This replaces the standard Rostock Bowden hotend mounting that used the M8 nut. I did not like the way it threaded on and would come loose. I modified this derivative from an Ultimaker bowden assembly and the original Rostock and this is what I ended up with. It works with a 4mm tube. The insert / collet could be a little tighter but I just used a piece of tape around the bottom of the tube.Print bottom in ABS, 100% fill at .20-.25mm layers. Use ABS if possible to protect from the effects of heat.I printed the cap on my Rostock with PLA because I liked the look. Source files to come soon.Derived from: http://www.thingiverse.com/thing:11864 http://www.thingiverse.com/thing:27840