The diameter of an imaginary cylinder touching the crests of an external thread or the roots of an internal thread. It sets the nominal size, but real bolts run under it: a 1/2-13 Class 2A thread has a maximum major diameter of about 0.4985 in and a minimum near 0.4876 in. Measuring the major diameter tells you the size family, while fit is controlled by the pitch diameter.
Major diameter mikes at point four nine one, so it's a half-inch bolt, it's just the allowance on a 2A thread.
Also heard as
- outside diameter
- OD
Do not mix it up with
- Pitch diameter: Pitch diameter sits partway down the flanks and is what actually controls how two threads fit.
Related terms
Minor diameter
The smallest diameter of a screw thread, at the roots of an external thread or the crests of an internal one.
Pitch diameter
The diameter at which thread thickness equals the gap between threads, the dimension that controls fit.
Thread class
A number-and-letter grade, such as 2A or 3B, that sets how much tolerance and allowance an inch thread has.