912 UL cylinder bore

Hi Mark,

I too had a 700hr UL that was burning oil. I had it stripped and rebuilt. It was discovered that the oil control rings had worn in such a way that the spring in the back of them was acting as a ratchet and so they were not pushing hard against the cylinder walls. I had new rings fitted but this did not solve the problem. I figured that I needed to run it in to get the new rings to seat properly, which they never did.

Before the strip down I was burning about 60ml per hour and afterwards I was burning more than double that. It was almost like owning a 2 stroke where I had to oil before most flights!

I had it stripped and rebuilt a second time (both with a Rotax agent) and the outcome was the same. In the end I just bought another secondhand engine and the problem went away.

My rebuilds cost me about half the price of the replacement engine so it was an expensive and very inconvenient exercise in the end. I don't know why the new rings did not bed in properly, it could have been a problem with cylinder honing. Looking at your photo, I didn't have the ring marks at the top that you have arrowed but my honing looked similar to yours.

If you are doing the work yourself then I would throw a new set of rings in and see if that solves the problem, if it doesn't then don't throw good money after bad, cut your losses and find another engine.

I would be interested to know the outcome since I still have my original engine sealed waiting for a potential solution which I suspect involves 4 new cylinders :-(

Evening Kevin,

After a long delay the engine has been rebuilt.The rings were soft, probably due an undocumented overheating event with the previous owner.

All components were still within acceptable wear limits and a new set of rings have been fitted. The cylinders were lightly honed.

After approximately 5 hours of running the oil consumption, while not zero, appears to be significantly lower. The AME believes it should reduce as the rings bed in.

I will post an update as the hours build.

It does seem, however, as you have experienced one problem has been replaced by another. See the new thread 'Rough running 912 UL'

Thanks for the update Mark. I have a couple of questions:

1. How did you determine that the rings were soft?

2. How did you hone the cylinders?

Let me know how you get on as I might revisit my engine and fit a new set of rings and lightly hone the cylinders.

I hope you resolve your rough running engine problem as it can get very tedious when you have a string of consecutive faults.

Morning Kevin,

I have not received the old rings from the AME yet.

He told me the old rings did not have the same spring force, radial and torsional, as the new rings. I have accepted his analysis based on his experience and will check them once they arrive.

I am not sure how the cycliders were honed, but will enquire.