago

Roger Lee

126 5.13k 4.56k

Hi Ken,

I figured the silicone wouldn't work. If you can move the muffler a little left or right to better align the knuckle that is best. It may be time for new springs or use a little heavier tensioned spring. We don't know what's on there right now and at 750 hrs. It is time for some new springs if those are originals. Baring that the header wrap works very well.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Leaking exhaust knuckles on 912..

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