A printable upgrade to allow increasing the tension on the X axis belt / pulley by turning a screw. Unlike the Y axis tensioner, ( http://www.thingiverse.com/thing:1618 ) the X axis tensioner only requires a single piece. The Sketchup file is included if you want to modify.
Make sure your X pulley is assembled as shown to the left and is adjusted to the correct height so that the X belt is totally level all the way through ( http://wiki.makerbot.com/cupcake-xy-stage-installation#toc3 ). The tensioner may work if you have your pulley assembled differently, but you'll have to make sure the pulley doesn't rub against the tensioner.
Special thanks to SSD ( http://www.thingiverse.com/ssd ) for testing and feedback. Version 3 has a slight change suggested by SSD which may help some people.
If you're trying this for the first time, use Version 2.
Loosen the X axis pulley, remove from the X axis slot, and remove the M5 nut from underneath.
Fit the tensioner around the M5 nut still on bolt.
Replace the X axis pulley with the M5 nut around the assembly.
Drop an M3 nut into the slot in the tensioner.
Drill a small M3 hole in the side of the MakerBot in line with the pulley.
Insert a M3x? bolt and tighten. (I am not positive as to the length of bolt required. This may differ slightly)