It’s a little creaky, and doesn’t yet have hours and hours and hours on the clock, but the hardened steel 17tooth gear hasn’t caused an extruded failure yet! #saintflint there’s a brake line spacer and a printerbot gear acting as spacing and retention, respectively.
Correct. Interestingly there is a shoulder on the gear that falls off when you drill out to the proper diameter. Normally using a drill bit to drill a 5 mm hole is not a good idea because it doesn’t really come out to5mm. In this case that makes it just smaller than 5 mm and creates an interference fit. Granted I’ve only done it once, but it turned out pretty good this time.
Mike, since your loop is not a fixed length tube, I fail to see how your loop drive would be any better than if you were to feed the filament in only from the bottom. Besides you just unnecessarily score the filament on the first pass. Am I missing something here?
Yeah…my filament is purple 
You’re seeing teflon tubing at the top and bottom of the extruder. #saintflint pulls the filament in on one side of the frictionwheel, and additionally pushes it out on the other side of the wheel, effectively halving the pressure required to drive the filament. There’s little to no deformation of the filament because there’s never really enough pressure to do so.
Sorry, I did not realize that that was teflon tubing on the bottom loop.
Fwiw, the gear gave it up after about six weeks of printing.