This is a tension arm for the QU-BD MBE Extruder. It works with a 608 bearing that is commonly used for skates, roller blades and other similar things. The bearing can be inexpensive, since this is a low speed operation you don't need high ABEC ratings. This will work with the standard raptor gear in i'ts normal position using the groove. The clearance is tight but it works. The head of the feed tube screw hits the bearing with no filament but once the filament it is in it clears fine and runs smoothly. I started getting my best prints once I put this in place. This arm is stronger than some of the others that I have seen, printed and tried to use. Especially if you are going to print this with PLA. I found other designs to warp easily from the heat of the extruder and bed rising up. This design is inspired by the spring tensioner from Barton Dring. http://www.thingiverse.com/thing:38281I printed the arm shown using PLA at 400 microns (.4mm). Use a 608 bearing and the screw is there to provide lateral strength against the whole arm, it's not there to keep the bearing in place. The bearing is not going anywhere between the arm and the heatsink. Of course if you don't have the heat sink you can use a button head bolt and washer. Depending on your printer and settings the pivot hole might be tight, and you may need to drill it out a bit so that it pivots smoothly. This works with the spring block from Barton Dring. http://www.thingiverse.com/thing:38281 For arms mounted on the opposite side, use a scale of -1 in your printer settings across the proper axis to flip it. I do this using Repetier Host, make sure you unlock the scales so they don't all flip. I was using a raptor gear but the MK7 works much better with this arm. My original arm was printed at 400 microns and works great, no warping or deforming after many hours of printing. If you are using PLA and want some peace of mind, increase to 200 microns and 80% infill. The top fill pattern is concentric.