Theory and parts: This is a completely new extruder that combines the idea of a saint flint (https://www.thingiverse.com/thing:979113) double sided extruder with the parts for a wades (http://reprap.org/wiki/Wade's_Geared_Extruder) style extruder. I used the gears from an early gregs wades (https://www.thingiverse.com/thing:6713). Herringbone gears will work unless they are printed with one gear mirrored. Some spacer washers will likely be required on the hobbed bolt to align it with the filament. How it works: The filament enters the extruder, is run through a loopback of bowden tube, and returns again so it is gripped on both sides of the hobbed bolt. It has very high traction. The bearing blocks do not require an axle shaft to be inserted, just dunk the bearing block in boiling water, press in the bearing, and then pinch to set it in place. Use m4 nuts threaded on the tube ends to retain the main bowden tube and the loopback tube under the bearing block fingers. Use 3" drywall screws or whatever is on hand to hold the bearing blocks in place. It could be possible to use long m3 bolts and nuts instead.