I have combined the extended X upgrade and the Double precision upgrade previously only available for the MXL belt. I will apologize in advance for the poor documentation, I am simply out of time.
I have been running this on my machine since early May, 2014 and all is well. I have also converted my printer to run 1/4 microsteps instead of 1/16 microsteps. See the MXL double precision for speed limits on the stock 1/16 microsteping.
http://www.printrbottalk.com/ is a fantastic resource for all questions Printrbot. There are a few threads discussing my upgrades.
Please refer to the source files for details, there is really not too much new here.
2014-06-13: Added file for RH where the rods go thru. to allow for adjustment of bed with. "RodEndClamp2X-Mirror-RodThru.stl"
2014-05-29: Added files for the L&R ends without the mounting ears to anchor the machine.