This is a replacement part for the Y belt tensioner on your Printrbot Simple Metal. My current version uses press fit 3mm nuts. The current version posted here is slightly slimmer than the photos to give you better tension range. 

I've included two files, the one labeled basic is a tighter fit, depending on how accurate your printer prints you can pick the best match.  If you don't have very good cooling fans, print both to give time for each piece to cool before the next layer.