1/4-28 UNF thread pitch and dimensions
The 1/4-28 UNF thread has 28 threads per inch, a pitch of 0.0357 in (0.91 mm), on a 0.250 in major diameter (ASME B1.1). Its basic pitch diameter is 0.2268 in and its basic minor diameter is 0.2113 in.
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| Dimension | Formula | inch | mm |
|---|---|---|---|
| Major diameter D | nominal | 0.2500 | 6.35 |
| Pitch P | 1 / TPI | 0.0357 | 0.91 |
| Pitch diameter E | D − 0.649519 / n | 0.2268 | 5.76 |
| Minor diameter (internal) | D − 1.082532 / n | 0.2113 | 5.37 |
| Thread height (basic) | 0.541266 / n | 0.0193 | 0.49 |
| Tensile stress area As | 0.7854 (D − 0.9743 / n)² | 0.0364 in² | 23.5 mm² |
| Thread | TPI | Pitch Ø E | Stress area As | Tap drill |
|---|---|---|---|---|
| 1/4-20 UNC | 20 | 0.2175 in | 0.0318 in² | #7 |
| 1/4-28 UNF (this page) | 28 | 0.2268 in | 0.0364 in² | #3 |
| Thread | TPI | Pitch (mm) | Minor Ø | Stress area As |
|---|---|---|---|---|
| #8-36 UNF | 36 | 0.71 | 0.1339 in | 0.0147 in² |
| #10-32 UNF | 32 | 0.79 | 0.1562 in | 0.0200 in² |
| 1/4-28 UNF (this page) | 28 | 0.91 | 0.2113 in | 0.0364 in² |
| 5/16-24 UNF | 24 | 1.06 | 0.2674 in | 0.0581 in² |
| 3/8-24 UNF | 24 | 1.06 | 0.3299 in | 0.0878 in² |
Practical notes
Unified threads are named diameter–TPI–series: 1/4-28 UNF is a 1/4 nominal diameter with 28 threads per inch in the fine series.
These are basic (zero-tolerance) dimensions. Most screws are made to class 2A and nuts to 2B (ASME B1.1), so measured sizes sit slightly below the basic values on the screw.
28 TPI is a pitch of 0.91 mm, which does not match a metric ISO pitch exactly; metric and Unified fasteners of similar size are not interchangeable.
Tensile stress area As is the area used for bolt strength (proof load = As × proof stress for the SAE J429 or ASTM grade). For 1/4-28 UNF it is 0.0364 in².
FAQ
How many threads per inch is 1/4-28 UNF?
28 TPI, a pitch of 0.0357 in (0.91 mm), in the Unified fine series (ASME B1.1).
What is the minor diameter of 1/4-28 UNF?
The basic minor diameter of the internal thread is D − 1.082532/n = 0.2113 in (5.37 mm).
What is the tensile stress area of 1/4-28 UNF?
As = 0.7854 × (D − 0.9743/n)² = 0.0364 in² (23.5 mm²).
What tap drill does 1/4-28 UNF need?
The standard chart drill is #3, 0.2130 in (5.41 mm), about 80% thread.
What is the difference between 1/4-28 UNF and 1/4-20 UNC?
Same 1/4 diameter, different pitch: 28 TPI versus 20 TPI. They are not interchangeable.
Sources: ASME B1.1 — Unified inch screw threads (UN and UNR thread form): basic dimensions, tensile stress area · ASME B18.2.1 — square, hex, heavy hex and askew head bolts and hex, heavy hex, hex flange screws (width across flats)
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