The top and bottom pieces on my Ultimaker 2 printhead assembly were falling apart after cracking in several places from over-tightening the thumbscrews. I searched YouMagine for direct replacements but, surprisingly, found nothing (everything I found was a mod for some specialized purpose). So, I downloaded the STEP files that Ultimaker generously provided on GitHub and converted them to STL, then divided each part into two pieces for easier installation. I've also made the non-divided parts available for download.

Update (7/15/2016): Over time, I started getting a bit of rattling and occasional slight left/right head shifts resulting from my poor decision to split the parts along the x-axis rod (especially the top piece!). I've reconfigured the split parts to be more stable. The new files have built-in supports which work very well using 0.1 mm layers. I apologize if anyone experienced any problems with my old files.