This is a modification of the z-axis top holder for the Prusa i3 Original. The design is based off of the original mount for the i3 with 8mm smooth rod and 5mm thread. It is a tight fit with the smooth rod but is loose enough with the 5mm to allow movement.
I was having problems with prints shifting the higher the print and noticed that there was a significant wobble within the z-axis rods. since installation prints have gone very smooth