Originally Posted by
Steel Charlie
indeed - 1mm difference and 24tpi over the 25.4tpi
seems like 30-40 years ago this was done for blitzed French and considering if I heard about it then it must have been fairly common knowledge
...it is still done for any of the other BB threadings that are smaller when they are beyond redemption. I have done it myself recently, with a rusted BB in English threading without much threading left. But it's pretty hard on your taps to run an Italian set through a French BB without first reaming it out a little. I guess you can do it, but it's not standard practice.