6mm Tube Makergear M2 Filament Guide Support Arm for 1.75mm filament
Two sizes, 6mm works great with 6mm OD teflon tubing which I highly recommend as it will always provide the least restriction even when using 1.75 mm filament. If your filament happens to have any bends when coming off the spool that puts restriction on the extruder as its being pulled through a small tube, give the extruder an easier time by using the 6mm OD tubing.
Buy the tubing from Mc-Master Carr, this is known good quality tubing, unlike the unknown stuff you can get on Ebay. Only $2.75 a foot as of March 2017.
https://www.mcmaster.com/#52335K34
The other legacy file is designed for the 3mm tubing that is stock on the makergear M2.
Also included is a simple adapter that allows the 6mm tubing to attach to this extruder:
http://www.thingiverse.com/thing:344950
Youtube video covering my various Makergear M2 Mods:
https://youtu.be/jD6ZTcCd0j8