Last updated 10 August 2026
A 1/4-20 UNC-2B tapped hole runs .2175 / .2223 on pitch diameter, and the minimum is the basic value exactly because internal threads carry no allowance.
This is the asymmetry between a bolt and the hole it goes into, and it is the reason the two are not mirror images. The allowance is applied to the external thread only, so an internal thread starts at basic and the whole of its tolerance opens upward. A 2B hole and a 2A bolt of the same size therefore do not straddle the same number.
It also means a tap is a fixed tool making a variable feature. What comes out depends on the material, the hole size before tapping and how the tap is running, so the limits are what you inspect against rather than what you set. A thread gauge is the honest check; a tap drill chart tells you where to start.
| Thread | 0.25 inch nominal, 20 threads per inch, class 2B |
|---|---|
| Pitch | 0.05 in |
| Basic pitch diameter | 0.2175 in |
| Allowance | none for this class |
| Tolerance | 0.0048 in |
| Pitch diameter | 0.2175 / 0.2223 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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