Caution, this is experimental. Not sure if the Nema17 will have enough torque to push 3mm PLA into the hotend but I'm going to have a go. Influenced by a number of very good designs already out there this has the mounting base integral to allow it to directly replace the existing Wade type extruder. Uses a 688AZ bearing which measures 16mm OD, 8mm shaft and is 5mm wide. Nema17 motor, the big one. Makerbot Mk7 hobbed pulley like this :http://store.makerbot.com/mk7-drive-gear.html