This is a geared extruder for Rostock, or any other RepRap printer. It is flatteringly identical to RichRap's recent remix. My changes amount to converting the extruder to accept 3mm filament, 1/4" to 6mm Pneumatic fittings and 4 ID - 6 OD mm PTFE tubing.
Assembly: Apply a little acetone or preheat the ABS locally before inserting the pneumatic fittings. This will ensure the snug fit and prevent the fitting from cheesing your ABS part.
Use: Consider using a light lubricating oil spray in the bowden tube if you are having trouble with filament friction.
UPDATE 27/03/13: The larger pneumatic fitting creates a smaller collar on the Hotend Mount. I have added another file (v2) that will be stronger as a printed part. 2 more holes need to be drilled out.
UPDATE: 29/03/13: More generous sockets have been added to all three printable parts. The original sockets were very tight indeed. The larger sockets still grip well while also making assembly a lot easier.
Remixed Sketchup files included. Remix on!