Summary

Turn your Prusa Mendel World upside down!

And increase your Z-axis build space by 40% !!!

I've been printing this way for over a month now.

The stl and scad files below are for the 126mm tube that is printed in the picture.

Here's a YouTube video of the last part of the print:
http://m.youtube.com/watch?v=O6T0TAdobz4

Instructions

OK, it's just flipping the x-axis upside down, but here's why you're going to dismantle your z-axis this weekend and flip your x-axis upside down.

I'm always happy to push the envelope on anything, but sometimes necessity is the mother of invention. When my son said that he needed a 114mm tube for a Nerf Mod, and the most I could ever get from my Prusa Mendel kit from MakerGear was about 92mm. A day later he said, "Actually Dad, I could really use a 126mm tube instead." What's a 3D printer to do?

How about increasing the vertical build space by 40%?

Well, I had been looking for ways to expand the z-axis build area, including a thinner print bed, and of course, just using painter's tape for adhesion with PLA, and dumping the PCB for a few mm more height. I'm still working on dropping the y-carriage lower in the frame to gain a few more millimeters of z-axis build space. However, one day I was looking at the printer and realized that the Makergear Prusa-Mendel x-carriage was relatively symmetric about its own axis. I needed to adjust my z-axis anyway, so why not just flip the x-axis while I was doing the rebuild?

I did a little more planning and part building before committing to the rebuild, including this new thing here to maintain the micro-adjustment of the z-axis:

http://www.thingiverse.com/thing:40230

Other modifications to make to your Prusa Mendel in order to maximize the effectiveness of this rebuild include the following (Please read all of them before dismantling your z-axis):

When I went back to a heated print bed, I wanted to clip my glass onto the PCB using the common metal binder clips. The large ones I had been using were too big for the x-axis belt to clear them at one end. So I used the really small ones. They fit my standard Prusa PCB with my 1/8th inch glass perfectly. Thicker glass could use the next size up. Or try this solution with silicone: http://www.thingiverse.com/thing:39475
Go into Slic3r, or wherever you generate gcode and make sure that you eliminate the automated homing of the x-axis at the end of your prints. In my current configuration of my Prusa Mendel, homing the x-axis would produce a catastrophic continual forcing of the extruder motor into the frame of the printer and a whole lotta shakin' goin' on. Some time this month I plan to solve this problem by printing out a new extruder mount, but my design is not yet finished.
On the Makergear Prusa Mendel there is considerable wiggle room for the bolts of the x-axis motor mount. In order for the x-axis belt to have as little chance as possible of rubbing against the binder clips mentioned in #1, the x-axis motor should be moved as high up as it will go in the mount. As soon as I did this, all belt/clip contact ceased entirely, even when the z-axis is homed.
Make sure that your z-axis rods are REALLY parallel, from all the way at the top to all the way at the bottom. I thought mine were until I had flipped the x-axis, but the z-axis motors were grinding away trying to lift the x-axis as high as it had clearance for in the new configuration. Ultimately, I needed to use Brazen Artifice's advice about freeing up the 8mm nut traps a little on the x-axis mounts so that the rods could all align better. In addition, when realigning the x-axis mounts through these rods, it is necessary to get rid of the springs that seemed to be designed to support the top 8mm nut in the mount. The springs combined with gravity make it nearly impossible to keep the now flipped 8mm nut in place while you thread the rod through the mount. I have not missed them.
Finally, make sure that all of the wiring that you may have clipped or wound together (especially for the x-axis) will be able to make it to the top of the frame. I initially stretched my x-axis endstop wiring to the limits of the solder before realizing what the problem was.

One last concern: when I started using the PCB again, I was very concerned that the x-axis belt might become heated and lose tension being so close to the print bed, especially for prints that would be horizontal. In order to limit this possibility, I keep the x-axis about 30 or 40 mm above the print bed during heating, and home the z-axis as I always have at the beginning of the print. I am currently experimenting with a switch to using synchromesh cables on both the x-axis and the y-axis for greater precision and even more build height. I will post the results when I am finished.

Flip It Good!