The delrin plunger, while an elegantly simple idea, bothers me a little because of the added drag on the filament. Even more so with the stepper driven mk6 extruder since for me at least there seems to be a pretty fine line between having it tight enough and too tight such that you are losing steps.
Like jstkatz and his http://www.thingiverse.com/thing:7037 I didn't like having the 4 screws and springs. I wanted something a little little simpler and could more easily pivot out of the way.
jstkatz's design definitely came out a little more elegant but functionally I think they are pretty similar (except mine uses the mk6 nema 17 motor).
Print the two parts. You shouldn't need support, I added some into the files already.
There is now a quick release lever for this extruder ( http://www.thingiverse.com/thing:7449 )
I seem to be having good success with the lever and no spring at all. It looks like an M4x40 bolt would be the right one for use with the lever.
Cut the support off the pivot location on the main body (on the lower right).
Cut the support material out of the pivot arm where the bearing mounts.
Clean up as necessary. It may help to run an appropriately sized drill through all of the holes.
Mount the bearing to the pivot arm with an M6 bolt & Nut (or you can print out http://www.thingiverse.com/thing:11424 )
Mount the pivot arm to the body with an M3 bolt & nut on the bottom and an M4 bolt, nut, washer and spring on the top.
You may need to cut out a little material to fit the M4 nut in there. I'll make slot a little bigger for the next version.
Insert the 4 M3 nuts into the provided slots. The one by the idler pivot can be a little fiddly to get in there.
Bolt to the extruder support with 4 M3x12 Bolts.
Bolt the stepper, cork gasket & fan mount with 4 M3x12 Bolts.
Let me know if I missed anything!
Change Log (started on 5/8/11)
5/8/11 Uploaded new idler arm (stl and alibre part) that works with 1.75mm filament (and 3mm)
6/28/11 Scaled the body part so that it doesn't need to be scaled before printing (inches->mm)