I recently noticed a damp patch on top of my Rotax 912uls engine while doing a preflight check. I couldn’t see any obvious source of the leak. After doing a ground run it took a few minutes for oil to begin weeping out of the crankcase join at the top of the engine.

I thought about sealing it with JB weld but knew I would never be happy with that solution. So, I removed the engine and took it to a Rotax repair centre. When the crankcase was split there was evidence of fretting between the two halves.

The end result is a new crankcase and a much diminished bank account.

The engine was manufactured in Spring of 2002 and has 580 hours. It has been well looked after and well maintained since new and was running fine.

Photos attached below.

Views: 1043


Reply to This

Replies to This Discussion


Two questions, please.


Were you given an explanation by the Rotax repair center for why the fretting occurred?   What is the root cause?


Was a new crankcase the only solution, meaning had you kept going with a temporary fix of JB Weld, did you risk engine failure in flight?

Sorry to hear of your trouble.  Airplanes have a tendency to go after your $$ reserves.   Ask me how I know...or maybe don't remind me.

This is a well known problem for older engines.  Some reports by owners indicate that Rotax may help with the cost of parts.  You have to file an official Customer Service Information Report.

Here is a write up on causes.   





Before everyone assumes in all cases crankcase fretting is strictly a Rotax design issue, please consider there are many influencing factors that ultimately can lead to a certain destructive harmonics being transmitted through the engine, resulting in loss of crankcase stud torque leading ultimately to crankcase fretting!

I've seen this issue from time to time in our Diamond Katana Commercial Flight School fleet and in all cases we found it attributable to a series of compounding issues which if not resolved ultimately lead to the fretting issue. Things such as proper carb synchronization, intake induction design influence on mixture settings, worn carb parts such as slide needles, slide bores, float arms, etc., bad engine mounts(worn out, too stiff, too soft, wrong design for isolating first order natural frequency of engine), poor engine mount design(again fails to totally isolate engines first order natural frequency), propeller balance and tracking, mass inertia, blade flex influence etc., gear box maintenance including friction torque settings, gear dog spring pack pressure etc., and gear box gear wear(pitting caused by poor oil quality with inadequate zinc content to prevent gear tooth pitting etc.).

As well, and as important is prop loading. Many people are over propping their engines by adding too much pitch, in essence running to high a gear(too much load), for the amount of throttle they are loading the engine too! I've seen a lot of chat on this site about prop loading and can tell you one rule of thumb that works for one aircraft/prop/engine combination may not necessarily work for another as there are many floating variables here also.

Too many people run over square on engine load versus throttle setting like running your car in fourth gear with a boot full of throttle trying to climb a steep hill when they should be in second gear! The Rotax 912 series can run it's entire life wide open throttle at 5500rpm and will love you for it and at this loading has the absolute least natural engine vibration!

Lastly, if you are sure you have discovered a safety issue with your engine, you can file a Customer Service Information Report(CSIR) by going to the "Service" button on this web site, then scroll down and click on "File a Customer Service Information Report". Make sure you fill in completely this form with all required information and then submit it. Copy of your submission will be sent to the factory and the other to the applicable Rotax Distributor in your area but only if it is deemed legitimate and not a fart in a mitt! Upon submission you may be further contacted for more information and follow up depending on the issue and circumstances. The BIG advantage is the system is designed to pick up repetitive problems allowing the factory to track trends and act on them! I've used this CSIR reporting system many times and it is the absolute best way to report issues and get action on them!

In Canada here where I operate we're pretty lucky to have the Rotax Distributor Rotech, these guys are responsible for all "Certified" Rotax engines in North, Central and South America, most of which are run in high utilization flight schools meaning they see many problems and find solutions to them in most cases years before the non-certified fleet does! They also have a Hugh store of Used serviceable parts including crank cases, gear boxes, cylinder heads etc., for those looking for saving some money, as well they have the best Rotax factory approved engine overhaul shop in the industry. Their web site is www.Rotech.ca , I use these guys all the time to solve weird problems and they always seem to have the answers, many which I've reflected in this writing!

Thanks for the reply Michael,

Yeah, my engine is one of the early’ish models and this is a known problem with that vintage of engine. There’s also an SB to check the crankcase for cracks. From about 2006 onwards the crankcase was improved to eliminate (hopefully) both of these problems.

With regard to the JB weld plan: I only fly about 50 hours a year so it might have kept me going for a year or maybe even two.

I would never have felt very happy with that solution though, especially if I had a passenger on board.

The fretting was never going to get better, only worse, and if I ended up in a muddy field upside down the cost would be higher.

I’ll fill out one if those Rotax CSIRs just to see what happens.

Thanks again and safe flying,


I found an honest Rotax Mechanic in Glendale Arizona. His name is Steve Beatty. Aircraft Screw performance. I had a bad experience with Rotech of Canada. Michael Moore likes them , I think there crooks.


What I was interested in was the actual cause and facts behind the problem.  I provided a link and a quote from the Rotax owner's forum.   It should have been totally obvious that nothing there infers my opinion one way or another.  

Please avoid reacting and going after someone personally when there is no basis. 


And if I may say so, bashing a company while providing no facts is poor form and informs no one.   Just because this is the internet and that is so easy to do does make it fair practice.    

I bought an engine from Rotech. They have been superb in every way. Five stars (out of five) for Rotech.


New from Zenith:

Zenith Planes For Sale 

Classified listing for buying or selling your Zenith building or flying related stuff...

Custom Instrument Panels
for your Zenith

Custom instrument panels are now available directly from Zenith Aircraft Company exclusively for Zenith builders and owners. Pre-cut panel, power distribution panel, Approach Fast Stack harnesses, Dynon and Garmin avionics, and more.

Custom Upholstery Kits for your Zenith Aircraft:

Zenith Vinyl Upholstery Kits

Zenith Apparel from EAA:

Zenair Floats

Flying On Your Own Wings:
A Complete Guide to Understanding Light Airplane Design, by Chris Heintz

Builder & Pilot Supplies:

How to videos from HomebuiltHELP.com

Developed specifically for Zenith builders (by a builder) these videos on DVD are a great help in building your own kit plane by providing practical hands-on construction information. Visit HomebuiltHelp.com for the latest DVD titles.

Aircraft Insurance:


West Coast USA:

Pro Builder Assistance:


Transition training:

Golden Eagle Aviation

Pianosa Flying Farm

Aircraft Spruce & Specialty for all your building and pilot supplies!

© 2021   Created by Zenith.Aero.   Powered by

Badges  |  Report an Issue  |  Terms of Service