For hex head cap screws, unc, ASME B18.2.1 fixes the geometry and the tolerances. The sections below describe the scope of the document, the properties it deliberately leaves to companion standards, and the equivalent designations across standards bodies.
ASME B18.2.1 — Scope
ASME B18.2.1 is "Square, hex, heavy hex, and askew head bolts and hex, heavy hex, hex flange, lobed head, and lag screws (inch series)". An inch-series document.
Material, property class, surface finish and packaging are governed by separate documents — ISO 898-1 and ISO 898-2 for mechanical property classes, ISO 4042 for electroplated finishes.
Tolerance & Product Grades
ASME B18.2.1 does not divide hex head cap screws, unc into product grades. Tolerances are given as limits on the individual dimensions in the table.
Where a general-tolerance class is cited on the drawing instead, it applies to the dimensions not otherwise toleranced.
Dimensions Specified by ASME B18.2.1
| Size | b (mm) | Pitch P (mm) | c (mm) | dw (mm) | Width across corners e (mm) | Head height k (mm) | r (mm) | Width across flats s (mm) | Length range l (in) | Reference standard |
|---|---|---|---|---|---|---|---|---|---|---|
| 1/4" | 19.05 | 1.27 | 0.406 | 10.557 | 12.832 | 4.366 | 0.37 | 11.113 | 3/8"~12" | ASME B18.2.1 |
| 5/16" | 22.225 | 1.411 | 0.406 | 12.065 | 14.665 | 5.556 | 0.423 | 12.7 | 1/2"~12" | ASME B18.2.1 |
| 3/8" | 25.4 | 1.587 | 0.406 | 13.573 | 16.498 | 6.35 | 0.476 | 14.287 | 1/2"~12" | ASME B18.2.1 |
| 7/16" | 28.575 | 1.814 | 0.406 | 15.081 | 18.331 | 7.541 | 0.529 | 15.875 | 3/4"~12" | ASME B18.2.1 |
| 1/2" | 31.75 | 1.954 | 0.406 | 18.098 | 21.997 | 8.731 | 0.635 | 19.05 | 3/4"~12" | ASME B18.2.1 |
| 5/8" | 38.1 | 2.309 | 0.406 | 22.622 | 27.496 | 10.716 | 0.794 | 23.812 | 3/4"~14" | ASME B18.2.1 |
| 3/4" | 44.45 | 2.54 | 0.406 | 27.146 | 32.996 | 12.7 | 0.953 | 28.575 | 1"~14" | ASME B18.2.1 |
| 7/8" | 50.8 | 2.822 | 0.406 | 31.671 | 38.495 | 14.684 | 1.111 | 33.337 | 1 1/2"~14" | ASME B18.2.1 |
| 1" | 57.15 | 3.175 | 0.406 | 36.195 | 43.994 | 17.066 | 1.27 | 38.1 | 1 1/2"~14" | ASME B18.2.1 |
| 1 1/8" | 63.5 | 3.629 | 0.406 | 40.719 | 49.493 | 19.05 | 1.429 | 42.862 | 1 5/8"~14" | ASME B18.2.1 |
| 1 1/4" | 69.85 | 3.629 | 0.406 | 45.244 | 54.993 | 21.431 | 1.587 | 47.625 | 1 5/8"~14" | ASME B18.2.1 |
| 1 3/8" | 76.2 | 4.233 | 0.406 | 49.768 | 60.492 | 23.019 | 1.746 | 52.388 | 1 3/4"~14" | ASME B18.2.1 |
| 1 1/2" | 82.55 | 4.233 | 0.406 | 54.292 | 65.991 | 25.4 | 1.905 | 57.15 | 1 3/4"~14" | ASME B18.2.1 |
| 1 3/4" | 95.25 | 5.08 | 0.406 | 63.341 | 76.99 | 29.369 | 2.223 | 66.675 | 2"~14" | ASME B18.2.1 |
| 2" | 107.95 | 5.644 | 0.406 | 72.39 | 87.988 | 34.131 | 2.54 | 76.2 | 2 1/2"~14" | ASME B18.2.1 |
| 2 1/4" | 120.65 | 5.644 | 0.406 | 81.439 | 98.987 | 38.1 | 2.857 | 85.725 | 2 1/2"~14" | ASME B18.2.1 |
| 2 1/2" | 133.35 | 6.35 | 0.406 | 90.487 | 109.985 | 42.069 | 3.175 | 95.25 | 3"~14" | ASME B18.2.1 |
| 2 3/4" | 146.05 | 6.35 | 0.406 | 99.536 | 120.984 | 46.038 | 3.493 | 104.775 | 3"~14" | ASME B18.2.1 |
| 3" | 158.75 | 6.35 | 0.406 | 108.585 | 131.982 | 50.8 | 3.81 | 114.3 | 3 1/2"~14" | ASME B18.2.1 |
| 3 1/4" | 171.45 | 6.35 | 0.406 | 117.634 | 142.981 | 55.562 | 4.128 | 123.825 | 3 3/4"~14" | ASME B18.2.1 |
| 3 1/2" | 184.15 | 6.35 | 0.406 | 126.683 | 153.979 | 58.737 | 4.445 | 133.35 | 3 3/4"~14" | ASME B18.2.1 |
| 3 3/4" | 196.85 | 6.35 | 0.406 | 135.731 | 164.978 | 63.5 | 4.763 | 142.875 | 4"~14" | ASME B18.2.1 |
| 4" | 209.55 | 6.35 | 0.406 | 144.78 | 175.976 | 68.263 | 5.08 | 152.4 | 4"~14" | ASME B18.2.1 |
Equivalent Standards
Where direct equivalents exist (ISO ↔ DIN, ISO ↔ ASME), they are listed on the parent Hex Head Cap Screws, UNC page.
Always confirm against the dimensions on this page, since some "equivalent" standards differ in tolerances and head geometry.
Companion Standards
ASME B18.2.1 controls only the geometry of hex head cap screws, unc. The mechanical strength is set by ISO 898-1 (bolts, screws and studs) or ISO 898-2 (nuts); the surface finish is governed by separate finish standards such as ISO 4042 for electroplating. For corrosion testing, the relevant document is ISO 9227 (neutral salt spray).
Custom manufacturing · Made to drawing
Need a bolt to this standard in a non-standard size?
Parts built to ASME B18 2 1 are already in the catalog, so a special is usually a variation on existing tooling rather than a new design — which is what keeps the cost down. Made by cold heading or hot forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
