Gentlemen
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 neccesarily 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!
Hope this info helps.......
