This is the ebay seller that I got it from: http://www.ebay.com/itm/Robo-3D-Hob...673?pt=LH_DefaultDomain_0&hash=item33962860a1. The important measurement is that the hobb is centered on 15mm from the base of the bolt head (not the end). I probably trimmed off any excess at the end of the bolt with a hack saw. You will need to add one or two washers next to the bolt head to center the hobb at the filament hole. I also suggest that you check the nut in the extruder gear to make sure it has no play on retraction. If it does put a drop of red Locktite on each of the six surfaces of the nut that touch the gear. Then let it dry 24 hours. Try not to get it on the threads or the bearing. I also added about four washers on each of the two the spring loaded bolts to increase the tension.