This is a replacement for the bottom part of the Printrbot Alu extruder to better mount an E3D hot end (or perhaps groove-mount hot ends in general).  This holds the hot end firmly, without relying just on pressure.  It also fixes the hot end to a level where the bed level sensor can still be mounted.To install:Insert M3 nuts into the 3 holes around the bottom, two on the rear piece and one in front.  Make sure they line up with the holes on the bottom.Insert the PTFE tube into the E3D hot end, but leave about 3cm extra, if not more, extending from the top.  When mounted, the PTFE will guide the filament through the empty space above the hot end.Remove the old extruder and support the motor to avoid damaging the wires.  Insert the E3D hot end through the opening, then hold the front and rear pieces of the mount around it.  The PTFE tube should fit between the sides, in the round cutout in the middle.While still holding the mount, screw the motor into place.  The screws from the original mount should fit.  Once done, this should be effectively a single, rigid piece.Screw the mount into place from the bottom, with the three short M3 screws from the old mount.Finally, reinstall the rest of the extruder.  Cut the PTFE tube so it ends just below the drive gear.  Getting the PTFE as close as possible, even angling the cut a bit, will improve your ability to print flexible filaments, since there'll be less (if any) room for the filament to be pulled into the gear.I was able to print this unsupported with the orientations used in the STL files.  This is remixed from Printrbot's Gearhead v3 design.