From the beginning the z-axis of my geeetech prusa i3 has been causing problems.
First I cut off the guiding hole on the top of the prusa frame (where the threaded rod is put in), which allowed movement of the axis. [Note: When you are facing the same problems on your printer and you want to try the mod with this z-nut-spacer, cutting of the guiding hole on top of the printer is absolutely necessary!]
To be able to print at all, I had to additionally loosen the mounting screws of the M8 nuts a bit (and used just 2 of the 4 screws). This way I was able to get pretty good prints, but I always wanted to improve this situation, because the already loosened srews of the nut repeatedly got more loose after a few printing hours.
So now I designed this z-nut distance pieces (spacers) to increase the distance from the nut to the stepper motor. When they are close together, what is always given in the first layers of the print, there always was high friction in the mechanic. On my uncles Geeetech printer the z-axis doesn't move at all, because of this.
So with the printed spacer the nut is further away from the stepper and imperfections of the coupling and axes not falling in line 100% is no longer super dramatic.
Also (on my printer) you can see that the top of the threaded rod doesn't wobble as much as before. The wobbling is reduced to about a third, which is pretty much!
Print like shown in the picture. This way the lower strength between the z-layers (compared to xy-strenght) is irrelevant.