The RP parts I have are for Adrian's geared extruder see the wiki here. For this you need some pats not listed on the original BOM list. There are a couple of different screws you need, nothing hard to find on ebay. Main parts you need are a short piece of M4 threaded rod. An M4 "Insert" - this is a splined piece used to grip the plastic filament. An M4 wing nut, and 3 bearings. The splined piece I got from ebay here is a bit different to the one shown in the wiki but serves the same purpose. It doesn't have a grub screw but it is internally threaded so should stay in place with bolts either side of it. It was listed as "Ripmax DYCO M4 x 0.7mm Coupling Insert" looks like it is used in the clutch mechanisms of model cars, trains etc.
After a bit of drilling and shaping the parts seem to fit together ok.