I'm having problems keeping the motor from slipping back and stripping the small gear.

From : Don Stackhouse

We have a new version of the MPS-1 that we're calling an MPS-1A, which does a better job of avoiding this. Both the front and rear frames are the same, similar to the rear frame in the MPS-1, so that it grips the motor can around the outside at both ends, instead of just that little bushing at the front end. There are other changes as well, including a different fit in the shaft bearings and a different prop (it's even possible to fit larger diameter/higher pitch props for high altitude operations). We'll start phasing this new version into all of the kits, starting with the new Piper J-3 Cub, the Curtiss-Wright Junior and the B-17.

In the meantime it's pretty easy to fix the problem you're having with your MPS-1's. Just get everything seated and aligned properly, then put a couple drops of thick or medium cyanoacrylate between the motor can and the plywood plate. This locks the motor in place, so that even if the front frame slips off the bushing, the gear mesh is maintained. If you need to get the motor out, just slip a #11 hobby knife blade between the motor can and the plywood plate and pop the motor loose. Scrape off the residual C/A and re-glue when you reinstall the motor.

Don Stackhouse
DJ Aerotech