5/16-18 UNC thread pitch and dimensions
The 5/16-18 UNC thread has 18 threads per inch, a pitch of 0.0556 in (1.41 mm), on a 0.3125 in major diameter (ASME B1.1). Its basic pitch diameter is 0.2764 in and its basic minor diameter is 0.2524 in.
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| Dimension | Formula | inch | mm |
|---|---|---|---|
| Major diameter D | nominal | 0.3125 | 7.94 |
| Pitch P | 1 / TPI | 0.0556 | 1.41 |
| Pitch diameter E | D − 0.649519 / n | 0.2764 | 7.02 |
| Minor diameter (internal) | D − 1.082532 / n | 0.2524 | 6.41 |
| Thread height (basic) | 0.541266 / n | 0.0301 | 0.76 |
| Tensile stress area As | 0.7854 (D − 0.9743 / n)² | 0.0524 in² | 33.8 mm² |
| Thread | TPI | Pitch Ø E | Stress area As | Tap drill |
|---|---|---|---|---|
| 5/16-18 UNC (this page) | 18 | 0.2764 in | 0.0524 in² | letter F |
| 5/16-24 UNF | 24 | 0.2854 in | 0.0581 in² | letter I |
| Thread | TPI | Pitch (mm) | Minor Ø | Stress area As |
|---|---|---|---|---|
| #10-24 UNC | 24 | 1.06 | 0.1449 in | 0.0175 in² |
| 1/4-20 UNC | 20 | 1.27 | 0.1959 in | 0.0318 in² |
| 5/16-18 UNC (this page) | 18 | 1.41 | 0.2524 in | 0.0524 in² |
| 3/8-16 UNC | 16 | 1.59 | 0.3073 in | 0.0775 in² |
| 7/16-14 UNC | 14 | 1.81 | 0.3602 in | 0.1063 in² |
Practical notes
Unified threads are named diameter–TPI–series: 5/16-18 UNC is a 5/16 nominal diameter with 18 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.
18 TPI is a pitch of 1.41 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 5/16-18 UNC it is 0.0524 in².
FAQ
How many threads per inch is 5/16-18 UNC?
18 TPI, a pitch of 0.0556 in (1.41 mm), in the Unified coarse series (ASME B1.1).
What is the minor diameter of 5/16-18 UNC?
The basic minor diameter of the internal thread is D − 1.082532/n = 0.2524 in (6.41 mm).
What is the tensile stress area of 5/16-18 UNC?
As = 0.7854 × (D − 0.9743/n)² = 0.0524 in² (33.8 mm²).
What tap drill does 5/16-18 UNC need?
The standard chart drill is letter F, 0.2570 in (6.53 mm), about 77% thread.
What is the difference between 5/16-18 UNC and 5/16-24 UNF?
Same 5/16 diameter, different pitch: 18 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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