The gear is breaking either due to printing errors (contact Shapeways customer service) or most likely due to too much internal pressure from an excessively tight fit. The gear should drop over the motor shaft, little to no assembly pressure should be required. I Informed you via Facebook that I'd recommend sanding the shaft of the motor since I can't speak of the diameter tolerance of the motors available in France. Saying "It's the same diameter of your recommandation" does not cover the variance of manufacturing of both motor shaft and gear internal diameter. This is a kit: sanding/tweaking may be required especially if you are not using the same motors as I (and my other happy customers) have used.