3/8-16 UNC thread pitch and dimensions
The 3/8-16 UNC thread has 16 threads per inch, a pitch of 0.0625 in (1.59 mm), on a 0.375 in major diameter (ASME B1.1). Its basic pitch diameter is 0.3344 in and its basic minor diameter is 0.3073 in.
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| Dimension | Formula | inch | mm |
|---|---|---|---|
| Major diameter D | nominal | 0.3750 | 9.52 |
| Pitch P | 1 / TPI | 0.0625 | 1.59 |
| Pitch diameter E | D − 0.649519 / n | 0.3344 | 8.49 |
| Minor diameter (internal) | D − 1.082532 / n | 0.3073 | 7.81 |
| Thread height (basic) | 0.541266 / n | 0.0338 | 0.86 |
| Tensile stress area As | 0.7854 (D − 0.9743 / n)² | 0.0775 in² | 50.0 mm² |
| Thread | TPI | Pitch Ø E | Stress area As | Tap drill |
|---|---|---|---|---|
| 3/8-16 UNC (this page) | 16 | 0.3344 in | 0.0775 in² | 5/16" |
| 3/8-24 UNF | 24 | 0.3479 in | 0.0878 in² | letter Q |
| Thread | TPI | Pitch (mm) | Minor Ø | Stress area As |
|---|---|---|---|---|
| 1/4-20 UNC | 20 | 1.27 | 0.1959 in | 0.0318 in² |
| 5/16-18 UNC | 18 | 1.41 | 0.2524 in | 0.0524 in² |
| 3/8-16 UNC (this page) | 16 | 1.59 | 0.3073 in | 0.0775 in² |
| 7/16-14 UNC | 14 | 1.81 | 0.3602 in | 0.1063 in² |
| 1/2-13 UNC | 13 | 1.95 | 0.4167 in | 0.1419 in² |
Practical notes
Unified threads are named diameter–TPI–series: 3/8-16 UNC is a 3/8 nominal diameter with 16 threads per inch in the coarse 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.
16 TPI is a pitch of 1.59 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 3/8-16 UNC it is 0.0775 in².
FAQ
How many threads per inch is 3/8-16 UNC?
16 TPI, a pitch of 0.0625 in (1.59 mm), in the Unified coarse series (ASME B1.1).
What is the minor diameter of 3/8-16 UNC?
The basic minor diameter of the internal thread is D − 1.082532/n = 0.3073 in (7.81 mm).
What is the tensile stress area of 3/8-16 UNC?
As = 0.7854 × (D − 0.9743/n)² = 0.0775 in² (50.0 mm²).
What tap drill does 3/8-16 UNC need?
The standard chart drill is 5/16", 0.3125 in (7.94 mm), about 77% thread.
What is the difference between 3/8-16 UNC and 3/8-24 UNF?
Same 3/8 diameter, different pitch: 16 TPI versus 24 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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