The diameters are sometimes quoted in mm but the thread pitches are in rational units of threads per inch (tpi) – that should make Paul smile
Oh yes, I remember details like that for when somebody tries to tell me that "everything" is made to metric specifications these days!
![[Linked Image]](https://www.electrical-contractor.net/ubb/biggrin.gif)
Copper water pipe in Britain underwent the change from inch to millimeter specifications, and in doing so also change from inside bore to outside diameter measurements. Hence an old 1/2-inch pipe is almost the same as a modern 15mm type, and is certainly close enough to be interchangeable in a compression fitting. Unfortunately, the changes to some of the larger size were not so convenient and necessitate the use of adapters when coupling up new pipes to an old system.
