Broken exhaust valve spring

It is possible to change them to singles as long as you change the spring retainers.  There is no requirement to replace the cotters.  Don't mix single and double on an engine.  The spring support, unless damaged, does not require change. 

Given you broke 2 I would also check the lifters to be sure they are not failing.  (not pumping up) 

Cheers

Just as I thought. 

It only happened this one time. Just hit 110 hours on a fresh overhaul and Zipper big bore kit install. It has been running very well with no other issues. Again, I was lucky to be somewhat close to home and in my newly built Kitfox 5 (Sep 2023). I never had an engine break a valve spring before. Not a good feeling and not so lucky to be at 2500ft and 10 miles from the nearest airport and over thick woods and hills. Was able to trailer it to my shop and start the repairs. Now that this happened, I've been hearing a lot more Rotax valve spring stories. Coincidence maybe?

I'll get the hundred-hour inspection done and possibly the annual as well while awaiting parts. My trip to Oshkosh will just have to wait till next year.

Thanks, RW for the reply.

Cheers

Hi Brett,

For future reference (hoping never to use) please describe inflight symptoms and subsequent diagnostic path. Thanks

The inflight Symptoms were a pronounced vibration followed by a loud knocking up forward.

Once on the ground there was no sign of oil or coolant leaking. Oil level was full as was the coolant. Propeller and hub were checked, and all was normal.

Trailered the plane to my shop and upon further inspection found the remains of the phenolic thrust washer in the bottom of the oil tank. Cutting the oil filter open revealed a little plastic debris and little else. The lugs looked good, and the wear from the knocking was within tolerance. The gearbox was clean with only 110 hours on it since rebuild. No metal debris on the magnetic chip detector. I replaced the thrust washer, installed new springs and re-shimmed the stack IAW the Rotax HMM. Did an oil/filter change and we were ready to start!

Upon start-up we still had a bad vibration and upon shutdown discovered the #4 cylinder had no compression. Shot compressed air into the spark plug hole and confirmed the exhaust valve issue as air shot out of the muffler.

My guess is that I lost the valve spring first and the vibration destroyed the thrust washer. Without the thrust washer, the lugs of the slipper clutch were making the noise.

Was a bit scary but my recent training really paid off. I had a twenty-year absence from flying until finding this Kitfox unbuilt in a barn in South Carolina 5 years ago. 

I had accumulated 110 hours in the last three months preparing to go to Oshkosh.