We ran into an issue where our NEMA8 steppers for the nozzle tips were not powerful enough to twist the rubber tube attached to it, even with the 180degree limit enabled. Looks like a couple people have gone with NEMA11 and special (expensive) adapters. We opted to print a little boot for the top of the motor that leaves the shaft to freely rotate inside. Problem solved!

The part is up on thingiverse here http://www.thingiverse.com/thing:1280038

3D files can be downoaded here http://a360.co/23dJGMF