1"-8 UNC thread pitch and dimensions
The 1"-8 UNC thread has 8 threads per inch, a pitch of 0.1250 in (3.18 mm), on a 1.000 in major diameter (ASME B1.1). Its basic pitch diameter is 0.9188 in and its basic minor diameter is 0.8647 in.
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| Dimension | Formula | inch | mm |
|---|---|---|---|
| Major diameter D | nominal | 1.0000 | 25.4 |
| Pitch P | 1 / TPI | 0.1250 | 3.18 |
| Pitch diameter E | D − 0.649519 / n | 0.9188 | 23.34 |
| Minor diameter (internal) | D − 1.082532 / n | 0.8647 | 21.96 |
| Thread height (basic) | 0.541266 / n | 0.0677 | 1.72 |
| Tensile stress area As | 0.7854 (D − 0.9743 / n)² | 0.6057 in² | 391 mm² |
| Thread | TPI | Pitch Ø E | Stress area As | Tap drill |
|---|---|---|---|---|
| 1"-8 UNC (this page) | 8 | 0.9188 in | 0.6057 in² | 7/8" |
| 1"-12 UNF | 12 | 0.9459 in | 0.6630 in² | 59/64" |
| Thread | TPI | Pitch (mm) | Minor Ø | Stress area As |
|---|---|---|---|---|
| 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 |
| 5/8-11 UNC | 11 | 2.31 | 0.5266 in | 0.2260 in² |
| 3/4-10 UNC | 10 | 2.54 | 0.6417 in | 0.3345 in² |
| 1"-8 UNC (this page) | 8 | 3.18 | 0.8647 in | 0.6057 in² |
Practical notes
Unified threads are named diameter–TPI–series: 1"-8 UNC is a 1" nominal diameter with 8 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.
8 TPI is a pitch of 3.18 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"-8 UNC it is 0.6057 in².
FAQ
How many threads per inch is 1"-8 UNC?
8 TPI, a pitch of 0.1250 in (3.18 mm), in the Unified coarse series (ASME B1.1).
What is the minor diameter of 1"-8 UNC?
The basic minor diameter of the internal thread is D − 1.082532/n = 0.8647 in (21.96 mm).
What is the tensile stress area of 1"-8 UNC?
As = 0.7854 × (D − 0.9743/n)² = 0.6057 in² (391 mm²).
What tap drill does 1"-8 UNC need?
The standard chart drill is 7/8", 0.8750 in (22.23 mm), about 77% thread.
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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