The bottom bracket converts the rotational power supplied by the legs, via the cranks, into athwartships torque via the chain to the free wheel and then to the wheels. Pre 1980, French bottom brackets had a diameter of 35mm and 1mm threads(25.4threads per inch aka TPI). Most other bikes had British ISO dims of 1.37" x 24 TPI. Replacing an old and tired French bottom bracket is no great hassle,it just takes a little time on the interwebs.The best part of this exercise is that you can then replace those rusty old cottered cranks with some shiny and new ones.Sugino cranks are my favourite.
Note: 35mm= 1.37". Diameter is not the issue here,just threading size. Note also, that the majority of bottom brackets have a length of 68mm
DO NOT DO THIS!.This is my first conversion.In ignorance I threaded an English ISO Shimano Alfine BB into a French BB shell.This functions really well, however as the threads have been 'crossed' I won't be able to use a threaded BB again. Below is a face saving solution to this problem by Velo Orange.
This threadless BB will work in any shell in which the threads have been stripped.As you turn the red side,the stainless steel 'shim'moves up the cartridge,locking it into the shell.A simple, yet elegant solution to save that old frame that's been lying around under the house.They come in various spindle lengths and 103mm is what you will need for Sugino cranks.
The Italian solution.We used this method when restoring the school schlepper.Keep the original bottom bracket cups and just replace the bearings and spindle.If the cups were Japanese, they have a slightly thinner wall and will require a 70mm Italian spindle rather than the usual 68mm. You can obtain one,NOS, without too much hassle on eBay.Slow schlepper had Sugino cups and this method worked perfectly.Thanks to the late Sheldon Brown for this tip.
The Shimano UN72 solution.A Shimano UN72 bottom bracket cartridge will work with 'Phil' French theaded rings.UN72s are no longer in production but you can obtain one,NOS, without too much hassle on Ebay.The Phil rings cost around $50 US.First you will need to remove the black,threaded end shells(the top cartridge above).You can do this,gingerly, with a hack saw. Once you have made the cut, just prize it apart with a screw driver and it will snap off.With this method you will also require a 'Phil' wrench to tighten the rings.Phil rings come supplied with a small bottle of 'loc tight' to prevent them loosening while being ridden. (I kept the original French locking ring and used this instead)
NOTE: this method has the advantage of athwartships adjustability.You can adjust the BB up to 5mm left or right to obtain the perfect chainline.
The 'Phil'ultimate solution.Same as above but use a shiny,expensive new 'Phil'BB cartridge.If you plan to keep the bike for a very long time, or just strive for perfection,do it.Heck, it's less than the cost of a tank and a half of petrol.
The VO 'budget' solution.VO produce a French threaded BB in various lengths. Cost is around $50 but don't be decieved by the low price.As with all VO products it's the quality goods. No athwartships adjustability and you will need a shimano BB tool to torque it in.