Re: User Error
BSP -- British Standard Pipe -- is a metric thread. The metric thread for pipes.
I can only imagine the negotiations and horse trading which led to that result (standards creation is notoriously corrupt), but out of all the thought that went into creating the metric standards, BSP was chosen without modification.
For nuts/bolts, a modern thread profile was wanted, which was why all the existing British bolt threads were abandoned, and Britain had the choice of going with Europe, or continuing with the new standard it had just developed in co-operation with the USA. Britain abandoned the new trans-Atlantic standard, and adopted the European standard instead. And Europe chose BSP for pipe threads. Was that the best choice for Europe? I have no idea what their pipe threads looked like before metric threads. But after metric threads, I know what their pipe thread standard is: BSP.