I really liked the Ultimaker extruder gear upgrade (http://www.thingiverse.com/thing:74839) by xifle (http://www.thingiverse.com/xifle), however after printing it I found that either our extruder gear hex nuts differ or my printer somehow managed to only make the hex nut hole way too small but left the gear at the perfect size.  

So - just in case anyone else runs into the same problem - here's the version that in the end worked for me. Gear is basically a 1:1 copy of xfile's version, I just widened the hex nut hole a bit. Made it an instant snap fit.

Originally hosted here: http://www.thingiverse.com/thing:151642, now migrated for good.