5/8-18 UNF thread pitch and dimensions
The 5/8-18 UNF thread has 18 threads per inch, a pitch of 0.0556 in (1.41 mm), on a 0.625 in major diameter (ASME B1.1). Its basic pitch diameter is 0.5889 in and its basic minor diameter is 0.5649 in.
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| Dimension | Formula | inch | mm |
|---|---|---|---|
| Major diameter D | nominal | 0.6250 | 15.88 |
| Pitch P | 1 / TPI | 0.0556 | 1.41 |
| Pitch diameter E | D − 0.649519 / n | 0.5889 | 14.96 |
| Minor diameter (internal) | D − 1.082532 / n | 0.5649 | 14.35 |
| Thread height (basic) | 0.541266 / n | 0.0301 | 0.76 |
| Tensile stress area As | 0.7854 (D − 0.9743 / n)² | 0.2560 in² | 165 mm² |
| Thread | TPI | Pitch Ø E | Stress area As | Tap drill |
|---|---|---|---|---|
| 5/8-11 UNC | 11 | 0.5660 in | 0.2260 in² | 17/32" |
| 5/8-18 UNF (this page) | 18 | 0.5889 in | 0.2560 in² | 37/64" |
| Thread | TPI | Pitch (mm) | Minor Ø | Stress area As |
|---|---|---|---|---|
| 3/8-24 UNF | 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 |
| 5/8-18 UNF (this page) | 18 | 1.41 | 0.5649 in | 0.2560 in² |
| 3/4-16 UNF | 16 | 1.59 | 0.6823 in | 0.3730 in² |
Practical notes
Unified threads are named diameter–TPI–series: 5/8-18 UNF is a 5/8 nominal diameter with 18 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.
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 5/8-18 UNF it is 0.2560 in².
FAQ
How many threads per inch is 5/8-18 UNF?
18 TPI, a pitch of 0.0556 in (1.41 mm), in the Unified fine series (ASME B1.1).
What is the minor diameter of 5/8-18 UNF?
The basic minor diameter of the internal thread is D − 1.082532/n = 0.5649 in (14.35 mm).
What is the tensile stress area of 5/8-18 UNF?
As = 0.7854 × (D − 0.9743/n)² = 0.2560 in² (165 mm²).
What tap drill does 5/8-18 UNF need?
The standard chart drill is 37/64", 0.5781 in (14.68 mm), about 65% thread.
What is the difference between 5/8-18 UNF and 5/8-11 UNC?
Same 5/8 diameter, different pitch: 18 TPI versus 11 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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