I’ve got a small exhaust leak on the front left (#2) manifold to muffler flange. It’s leaking on the inside. About the 9 o’clock position. 

Used an automotive smoke tester to find the leak. 

Any advice on how we can fix this? Aligning the muffler fixed the other small leaks but this one is very stubborn. I’ve temporarily fixed others with high temp antiseaze. 

I think part of the problem is the 2 part manifold is stuck together at the seam, clamp. 

Need some guru advice! 



10202_1_9EAAEC04-1BB2-45D4-A691-A0EDEA8CE1FD.jpeg (You do not have access to download this file.)
  • Re: Stubborn exhaust leak?

    by » 5 months ago

    Is this a Rotax 912?

    If so, remove the entire exhaust system and thoroughly clean all manifold flanges and slip joint coupling contact areas. Then reassemble, don’t tighten fully, using anti-seize paste at all inserts especially the slip joints, ensure all manifold nuts and tension springs allow movement of the entire exhaust assembly. Wiggle and nudge the entire exhaust assembly in unison side to side until all slip joints on both sides of the engine appear to be level and properly aligned. Then methodically tighten the brass manifold nuts (meaning snug each nut in turn evenly, a few turns at a time, moving from front to back or back to front, doesn’t matter, and side to side in turn, never fully tighten until all are snug equally). Wipe clean the excess anti-seize paste at each slip joint. If there’s no excess antiseize paste, you didn’t apply enough. Ground run and look for soot especially at the slip joints. Ground run means get the engine as warm as possible. Go taxi around the field if able. Then if any soot is found, and it’s normally at the slip joints, wipe it off and repeat the above- loosen the brass manifold nuts (yes, all of them- no cheating) using minor left-right alignment adjustment to the entire exhaust system. The key is you must wiggle and settle the entire exhaust system in unison. Persistence and patience is crucial. Eventually you will eliminate the leaks, and more importantly develop an eye for what “looks right” as far as slip joint alignment. Then one day, the leaks disappear forever. 

    Have fun!

    Thank you said by: Nick

  • Re: Stubborn exhaust leak?

    by » 5 months ago

    Hi Jim,

    yes, 912. Had an experienced rotax guy realign everything. Sort of embarrassed to bring it back because he spent quite a bit of time on it. I’m noticing the problem is only with #2. 

    The adjustable 2 piece pipe is stuck together, not sure if this is preventing the bulb from seating all the way in the flange on the muffler?

    I can disassemble it and cram some antiseaze in there, how long will this work for? 

    I think the solution is a new manifold and potentially a weld bead on the bulb or inside the muffler? 

    I’ve heard most rotax exhaust systems leak, is this true? Is it safe to operate with a small exhaust leak? A&p friend hates rotax and says no exhaust leak is acceptable but the design doesn’t seem possible to completely seal the system.


  • Re: Stubborn exhaust leak?

    by » 5 months ago

    How can anyone hate Rotax?  I own a O-200 on a Cessna & that exhaust was leaking when I bought it too.  The Continental has a chronic oil leak (that we haven’t found for sure yet), and the O-200 uses 6 gallons an hour (although has never quit).  The Rotax doesn’t leak, uses less fuel, is quiet & smooth.  Six years ago I was able to get a full exhaust system from Kitfox, if I needed a new one now, I’m not sure where I would get one though.

  • Re: Stubborn exhaust leak?

    by » 5 months ago

    Has anyone tried using lapping compound or fine sand paper to work the joints to make them fit better? 

    Also wondering if heat will cause the metal to expand and seal the gaps?


    how long will antiseaze seal the gaps for? 

  • Re: Stubborn exhaust leak?

    by » 5 months ago


    Do not use lapping compound on the joint.  The pipe is stainless and the head is aluminium.  Lapping compound will damage the 45 degree face of the exhaust port.  Anti-Seize is to lubricate the joint not to seal it.  The lube is required so it will not "seize" and allow it to align with the exhaust correctly as well as absorb vibrations from exhaust pulses.  

    The seal is created by the ball of the pipe having even pressure from the exhaust clamping plate or in the case of the cannister the even pull of the exhaust springs. (check alignment)  The pressure has to be even and never overtorque it to the point of contact of the plate to the head.  For the contact to the exhaust canister the alignment has to be correct as per the installation manual.  Note that almost all exhaust systems are assembled, welded, by the OEM and only an extreme few are fully made by Rotax.  (They did have one version done for the iS engine as an option and all turbo engines have full factory exhausts) 


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