I just performed maintenance on a 912ULS with 375 hours where the female socket on top of the muffler was destroyed from too little spring tension which allowed the exhaust pulsations to hammer on the socket. Since we are talking about this exhaust socket, or joint, I should bring up that this needs periodic lubrication. Use copper anti seize on this joint to help prevent early failure. This is the same anti seize used in the gearbox assembly. (You may have even seen what you thought was brass metal particles in the oil the first time the engine oil was changed. It was, most likely, just this copper anti seize.) When there is too little spring tension the spring hooks will vibrate excessively and wear at the spring hook where it comes in contact with the loop welded on the exhaust tube and muffler. (See attached pictures)
So, let's examine the solutions There are a few different types of springs available not to mention all the ones owners may buy at the local hardware store. I recommend springs approved by Rotax. So it may be hard to get the perfect tension addressed in this article so again, I will limit this discussion to just two types. (Pictures below are the two most common.) One is the Rotax spring part number, 938-795. The other one commonly seen is a stainless steel spring and is much heavier in construction. It isn't necessarily better, just different and the correct tension will prevent premature wear. The wrong tension can make any spring wear out or break in as little as 200 hours. If you use the heavy stainless steel spring an effective quick evaluation for the best tension is obtained when you can see a little daylight between the coils or you have enough room to insert your fingernail between coils easily. If you can't see any space, then the spring may not have enough tension. At least you will know that there is some tension on the spring if you see a little daylight and it won't be over stretched. The correct calculation, following Rotax’s maximum 10mm(.4”) stretch would be calculated as follows: Each spring has approx. 11 coils but the first coil is zero leaving 10 potential spring gaps. If you have ten spaces between coils, and maximum stretch is 10mm(.4”), your maximum allowed distance between coils is 1mm(0.039”). Let’s call it 0.040” and use a feeler gauge to measure it, straight forward, simple!
Get back in your seats; we aren’t finished yet. We need to apply some high temperature RTV silicone. This helps prevent vibration from wearing on the spring and helps give it a little extra support. There are two ways to apply the RTV silicone. The least common and least desirable way is to fill the spring’s interior with the silicone. I don’t particularly like this because you need to safety-wire the spring and remove it at times and then it becomes a problem trying to get all the RTV out. The best, and most common way, is to lay a bead of the silicone from top to bottom on the outside of the spring. Here is where it was important to see a little daylight between coils because you want to work the silicone in between the coils as you apply your bead down the side. The bead should be at least 3/8”- 1/2 inch wide and 3/16” – 1/4 inches tall. This gives the silicone some body and strength. Applying a pencil thin bead, or a line down a fully collapsed shut spring is close to worthless.
One last thing you should do is safety-wire those springs. Too many have gone through props on pushers. You can run the wire down through the center from welded loop to loop, or through the spring and back around the outside, but still through the welded loops. Do not make the safety wire tight. Leave a little play in it so it can move. It isn’t there to hold the exhaust together, but only keep the spring in place in case it comes off or breaks. If you make the wire tight the vibration will just wear through the wire quickly. My personal choice is to use .041 wire and not .032.
I have attached a couple of pictures to show what excessive wear will do to a heavy, stainless steel spring hook and a muffler socket. The red RTV silicone was applied well, but isn’t between the coils. There is a picture of the Rotax spring connected to the exhaust with the RTV silicone and safety wire already applied.
For readers reference I’ve attached page 43 of the Rotax installation Manual which states Rotax’s official technical position on proper muffler spring tension.