The ISO fitting has been the worldwide standard long before ISO even existed. It is originally the BSA standard that was the de facto worldwide standard until the US departed from that uniformity by developing the slightly different version called today NP. Both the US and BS fittings are inch based. All major manufacturers of fittings make them with any type of pipe threads (taper or parallel) and any number of ordinary threads. Whether they sell them all in any one country is a different question. A US (NP) and ISO (BS, DIN or any other) inch threads do screw together in most sizes as you experienced. But they cannot seal on the threads in principle. If they do, it is due to the extraordinary muscle size of the pipe fitter, the high strength of the goop/stuff put on the threads, and the undemanding performance requirement (low pressure, no vibration, etc.) Either inch-based fitting does not screw readily into a metric fittings. Stan Jakuba, www.awc.com/metric PS: The term fitting was used here in a loose, all encompassing sense, as you named it.
