I need a part that is not sold separately from the radiator.

It is just the upper neck that the upper hose fits over. It's attached to the radiator with two 8mm bolts and has an o-ring between the radiator and itself.

Someone out there has got to have an old trashed radiator or something they can grab this part off of!

 

Damn, up until a couple weeks ago I would have had one for you...finally got around to trashing the OEM radiator from my old R53 when I replaced it with a Mishimoto several months ago.

Check threads for people doing DIY cooling system repairs. If they're replacing the radiator, see if they'll send you the part.

 

Succubus, any chance you know what size that o-ring is? I've got a leak there and want to be sure I can get a replacement before I go pulling things apart.