This is a remake of my self-centering z-coupler (http://www.thingiverse.com/thing:396781)  
The thread is now optimized for 3D printing and the overall length is way smaller. It's also a parametric design, so i can easily generate different sizes.  
I made 4 different sizes for 5mm motor shaft and 5, 6, 8 or 10mm threaded rod.
You can just ask me, if you need different sizes.  
12/24/14: Added coupler_5_12, coupler_6_12 and nut_12
10/17/15: Added [V2.5] coupler 6.35/8mm + nuts
12/16/15: Added [V2.5] coupler 5/3, 5/4, 5/7 + nuts
02/04/16: Added [V2.5] coupler 5/6.35
03/12/16: Added [V2.5] coupler 5/5
05/26/16: Added [V2.5] coupler 5/8