Specification summary of ASME B18.3 as applied to flat head socket cap screws, unc: the dimensions the standard controls, the full specified size range, and the equivalent DIN/ISO/ASME designations.
ASME B18.3 — Scope
ASME B18.3 is "Socket cap, shoulder, set screws, and hex keys (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 B18.3 does not divide flat head socket 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 B18.3
| Size | Pitch P (mm) | b (mm) | Head diameter dk (mm) | dk_mean (mm) | Head height k (mm) | r (mm) | Hex socket s (mm) | t (mm) | Length range l (in) | Reference standard |
|---|---|---|---|---|---|---|---|---|---|---|
| #0 | 0.318 | 6.096 | 3.505 | 3.239 | 1.118 | 0.076 | 0.889 | 0.635 | 1/16"~1 1/2" | ASME B18.3 |
| #1 | 0.397 | 7.417 | 4.267 | 3.95 | 1.372 | 0.093 | 1.27 | 0.787 | 1/16"~1 1/2" | ASME B18.3 |
| #2 | 0.454 | 8.738 | 5.004 | 4.636 | 1.626 | 0.109 | 1.27 | 0.965 | 1/16"~2 1/2" | ASME B18.3 |
| #3 | 0.529 | 10.058 | 5.74 | 5.321 | 1.854 | 0.126 | 1.575 | 1.118 | 1/8"~2 1/2" | ASME B18.3 |
| #4 | 0.635 | 11.379 | 6.477 | 6.007 | 2.108 | 0.142 | 1.575 | 1.397 | 1/8"~4" | ASME B18.3 |
| #5 | 0.635 | 12.7 | 7.137 | 6.617 | 2.286 | 0.159 | 1.981 | 1.549 | 3/16"~4" | ASME B18.3 |
| #6 | 0.794 | 14.021 | 7.798 | 7.239 | 2.464 | 0.175 | 1.981 | 1.676 | 1/8"~4" | ASME B18.3 |
| #8 | 0.794 | 16.662 | 9.119 | 8.509 | 2.845 | 0.208 | 2.388 | 1.93 | 1/8"~5" | ASME B18.3 |
| #10 | 1.058 | 19.304 | 10.439 | 9.779 | 3.226 | 0.241 | 3.175 | 2.21 | 3/16"~8" | ASME B18.3 |
| 1/4" | 1.27 | 25.4 | 13.487 | 12.84 | 4.089 | 0.318 | 3.962 | 2.819 | 3/8"~10" | ASME B18.3 |
| 5/16" | 1.411 | 31.75 | 16.662 | 15.951 | 5.029 | 0.397 | 4.775 | 3.429 | 1/2"~10" | ASME B18.3 |
| 3/8" | 1.587 | 38.1 | 19.837 | 19.063 | 5.944 | 0.476 | 5.563 | 4.039 | 1/2"~12" | ASME B18.3 |
| 7/16" | 1.814 | 44.45 | 21.438 | 20.637 | 5.944 | 0.556 | 6.35 | 4.039 | 3/4"~12" | ASME B18.3 |
| 1/2" | 1.954 | 50.8 | 23.825 | 22.987 | 6.375 | 0.635 | 7.925 | 4.369 | 3/4"~12" | ASME B18.3 |
| 5/8" | 2.309 | 63.5 | 30.175 | 29.21 | 8.23 | 0.794 | 9.525 | 5.588 | 3/4"~14" | ASME B18.3 |
| 3/4" | 2.54 | 76.2 | 36.525 | 35.471 | 10.058 | 0.953 | 12.7 | 5.588 | 1"~14" | ASME B18.3 |
| 7/8" | 2.822 | 88.9 | 42.875 | 41.808 | 11.887 | 1.111 | 14.275 | 6.299 | 1 1/2"~14" | ASME B18.3 |
| 1" | 3.175 | 101.6 | 49.225 | 47.993 | 13.716 | 1.27 | 15.875 | 7.544 | 1 1/2"~14" | ASME B18.3 |
| 1 1/8" | 3.629 | 114.3 | 55.575 | 54.191 | 15.519 | 1.429 | 19.05 | 8.255 | 1 5/8"~14" | ASME B18.3 |
| 1 1/4" | 3.629 | 127 | 61.925 | 60.376 | 17.348 | 1.587 | 22.225 | 9.093 | 1 5/8"~14" | ASME B18.3 |
| 1 3/8" | 4.233 | 139.7 | 68.275 | 66.561 | 19.177 | 1.746 | 22.225 | 10.211 | 1 3/4"~14" | ASME B18.3 |
| 1 1/2" | 4.233 | 152.4 | 74.625 | 72.758 | 21.006 | 1.905 | 25.4 | 11.049 | 1 3/4"~14" | ASME B18.3 |
Companion Standards
ASME B18.3 controls only the geometry of flat head socket 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).
When to Specify ASME B18.3
ASME B18.3 is the inch-based reference; for metric programs choose the ISO equivalent instead.
Custom manufacturing · Made to drawing
Need a screw to this standard in a non-standard size?
Parts built to ASME B18 3 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 warm forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
