Bob
From my experience in the States most of their plumbing fittings are our equivalent of 1/4 inch as they fit isolating valves under every tap outlet.
I also think you wuill find that our Plumbing fittings are all threaded in BSP and the Americans are all in NPT which is not an easy conversion to couple to.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians