My previous extruder broke so I had to make a new one, I decided to go with a new design since I was planning to hang it from the top of the printer. It became a mix from some previous extruders I used. The acrylic one from http://www.blueeaglelabs.com that came with their Kossel Clear printer and another one also here on Thingiverse from ChiggerPepi http://www.thingiverse.com/thing:245677
Until now it works really well for me, but I'm still tweaking the design for easier and more all round use. If you decide to build it I would love to know how it works for you, so please let me know if you think there is stuff that could be improved. You can download the print ready slt files or the zip files if you want the parts separate.
For information about what version you need, MK7 or MK8 check this:http://reprap.org/wiki/Drive-gear
I printed the parts with .5mm bottom, top and shell thickness, a layer height of ).15mm and a infill of 30%. Be careful not to drill all holes up to 3 mm after printing. Two holes in the barring_plate need to be threaded with M3. It should work to thread with the M3 bolds directly. Also the holes where the tubing goes are now 4.2mm to allow the use of the tube-nuts version, you will need to drill these out to 4.5mm if you want to use tube connectors with M5 threading.
The design is made for use of 1.75mm filament but you could possibly use it for 3 mm filament also if you drill out the holes were the tubing goes.