Summary

Rewrite from http://www.thingiverse.com/thing:155175 using OpenSCAD. Slightly modified for easy printing. Fully parametrized for easy adapting on other printers. See comments in the scad file.

Same instructions and BOM as the original.

Instructions

Copied from the original source http://www.thingiverse.com/thing:155175. The instruction is for installation on Portabee.

To install, carefully loosen the bolts at the back of left x-rods-clamp until the left z-bearing holder can be removed. The x-rods-clamp and x-end-motor should remain in place as shown in the last image. This case you do not need to re-install the whole x axis assembly.

After installed, leave about 5mm space between the base clamp and the z-endstop holder. Locate the z-endstop holder as usual and tighten the 16mmM3 bolt.

Now you can fine tune the position of the z-endstop by the adjustment wheel. Lock the position by the switch underneath after the adjustment to prevent the effect of vibrations.

work with:
M3 10mm bolt x 2
M3 16mm bolt x 1
M3 nut x 3
M3 washer x 1
M3 nylon washer x 2
M4 30mm screw x 1 (optional, M4 35mm screw)
M4 nut x 3
M4 washer x 3
M4 10mm spring x 1 (optional, insert between holder and base clamp)

OpenSCAD Z-endstop fine tuner