badBrick 40 Tooth Gear Bearing. Lego Technic compatible. If you don't have a 3D printer but you're interested in getting one of these we can print you one at http://www.badbrick.com/catalog Enjoy.In MakerWare, I modify the "Medium" settings: Quality: Layer Height = 0.18 All other defaults are the same. To Assemble: Lay the flat cover with center shaft face up and put the gear over it. Then load it with about 8 balls. Spin the gear some to get them seated. Then carefully pick up the gear by its shaft and hold it together by hand. Push another 5 ball bearings in the gear. Keep some back pressure on it so the center shaft and other balls pop out the other side. The gear will take 13 ball bearings. To make the balls: Print 13 balls. 5.96mm in diameter. Get 2 pieces of 10,000 grit sand paper and lay one on a hard surface. Put the balls on it. Get and another piece, put it on top of them. Get a heavy book, put it on top of the sand paper and rub it around to sand em smooth. Then turn on a frying pan warm it up, and put the balls in there to really polish them up. At this point your wife will start yelling at you... Turn it all off. Put every thing away. Go to the sports store and get a bag of Airsoft 6mm bb's and use them for the bearings. ;-)