New Exhaust Springs

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.

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.

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.

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.

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.