3/8-24 UNF thread pitch and dimensions
The 3/8-24 UNF thread has 24 threads per inch, a pitch of 0.0417 in (1.06 mm), on a 0.375 in major diameter (ASME B1.1). Its basic pitch diameter is 0.3479 in and its basic minor diameter is 0.3299 in.
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| Dimension | Formula | inch | mm |
|---|---|---|---|
| Major diameter D | nominal | 0.3750 | 9.52 |
| Pitch P | 1 / TPI | 0.0417 | 1.06 |
| Pitch diameter E | D − 0.649519 / n | 0.3479 | 8.84 |
| Minor diameter (internal) | D − 1.082532 / n | 0.3299 | 8.38 |
| Thread height (basic) | 0.541266 / n | 0.0226 | 0.57 |
| Tensile stress area As | 0.7854 (D − 0.9743 / n)² | 0.0878 in² | 56.7 mm² |
| Thread | TPI | Pitch Ø E | Stress area As | Tap drill |
|---|---|---|---|---|
| 3/8-16 UNC | 16 | 0.3344 in | 0.0775 in² | 5/16" |
| 3/8-24 UNF (this page) | 24 | 0.3479 in | 0.0878 in² | letter Q |
| Thread | TPI | Pitch (mm) | Minor Ø | Stress area As |
|---|---|---|---|---|
| 1/4-28 UNF | 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 (this page) | 24 | 1.06 | 0.3299 in | 0.0878 in² |
| 7/16-20 UNF | 20 | 1.27 | 0.3834 in | 0.1187 in² |
| 1/2-20 UNF | 20 | 1.27 | 0.4459 in | 0.1600 in² |
Practical notes
Unified threads are named diameter–TPI–series: 3/8-24 UNF is a 3/8 nominal diameter with 24 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.
24 TPI is a pitch of 1.06 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 3/8-24 UNF it is 0.0878 in².
FAQ
How many threads per inch is 3/8-24 UNF?
24 TPI, a pitch of 0.0417 in (1.06 mm), in the Unified fine series (ASME B1.1).
What is the minor diameter of 3/8-24 UNF?
The basic minor diameter of the internal thread is D − 1.082532/n = 0.3299 in (8.38 mm).
What is the tensile stress area of 3/8-24 UNF?
As = 0.7854 × (D − 0.9743/n)² = 0.0878 in² (56.7 mm²).
What tap drill does 3/8-24 UNF need?
The standard chart drill is letter Q, 0.3320 in (8.43 mm), about 79% thread.
What is the difference between 3/8-24 UNF and 3/8-16 UNC?
Same 3/8 diameter, different pitch: 24 TPI versus 16 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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