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Be careful with the threads on your engine


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These days I have some trouble with a 912 ULS, 1400 TSN. The engine has a defective crankshaft bearing. So far so bad. I tried to organize an overhaul at our local service center but Rotax denied to reuse the housing because of a damaged thread. The thread in question is the upper thread for the engine mount on the right side. This is where the long bolt comes in.

To my surprise Rotax does not allow to repair the thread using HeliCoil or equivalent. They told me, that even the thread for the crankshaft fixation tool may not be repaired. If the threads are questionable, the housing must be replaced. Unbelievable but true: If a thread is defective, your engine is probably scrap.

Don't ask me, why the thread is damaged. When the engine was mounted, the bolt has been tightened using a calibrated torque tool.

So be careful out there!

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Unfortunately no trade in value! Crankcase & crankshaft unusable --> No value! I negotiated a GoodWill of 900 Euros from Rotax because of the early defect. That's all! A replacement engine is on its way....

I can see no technical argumentation for being that strict with threads.


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OK, here we go.

On the pictures you cannot see the colour of the chips. The chips, pointed to with a blue cable strip, are bronze coloured. The very last picture is the smear of the magnet screw.

Due to this situation, I decided to ground the engine. After disassembling, one crankshaft bearing showed defective (the one in the middle).

All other parts of the engine were in healthy condition and an overhaul with a new crankshaft would be no problem, if only this shitty thread would be OK.

How does this come, what is the cause? Because there are no other findings, it is impossible to determine the cause of this defect. Rotax said something about lubrication problems in company with air in the oil tubes. My opinion is, that if there was a lubrication problem, it would have affected not only one bearing.










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Markus, thanks for posting pictures.  Good to know what "BAD" things look like when checking the filter.  Looks like a large quantity of fine metal filings too.  You ask a valid question when you ask "why just one bearing failure and not more failures if there was insufficient lubrication?".  Especially if the engine only has 1400 hours and you have done the recommended prop turn until pressure comes up at oil changes.  I think if Rotax maintains it was your fault then they owe you your old parts so you can see for yourself what failed.  I think there is a law here in the U.S. that the repair shops must return parts to the owner.

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Hi Dick,

Rotax did not say "it s your fault". They only speculated about the cause. I think speculation is just that: Speculation. Nobody wants to pay the cost for a real investigation of the cause. I accepted their offering of an engine in exchange. The old engine remains at Rotax.

The bill reads as follows:
1. Overhauled engine in exchange with your defective engine

2. Plus core credit because your engine is completely unusable

3. Minus goodwill from Rotax

I do not expect to get the old engine back because of 1.

The engine in exchange is ~ 500 Euro cheaper than the repair and overhaul would cost. And it is about 2500 Euro cheaper than a new engine. But: If I would buy a new engine, they would not give me the goodwill AND they would hand me over a second bill for the disassembly and the inspection of my old engine. Racketeers ;-)

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Thanks Corey for asking.

I discussed again with my Rotax representative and they persist in their view of things. My interlocutor told me, he had a discussion with the engineering of Rotax in Austria and they actually do not allow to use HeliCoil despite the fact, that there are HeliCoil inserts at some threads of the case (Parts catalog figure 72-20-00-1, item #2). He assumes, leading edge air foils is wrong with their assessment.

Now the ship has sailed. The replacement engine is on the way to me.

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