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Per the latest Illustrated Parts catalog, Rotax has replaced the black 938795 exhaust springs with 938796 stainless springs.  My understanding is the change was to move away from a less environmentally-friendly coating process to one that was more eco friendly.  The new ones have a gray plating.

I just ordered a complete set of the new 938796 springs.  I had never measured them before, but according to the Illuminated Parts catalog, the springs are 66 mm.  Not sure what dimension this is, but upon measuring the old and new springs, they both measure 51 mm inside hook-to-hook, and about 56 mm outside hook-to-hook.  So, my first question is if anyone knows what the 66 mm dimension supposed to be?

Second issue is the old vs. new spring tension.  The old ones, when stretched about 0.5” give a tension of around 27-28 lbs.  The new ones only give a tension of 20-21 lbs.  I’ve recently had issues with the heat muff surrounding the muffler in my RV-12iS coming apart due to what I assumed was vibration, so am sensitive to the strength of the exhaust pipe to muffler ball joint interface or never would have thought to take these measurements.  And my propeller dynamic balance is about 0.03 ips, so pretty good.

Attached are some photos.  Does anyone have any insights to these two issues?

 

9450_1_06DFB851-2BCF-48F7-995B-6AC1134349F4.jpeg (You do not have access to download this file.)
  • Re: New Exhaust Springs

    by » 3 months ago


    Bob,

    I believe the overall length of the steel and stainless steel spring is nominally 60mm, and the "hook distance" is 66mm.  While we typically think of hook length as the inside/inside distance between the hooks when the spring is at rest, in this case I believe it is the inside/inside measurement of the spring hooks when stretched to their ideal installed length (Length L1 in the attached drawing).  So simply stated, its the installed length as per Rotax design. 

    The spring rates may be different between spring steel and stainless steel, so if you are interested you should check the tension at the installed length of 66mm (about 1/4" of stretch)   Your test was with the spring stretched 1/2",  which does not represent the normal installation.  Let us know how the springs compare at that length. 

    I have had a ball joint shake apart between the header and muffler on one occasion, and it happened when starting the (912iS) engine.  I have since doubled up the springs in those locations.

    33921_2_Hook Length.jpg (You do not have access to download this file.)

  • Re: New Exhaust Springs

    by » 3 months ago


    Thanks, Jeff.  I tested at about 0.25” as you suggested and, as you’d expect, the difference dropped from around 130% to around 120%.  I’ll have to measure the final  length tomorrow after I install them, but based upon the relaxed lengths, it would have to stretch more than 0.25” to get to 66 mm.


  • Re: New Exhaust Springs

    by » one month ago


    Well, the installed lengths do run about 66 mm end-to-end.  It just seems odd that Rotax would cite the installed length against a replacement part, since any spring less than 66 mm will end up being 66 mm after it’s installed.  This means the springs are stretched about 10 mm, which puts the old spring tension to be about 25% higher than the new ones.  I guess time will tell if the lower tension has any impact on the installation.  I’m just super sensitive to this metric after the failures I’ve had.


  • Re: New Exhaust Springs

    by » one month ago


    If you are concerned with the springs stretching under use, especially  during Staring and Stopping,
    consider double safety wiring the spring internally.
    They will still have a little stretch but can't go too far.
    I had a similar problem and solved it like This.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


  • Re: New Exhaust Springs

    by » one month ago


    Thanks for the tip, Bill.  Looks like you used 0.041 safety wire, which I do have.  I don’t have near the clearance on the right side of my engine as you show on yours, but I’ll give it a try.  Thanks again. 


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