Inch and mm

Depending on the Schedule for the pipe, the outer diameter of a 1" nominal bore pipe may well be 33.7mm. It may be more or it may be less, depending on the Schedule selected.

Why dont the outer diameter of a 1" nominal bore pipe may well be 25.4?

Because then the actual ID (bore) would be much less than 1" and the 1" refers to the bore (as it says). As TVP45 says, the OD is the controlled dimension. The actual bore is usually slightly greater than nominal for thinner wall pipes, slightly less for thicker wall.

For metric pipe, and inch pipe above 12", the nominal size is the OD (without going into too much detail). It doesn't matter, as long as you check the details of the pipe you're using.

Cheers........Codey

The 1" is a historical nominal value about a hundred years old. It originally referred to the ID, but not anymore. The OD of pipe has to be the same so that fittings work OK. Here's a chart that might help.

Thank you for comments

i have understood . But i wish there is only a unit in the world and we have works more easly :)

Ask one of the UK fellows to explain Whitworth bolt sizes to you.

Thanks for the link. A very long time ago, a mate bought an old Triumph. It wasn't really running, so I offered to tune it for him. I had SAE and metric wrenches, but what was this? I think I rounded most of the bolts before somebody told me what I was doing.