Simplest, lowest hassle Zwobble elimination that I know of for the Cupcake.
1)Print 4 ACME Nut Holders, these work with 7/8" Wrench size nuts which is typical for 1/2" ACME nuts.
2)Print 4 Z extender Bases Direct from Zydac's Z extender kit (as in NOT my design), Thisincreases the base height and reduces the length of the rod which in turn moves the worst wobble spot (the center of rod) up away from your typical printing height and reduces the max wobble (straightness is expressed in 1/1000/s per inch, so less inches gives more straightness)
3) Cut 4 9" lengths of 1/2" -20tpi Good quality ACME rod I use Bright zinc tested for straightness on a glass table. (strength is no issue here, only straightness/ thread quality)
4) Lathe down 20mm to 7.8~7.9mm to slide fit a Skate bearing on one end of each rod. (what is normally used on the cupcake for Z bearings)
5) lathe down 34mm of the other end of each rod to the same size.
6) Bolt Z Extenders to cupcake "floor" using existing hardware
7) Press fit ACME its in to holders and bolt to BOTTOM of acrylic Z platform clearancing for Rods if necessary (mine fit, but only by 1/2mm on each side.
8) OPTIONAL If your ACME nuts are tight fitting you will need to swap your stepper pulley for the smaller size used on the Cupcake X + Y stages.
Calibrate your Z stage steps per MM depending on if you had to upgrade your pulley or not.
I do sell the Rods pre-Turned down on Ebay and include the nuts which fit perfect for use with the stock Z pulley if you don't have access to a Lathe, and the ACME rods are available from McMasterCarr.
Ebay Item #130661755008 http://www.ebay.com/itm/Makerbot-Cupcake-CNC-z-wobble-eliminator-ACME-rods-/130661755008?pt=LH_DefaultDomain_0&hash=item1e6c0c2880#ht_500wt_1413