If you make a Printruder II ( http://www.thingiverse.com/thing:1980 ), and use a standard hex headed M4 x 40mm bolt for the tension adjustment, then you can more easily adjust the idler tension if you have a knob on the end of the bolt. This is that knob.Print. Insert M4x40mm bolt through the central hole so that the hex head nestles into the hex shaped hole. Press fit if needed.