This is a modified version of: http://www.thingiverse.com/thing:96737 for V1 Printrbots. The lasercut part where the threaded rod goes through is rounded, the design was adapted for this reason.
I made 2 different length versions for those that have limited space when homeing the z axis. I used the long one as my springs where longer. The original solidworks file is included if you need further modifications.