Minimal extruder housing for use with the Printrbot Gearhead drive gears, 1.75mm filament, and a NEMA17 motor.
The secondary gear is inserted from the side, and the additional M3 screws or setscrews hold the gear in place and adjust the tension. It's designed to use a bowden coupler from RobotDigg. Filament is Emerald Green PETG from Atomic Filament.
I prefer to use 48mm NEMA17 motors with the Gearhead drive gears, but a 40mm version can be used to reduce weight or a larger one if more torque is needed. Make sure the setscrew doesn't stick out on the primary drive gear, especially if using a motor without a D-shaft.
There is also a NEMA14 version and info about several other dual drive bowden extruders I've just uploaded on thrinter.com