The diameter of an imaginary cylinder that cuts the thread where the width of the thread and the width of the groove are equal. It is the key dimension in thread class tolerances, since mating flanks bear on each other around it. It is checked with thread micrometers, thread plug and ring gauges, or the three-wire method, where wires of a best size sit in the grooves and the measurement over them is converted by formula.
OD's fine, but pitch diameter over the wires is a thou under the 2A minimum, so the whole lot goes on hold.
Also heard as
- PD
- effective diameter
Do not mix it up with
- Major diameter: Major diameter is the outside size over the crests; a thread can have a good major and still fit badly if pitch diameter is off.
Related terms
Thread class
A number-and-letter grade, such as 2A or 3B, that sets how much tolerance and allowance an inch thread has.
Thread plug gauge
A hardened GO and NO GO gauge for internal threads: the GO end must screw in, the NO GO end must not.
Major diameter
The largest diameter of a screw thread, over the crests of an external thread or the roots of an internal one.
Minor diameter
The smallest diameter of a screw thread, at the roots of an external thread or the crests of an internal one.