From Sheldon Brown; Threading/interchangeability Issues for Older Raleigh Bicycles

Re-tapping to B.S.C./ISO

This option is the most versatile, but also the most drastic. Only the top bike shops will have the necessary guided tap set required for this. The threads will not be as strong as they would be if the shell was made for standard threading, but this approach generally works well. You will still need to deal with the overall shell width. A shop that has the correct taps will also have the correct tool to face the ends of the shell, and this tool can be used to bring the width down to a more standard size (though it takes a lot of elbow grease! Beware, though, the welds to the bottom bracket shell of a Raleigh Twenty can be very hard, maybe harder than the taps.