This is the diameter of an imaginary cylinder coaxial with the thread, which has equal metal and space widths. It is often referred to as pitch diameter. Sometimes referred to as the simple effective diameter to differentiate from the virtual effective diameter.
Found on
http://www.boltscience.com/pages/glossary.htm
In metal bellows terminology it is the calculated diameter where the pressure penetrates between the metal plates. This number is used to determine the hydraulic balance diameter of the seal face.
Found on
https://www.encyclo.co.uk/local/21120
No exact match found.
