Hi there, Mike and John,
Pipe threads can be a bit confusing!
The key to the enigma is knowing a bit of the history.
Here's my take on the subject:
When drawn steel pipe was first standardised, the then current technology caused the pipe wall to be quite thick. (Maybe also, in those days, folded and seam-welded pipe.) The pipe was denoted by its bore but when pipe threads were standardised they obviously had to be based upon the outside diameter, NOT the bore.
Technology improved as time progressed and the pipe manufacturers learnt how to make pipe with thinner wall, more economical of material and lighter, hence easier to handle. However, by that time, there was a lot of pipe-threading tackle in wide use so the standardised pipe OD had to be maintained and the wall thinned from the inside outwards.
Hence the modern situation where the OD of a nominal 1½" pipe is actually 1_29/32" and nowhere on it will you find a dimension of 1½" !!!
(Pipe standards have proliferated as time progressed. I know that some branches of engineering (e.g. hydraulics) do, nowadays, use a range of pipe that's denoted by its OD - what I've written above relates to traditional steel pipe. As I write, I'm unsure as to how our modern domestic copper pipe is denoted, ID or OD.)
