Originally Posted by Poguemahone

"put a new Shimano STX headset ($25) into my old Motobecane conversion no problem."

Unless the HS is French thread, this is (very) bad advice. Seaden's bike is certainly French thread in the HS (70s Moto). The diameter of French fork steerer tubes is slightly smalller than the BSC standard. You can actually thread a BSC locknut onto a french fork, but it will be slightly loose. Enough to tear off at an inconvient moment. It is likely your bike is either late enough to be English thread (French manufacturers switched over in the eighties) or the fork has been replaced. If your fork is French thread I would abandon this HS setup immediately.

You do have other options aside from hunting down a French HS. Often, in the case of brinelling, it's the lower races that are the problem. You can mix headsets, using the bottom from a standard and the top from a frenchie, although you may have to file down the fork crown race or have it done at a shop. Older French fork crown races are 26.5, Campy Standard 26.4. Please note in the seventies and eighties, you will find french threaded bikes with the fork crown at 26.4 and 27.0 (JIS standard). You can also replace a french fork with an English threaded one; replacing the HS as well. These two fixes work because the interior of the head tube is the same on both a frenchie and a more standard bike, so you can install standard cups in a French frame.