Last updated 10 August 2026
Basic pitch diameter is .3344, and a class 2A external thread runs .3287 / .3331. The maximum sits below basic because 2A carries a .0013 allowance.
Basic pitch diameter is geometry: the major diameter less three quarters of the height of the thread form, which for a 60 degree thread is .649519 times the pitch. That figure belongs to the thread itself and every class shares it. What the class decides is where the band sits relative to it.
The allowance is the part people read as tolerance and it is not. It is clearance built into the external thread so a coated or slightly imperfect bolt still enters its nut, and it is spent before the machinist sees the drawing. The tolerance is the .0044 below it, and that is the whole of what there is to work in.
| Thread | 0.375 inch nominal, 16 threads per inch, class 2A |
|---|---|
| Pitch | 0.0625 in |
| Basic pitch diameter | 0.3344 in |
| Allowance | 0.0013 in, below basic |
| Tolerance | 0.0044 in |
| Pitch diameter | 0.3287 / 0.3331 in |
Open this case in the calculator, and change the numbers.
Computed by ctrlPlanner. ASME B1.1, Federal Screw Thread Specification H28.
This is one thread. Pair it with walk_back_tolerance to get the size to cut before plating, and note that ctrlPlanner does that pairing for every threaded feature on a print at once, and redoes it when the drawing revises.
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