3/8" hole is 15.173mm, bug or feature?

[Sluring]

Hello. I made some holes and made the thread in them BSP 3/8".

The strange thing is that the diameter became 15.173 mm, but 3/8" is 9.525 mm.

Is this a bug or is my understanding of BSP threads all wrong?

The fittings we use seems to be BSP 3/8". (The website says 3/8" imperial)

[seth.madore · Accepted solution]

A 3/8 BSP is MUCH bigger than 3/8", as the 3/8 identifies the ID of the pipe. Here's some light reading: https://www.newmantools.com/taps/bsp.htm

I don't think you have an actual 3/8" BSP in your hands, it's looking like they might be 1/8" BSP

[Sluring]

I see.. and the thread for 1/8" holes just happened to be 3/8". If the thread had been some other dimension I'd probably have caught on to my mistake earlier. 🙂

Thanks!