the sizing of pipe, and the associated NPT threads are really basic, and very well organized. As this has been an industry mainstay for probably 200 years now. It may be easier for me, as I work with this sort of stuff nearly every day. The part that gets me is the metric, along with those companies that for whatever reason, decide to use BSPP (british standard parallel pipe) on the same piece of machinery with SAE products. You have the "AMERICAN" metric, the "EUROPEAN" metric. Then of course you get the different schedules of pipe, the OD needs to remain the same to facilitate threading, and basic fitting with/to the various fittings. Schedule 160 has a lot smaller ID than schedule 40.
