Last updated 10 August 2026
3A drops the allowance and takes three quarters of the tolerance. On a 3/8-16 that is .3311 / .3344 against 2A's .3287 / .3331: a quarter less room, positioned higher.
Two things change and only one of them is the tolerance. 3A carries no allowance, so its maximum pitch diameter is the basic value exactly rather than sitting a thousandth below it. And its tolerance is three quarters of 2A's. The band is both tighter and higher, which is why a 3A thread will not always gauge in a fixture set up for 2A.
The practical cost is that 3A leaves nothing for plating. A 2A thread has an allowance that a thin coat can partly live in; a 3A thread at maximum material is already at basic, so any deposit at all puts it over. If a print calls 3A and a finish in the same breath, that is worth a question before the job runs rather than after.
| Thread | 0.375 inch nominal, 16 threads per inch, class 3A |
|---|---|
| Pitch | 0.0625 in |
| Basic pitch diameter | 0.3344 in |
| Allowance | none for this class |
| Tolerance | 0.0033 in |
| Pitch diameter | 0.3311 / 0.3344 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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