This extruder is derived from http://www.thingiverse.com/thing:275593. I purchased an FLSun i3 a Prusa I3 knock-off with a 2020 aluminum extrusion frame. The printer is pretty good, but of course had a few annoying quirks. One of these was that generally to load filament I had to remove the bowden tube and the fitting. The supplied extruder seems to be derived from the original, or one very much like the original. To over come the filament loading problem I reduce the distance from the drive gears to the filament guide and flared the opening. I also redesigned the lever. I broke the first one I built so I have also strengthened it. Finally I changed the size of the hole for the 3mm nut so that it would not rotate.

I use the 5mm threaded fitting for the bowden tube, I cut threads in the extruder for the fitting with a 5mm self-threading bolt I made.