Really open the crankcase for this?

Hi Alexander

I do not know how you would damage this is such low hours.  This seems very odd.  I have seen many engines where this has gone to TBO several times without that kind of damage shown on the video.

Cheers

A very old thread from years ago, same issue.  No solution without more details on what happened.

https://www.rotax-owner.com/en/912-914-technical-questions/1150-free-wheel-gear

RW,

In your opinion, can engine kickback cause this problem? The damage is certainly on the contact surface with the sprag clutch.  

it was the failure of the sprag clutch to disengage. When the engine caught on while starting, the sprag clutch didn’t disengage thus the engine run against the clutch. At the first few occurrences I did not know what to make about this. When I finally was advised of the problem, we (competent mechanic and me) thought that several more hours of flying (our main holiday) was acceptable. During the trip it became more severe, at the end it was really terrible to hear the engine running against the clutch until it finally disengaged. Imagine you were in Alaska on holiday by plane and want to go back to your home in Florida. Didn’t look at the prop reduction gear though… lessons learned

Hi Alexander

OK that makes some sense for sure.  At 250 hours it is still very odd however.  If this is a new engine, under warranty, and you still have all the parts then I would pursue a request for warranty.  Given you are in a rather distant place service is not going to be easy.  Go online to flyrotax.com and fill in a customer service information report, CSIR, that is the first step.  Follow up with the nearest independent Service Centre you can get to deal with it.  As a note the gear is put on the cam with an interference fit and Loctite.  It has a keyway also because it has a critical alignment for cam to piston stroke timing  of the cam.  The cam itself is not sold except as an assembly.  The free wheel gear is separate as you note.  

The question from Jeff is for sure this is from the sprag failure.   Normally the sprague fails due to kickback.  High mass props, poor gearbox shimming are the most common cause.  There is an SB from 2002 that outlines all the checks to look into with prop kickback.  The last item in that SI is the most important.  if you have starting issues check the DC amp draw on the starter motor, it must not be less than 75 amps (DC) If this failure was dragging the starter it will most likely be failed also due to the gear ratio on the starting motor to crank.  I can only speculate on that part without seeing the parts. A normal fail is from the kick causing the small spring to stretch and then the cams inside the device do not engage completely.  The video show damage on the free wheel gear that is unusual in that the cams in the sprague must have broken or have had debris inside to jam them into the free wheel gear.  The normal look, usually after hundreds or even thousands of hours, is the the free wheel gear surface to start to appear like a barcode.  If this is visually rather than something you can feel it is just fine.  If it is indeed imprinted into the gear it should be changed. 

Cheers