I just tried it in my new MicroSizer, and I still have the same problem. That 7-tooth KIT gear just has a bigger diameter than the standard Bit gear. I
was able to get the motor to spin if the heat sink isn't snapped in all the way, giving the motor a little play.
It fits just fine in my ZipZap, though,
Later on, I'll see if I can just file the ends of the gear teeth down a little bit, to make it fit my Bit chassis.