Here's a bowden extruder for the 42BYG48HJ50 1:50 geared stepper motor.I could not find one online so here's one.I use a flexible tube instead of springs (like on Air Tripper's extruder)I've made the screw holes small so I can directly screw in it without using a nut. But it might be different on other printers, there's always the possibility to place a nut anyway.The STL files aren't oriented properly for printing, you should rotate the Extruder so the supports are on the bed, and you should rotate the idler so the biggest surface is flat on the bed.Also a taper is needed to make the thread for the Pneufit connector.