Summary

This is an adaptation of Makergear's concept for micro-adjusting the z-axis height for a Prusa Mendel. I needed it to be able to adjust the height after I flipped my x-axis here:

thingiverse.com/thing:40192

See instructions below for a more detailed explanation.

And please forgive the lousy print in the photos, I had some loose bolts on the print bed when I printed it. Of course, it's such a simple part that it doesn't affect the operation of the bolt.

Instructions

This part is used to permit the flipped x-axis of a Makergear Prusa Mendel to use the
tallest vertical support of the x-axis (moving on the steel rod) to trip the z endstop.

For pictures and full explanation of the ease with which I flipped the x-axis, go here:

http://www.thingiverse.com/thing:40192

For the micro-adjustment, mount an M3 bolt in the hole and turn as needed to adjust the height.

While I adapted this from a fan mount for a Makergear Prusa Mendel with nicely-rounded supports, this same design concept can be applied to any reprap whose x-carriage is symmetric with respect to the support bars. In Makergear's Prusa Mendel x-axis motor mount, there is a place for a screw to make micro-adjustments to the z-axis height and I didn't want to give up that capability when I flipped the x-axis. I could have just hacked the motor mount with a drill, but it would require a super-long screw, and I don't need that!

Instead, I adapted a ridiculously simple mount for a screw that can be turned up and
down to adjust the z-axis height with a twist of the nice M3 hex key that came with the kit.

I derived this code from this parametric fan mount on Thingiverse:

http://www.thingiverse.com/thing:13343

Then I lined it up with the z-axis endstop (BTW, if you ever need to adjust your endstop holders, I recommend this excellent parametric OPENSCAD z-axis endstop from Triffid Hunter, also on Thingiverse: http://www.thingiverse.com/thing:13482)

For my Makergear Prusa Mendel, I was able to mount this clip onto the upside-down vertical support when I flipped the x-axis.

I made my mount pretty thick, and I closed the opening a little from the original mount
so that the overall friction is increased. The $fn (and the concept) can
be adjusted to make any polygon-shaped clip regardless of the x-axis motor mount
shape.

Ridiculously Simple Z-axis Micro-Adjuster