The explanation of inch 25.4 mm and the pipe or why a 1” pipe does not have an outer diameter of Ø 25.4 mm.

The reason lies in the inner diameter, because at the beginning of the 20th century, the technical production possibilities and steel options were even more limited than they are today. The inside diameter of 1”, i.e. 25.4 mm, required a corresponding wall thickness, so that the outside diameter extended to approx. 33 mm. This meant that the pipes had an inner diameter of 1”, i.e. 25.4mm, a wall thickness of approx. 3.8-4.0mm and an outer diameter of approx. 33mm.
The fittings and tools were now also manufactured on this basis. As the steel quality improved, the pipe walls could also become thinner. However, because the fittings and threading tools were manufactured according to the outer diameter, the inner diameters inevitably had to become larger.
In heating and plumbing technology, the inch values no longer correspond to the measurements of the time, as the pipe dimensions with inch specifications are now defined metrically. Of course, the terms are still used in everyday language. If you find a pipe with an outer diameter of ~33 mm, then it is and remains a “1 inch pipe.” 
The DIN EN ISO 228-1 standard specifies the thread designation. However, the inner diameter of the pipe is not described in the standard. It is therefore not possible to convert from inches to mm.
It is certainly not easy to maintain an overview, but the basic German style should be incorporated and it is better to base the application on somewhat more stable dimensions because we have all grown old with them.