I also have an old French bike that will probably be upgraded from cottered to cotterless sometime in the future. To summarize the early info in the thread: You can use your old cups and just get a new spindle but the new spindle must have the same measurements between the races, etc. Or you can replace it as a whole unit and all you have to worry about is that the replacement is French threaded. The French threads are lefty loosy tighty righty on both sides. So watch which way you turn. It is better to leave the fixed cup in there unless you are replacing it.

Is that right so far? If you are replacing the whole unit how do you determine what size of spindle goes in? I was going to put the cottered cranks/chainrings on the old spindle, take a measurement from there and convert to cotterless. I am going to use a Stronglight 93 cotterless crankset. My cottered spindle is 137mm with 56mm between the races. Does anyone know what that would be in cotterless?