I have a couple of motors and the mounting threads started to become stripped. I used the little aluminum "+" brackets that come with the motors in place of the fiber board discs that come in the 3D Robotics Y6 kit. Because they can mount flush, you can attach them to the bottom motor using the wider set of mounting holes. I had to file a half-moon in the end of the arm and drill a hole for the 4th screw to keep the plates parallel. I also cut notches in the spacer at the end of the arm.
All in all, a little fiddly to put together. Very solid and I can use my stripped motors. If you have a few brackets in your parts box and a stripped motor, this might be useful.