I really liked this extruder, but there were a couple of things I wished were different so I remixed it. I've replaced the gears with herringbone gears because I think they're more aesthetically pleasing, and I revised some of the hardware because I don't like ordering stuff that's not available locally. I had the 6001 bearings already, so I retained the M12 bolt.

The extruder will accommodate 1.75mm or 3mm filament, as long as you have the correct tube fitting and the bolt is hobbed correctly.

The M12 bolt is partially threaded, and the portion that the filament will pass over is part of the non-threaded portion so it can be hobbed easily.

The M8 bolt holding the 608 bearing in place is a regular cap head screw that I cut the head off of once it was in place. Otherwise it won't clear the main bearings.

The body and idler parts are unmodified but I have included them anyway.

I hope this revision helps. It's a great design and I hope more people use it.

Here's the revised BOM:

Bearings:
2x 6001 bearings
1x 608 bearings

Bolts:
M8x20 (qty 1) - Holds the 608 bearing
M3x20 (qty 1) - Holds the idler to body
M3x55 (qty 2) - Tensioner Bolts
M12x60 (partially threaded) - Main hobbed bolt

Nuts:
M12 (qty 1) - Nylon lock nut
M3 (qty 2) - Nylon lock nuts

Other:
Two springs
M3 Washers (qty 4)
One standard NEMA17 stepper motor