ASME B18.2.1 Hex Head Cap Screw — Inch Series
The inch-series hex head cap screw, supplied in UNC (coarse) thread as standard. This is the North American counterpart to DIN 933 and ISO 4017, but the head is held to inch dimensions rather than metric, so the two are not interchangeable across a wrench.
Where it is used
The inch-series hex cap screw is the default structural bolt across North America: building frames, machine bases, structural steel connections and general industrial assembly. It carries the same role that DIN 933 and ISO 4017 hold in metric markets. Because US drawings, bills of material and hardware catalogues are written in this size system, a metric substitute cannot be dropped in without redrawing the joint.
How to choose it
UNC coarse thread is the standard supply; UNF fine is available in the smaller diameters where vibration or thin-wall sections call for it. Strength is called out under SAE J429 rather than ISO 898-1: Grade 2 is unmarked low carbon, Grade 5 has three radial lines on the head, and Grade 8 has six. Grade 8 reaches roughly 1034 MPa tensile, close to metric class 10.9, but the two grades are not interchangeable because the thread forms and strength designation systems differ.
Dimensions8 sizes · inches
| Thread | TPI | Width across flats s (in) | Head height k (in) | Head diameter dk (in) |
|---|---|---|---|---|
| 1/4-20 | 20 | 0.4375 | 0.1719 | 0.505 |
| 5/16-18 | 18 | 0.5 | 0.2109 | 0.577 |
| 3/8-16 | 16 | 0.5625 | 0.25 | 0.65 |
| 1/2-13 | 13 | 0.75 | 0.3281 | 0.866 |
| 5/8-11 | 11 | 0.9375 | 0.4219 | 1.083 |
| 3/4-10 | 10 | 1.125 | 0.5 | 1.299 |
| 7/8-9 | 9 | 1.3125 | 0.5781 | 1.516 |
| 1-8 | 8 | 1.5 | 0.6563 | 1.732 |
Nominal dimensions in inches as published in ASME B18.2.1. Tolerances, material requirements and marking are defined separately by the issuing body.
Available sizeslinked to individual specification pages
Thread referencesused by the sizes above
Fitting and interchange notes
Wrench sizes follow inch tooling — a 1/4 in bolt takes a 7/16 in wrench, not an 11 mm. The two are close enough to look interchangeable on a bench and different enough to round a head under load. Grade 5 and Grade 8 should not be mixed in the same joint, and Grade 8 is sensitive to hydrogen embrittlement when electroplated, so plated parts must be baked per ASTM B633 or the applicable finish specification.