Hi, in the process of doing my 200 hour maintenance on my 912ULS I found some oil adjacent to the exhaust manifold for cylinder # 4. I traced it to the lower threads of the exhaust manifold and the nut also looks a little like oil baked on it (Picture shows what it looked like after I had just cleaned it up). Any thoughts on resolving this?

As part of the 200 hour service I did a differential pressure test and it came back 87/86 on all 4 cylinders



10209_1_IMG_1914.jpeg (You do not have access to download this file.)
  • Re: Rotax 912 Oil Leak from Rear Exhaust Manifold

    by » 4 months ago

    I am struggling to imagine how you would have an oil leak at this point - Oil getting past piston rings and or valves would be at least partially consumed in the combustion process - I would expect the main symptom would be smoky exhaust & fouled spark plug(s), which may show up as darker stain on the belly of the aircraft.

    Are you sure the material is oil and that it is from the inside of the exhaust?


    I am intersted in what would seem to be a plugged position for a threaded EGT probe - what is your rationale for not using this, in favour of the hose clamp type?

  • Re: Rotax 912 Oil Leak from Rear Exhaust Manifold

    by » 4 months ago

    The lower nut actually looks like a different shape. I’m guessing that someone put a locknut with a nylon insert in that spot by accident. That would melt and turn to a black goo on the nut.  



    I’ve seen a hose clamp type EGT probe alongside a perfectly good threaded exhaust bung quite often.  I’ve even seen manufacturers do it, and I’ve always assumed it was because the clamp type are so much cheaper.  But it does make you scratch your head.  

    Thank you said by: Rotax Wizard

  • Re: Rotax 912 Oil Leak from Rear Exhaust Manifold

    by » 4 months ago

    Well spotted Jeff - I thought it looked diffrent because he had "just cleaned it up".

  • Re: Rotax 912 Oil Leak from Rear Exhaust Manifold

    by » 4 months ago

    I am scratching my head on it also, which is why I posted the question. Nowhere else did I see any oil and the spark plugs / borescope for that cylinder  did not show anything (Spark plug same healthy color as the rest). I’m assuming there is some lubrication at the valve stem that is past the combustion chamber. Could this be the source? I will continue to monitor and see if anything develops further.

    To answer your question on the probe, the aircraft is a Remos GX and it came from the factory that way.

    I do have one more question:

    I went through the oil purging procedure and after I completed it I got the engine up to operating temperature and went through the process of checking the valve tappets . After putting at least 15lbs of force for the required 3 seconds on pushrod end of the rocker arm I could not get enough movement of the rocker arm to put any feeler gauge between it and the valve contact surface. Same results for all the valves in all 4 cylinders. I’m assuming for sure indicates that the tappets do not have any air in them. Is the lack of movement a concern?

    Thanks for the reply!



  • Re: Rotax 912 Oil Leak from Rear Exhaust Manifold

    by » 4 months ago

    Your tappets sound fine. The oily substance may have been blown back by wind force. I see this at times. I also here people claim they have an oil leak when it was only coolant mixed with dirt and grime.

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

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