I took http://www.thingiverse.com/thing:9460 and used the gear4a that imports well into openSCAD to build a coupler that an 11 tooth pinon gear can be inserted into.The gear I used was http://banebots.com/pc/P60P-GEAR/P60P-GM5023 - I meant to get the 3.2mm bore one, but I was able to drill it out just fine. I tapped a 3mm set screw hole too and then was able to drive it with an RS550 motor. As a test, it was easy to just put the gear into my hand drill too, but the motor direct drive is much faster.Video of operation, recorded at 1/8 speed: http://www.youtube.com/watch?v=9TFWST00E_8Should print just fine with whatever settings you used for the other gears. Download the gear4a.stl from the parent thing http://www.thingiverse.com/thing:9460Parametric, but doesn't work with Customizer because it needs the external STL file.