Bowden Direct Extruder for 1.75mm filament

This is a direct extruder for bowden and 1.75 mm filament (NEMA 17 stepper motor)

Bill of materials:

Bowden tube: ID = 2mm; OD = 4mm (e.g: http://reprapworld.com/?products_details&products_id=167&cPath=1616_1653)
Pneufit for 4 mm tube (e.g: http://reprapworld.com/?products_details&products_id=169&cPath=1616_1653)
Drive gear (e.g: http://www.ebay.es/itm/151122877988?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649)
623ZZ Bearing
"Strong" spring (about 20 mm long; is not strictly necessary, you can make pressure with the screw, without spring)
NEMA 17 stepper motor
M3 screws (different lengths)
1 M3 nut