In article , Upscale wrote:
On Sat, 26 Jun 2010 03:40:58 GMT, (Doug Miller)
wrote:

Think about it: if OD were "whatever the manufacturer feels like" you wouldn't

be able to use Manufacturer A's fittings with Manufacturer B's pipes. It's
precisely to ensure such interchangeability of pipes and fittings across
brands that the sizes are specified by OD, not by ID.

Would that not necessitate that ID also needs to be specified to the
fitting of one manufacturer will cover and fit the OD of another
brand? Sounds like it works both ways.

Short version:

The specs are for the OD of the pipe. Fittings are sized to match the pipe.

Long version:

Pipe sizes are specified by nominal diameter. Pipes of any given nominal
diameter in a particular material always have the same actual outside
diameter, although the inside diameter varies depending on the service
schedule. For example, Schedule 80 pipe is heavier-duty than Schedule 40 --
but 2" Sch 80 and 2" Sch 40 have the same OD. Sch 80 has a smaller ID because
it has thicker walls. The material matters too: 3/4" PVC, 3/4" steel, and 3/4"
copper pipes have distinctly different diameters.

Pipe fittings are specified also by nominal diameter. Female fittings of any
given nominal diameter in a particular material always have the same actual
*inside* diameter, because they need to fit on pipes with a specific *outside*
diameter. Male fittings of any given nominal diameter always have the same OD
as the corresponding pipe in the same material.

Threaded fittings of a particular nominal diameter always have exactly the
same actual diameter of the threaded portion, regardless of material.