The socket head cap screw (SHCS) is the workhorse of the socket screw family: a cylindrical head with a hex socket, a flat bearing surface, and a body that can be preloaded harder than any hex-head bolt of the same size. Because the head is only about 1.5× the screw diameter, it fits in counterbores where a wrench never could, which is why it dominates machine tools, dies and fixtures, pumps, and anything else built to be taken apart and put back together.
The table below gives the standard head dimensions for inch-series hex socket head cap screws per ASME B18.3, from #0 through 1-1/2″, with the key engagement and wall-thickness minimums most charts leave out. Cold Headers stocks socket head cap screws in alloy steel, 18-8 stainless, 316 stainless and brass, in coarse and fine thread, and in the odd lengths and diameters that most distributors don’t carry.
Socket Head Cap Screw Dimensions — ASME B18.3 (inches)
| Nominal Size | Threads/in (UNC / UNF) | D – Body Diameter | A – Head Diameter | H – Head Height | J – Hex Socket (nom.) | T – Key Engagement (min) | G – Wall Thickness (min) | |||
|---|---|---|---|---|---|---|---|---|---|---|
| Max | Min | Max | Min | Max | Min | |||||
| #0 | 80 UNF | 0.0600 | 0.0568 | 0.096 | 0.091 | 0.060 | 0.057 | 0.050 | 0.025 | 0.020 |
| #1 | 64 / 72 | 0.0730 | 0.0695 | 0.118 | 0.112 | 0.073 | 0.070 | 1/16 | 0.031 | 0.025 |
| #2 | 56 / 64 | 0.0860 | 0.0822 | 0.140 | 0.134 | 0.086 | 0.083 | 5/64 | 0.038 | 0.029 |
| #3 | 48 / 56 | 0.0990 | 0.0949 | 0.161 | 0.154 | 0.099 | 0.095 | 5/64 | 0.044 | 0.034 |
| #4 | 40 / 48 | 0.1120 | 0.1075 | 0.183 | 0.176 | 0.112 | 0.108 | 3/32 | 0.051 | 0.038 |
| #5 | 40 / 44 | 0.1250 | 0.1202 | 0.205 | 0.198 | 0.125 | 0.121 | 3/32 | 0.057 | 0.043 |
| #6 | 32 / 40 | 0.1380 | 0.1329 | 0.226 | 0.218 | 0.138 | 0.134 | 7/64 | 0.064 | 0.047 |
| #8 | 32 / 36 | 0.1640 | 0.1585 | 0.270 | 0.262 | 0.164 | 0.159 | 9/64 | 0.077 | 0.056 |
| #10 | 24 / 32 | 0.1900 | 0.1840 | 0.312 | 0.303 | 0.190 | 0.185 | 5/32 | 0.090 | 0.065 |
| 1/4″ | 20 / 28 | 0.2500 | 0.2435 | 0.375 | 0.365 | 0.250 | 0.244 | 3/16 | 0.120 | 0.095 |
| 5/16″ | 18 / 24 | 0.3125 | 0.3053 | 0.469 | 0.457 | 0.312 | 0.306 | 1/4 | 0.151 | 0.119 |
| 3/8″ | 16 / 24 | 0.3750 | 0.3678 | 0.562 | 0.550 | 0.375 | 0.368 | 5/16 | 0.182 | 0.143 |
| 7/16″ | 14 / 20 | 0.4375 | 0.4294 | 0.656 | 0.642 | 0.438 | 0.430 | 3/8 | 0.213 | 0.166 |
| 1/2″ | 13 / 20 | 0.5000 | 0.4919 | 0.750 | 0.735 | 0.500 | 0.492 | 3/8 | 0.245 | 0.190 |
| 5/8″ | 11 / 18 | 0.6250 | 0.6163 | 0.938 | 0.921 | 0.625 | 0.616 | 1/2 | 0.307 | 0.238 |
| 3/4″ | 10 / 16 | 0.7500 | 0.7406 | 1.125 | 1.107 | 0.750 | 0.740 | 5/8 | 0.370 | 0.285 |
| 7/8″ | 9 / 14 | 0.8750 | 0.8647 | 1.312 | 1.293 | 0.875 | 0.864 | 3/4 | 0.432 | 0.333 |
| 1″ | 8 / 12 | 1.0000 | 0.9886 | 1.500 | 1.479 | 1.000 | 0.988 | 3/4 | 0.495 | 0.380 |
| 1-1/4″ | 7 / 12 | 1.2500 | 1.2336 | 1.875 | 1.852 | 1.250 | 1.236 | 7/8 | 0.620 | 0.475 |
| 1-1/2″ | 6 / 12 | 1.5000 | 1.4818 | 2.250 | 2.224 | 1.500 | 1.485 | 1 | 0.745 | 0.570 |
Body diameter D applies to the unthreaded shank of partially threaded screws. Head diameter A is the maximum across the knurled or plain head. Key engagement T is the minimum depth a correctly sized hex key must engage. Wall thickness G is the minimum material between the socket and the outside of the head. The hex socket size J is also the hex key (Allen key) size required to drive the screw.
Thread length
ASME B18.3 sets the minimum thread length for inch socket head cap screws at LT = 2D + 0.50″, where D is the nominal diameter. Any screw shorter than LT plus the unthreaded body is supplied fully threaded. Examples: a 1/4″ SHCS has at least 1.00″ of thread, a 3/8″ SHCS at least 1.25″, and a 1/2″ SHCS at least 1.50″. Longer screws carry a plain body between the head and the threads.
Material and strength
- Alloy steel (ASTM A574): minimum tensile strength 180,000 psi for sizes through 1/2″, and 170,000 psi for sizes over 1/2″. Hardness Rockwell C39–45 (through 1/2″) or C37–45 (over 1/2″). Supplied with a black oxide (thermal) finish unless plated.
- 18-8 stainless steel (ASTM F837): austenitic 300-series stainless, typically 70,000–90,000 psi tensile depending on size and condition. Non-magnetic in the annealed condition, excellent general corrosion resistance.
- 316 stainless steel: for chloride, marine and chemical-process environments where 18-8 pits.
- Brass: non-sparking, non-magnetic, decorative; used in electrical and marine hardware.
How to specify a socket head cap screw
Give the diameter and thread pitch, the length measured from under the head, the material, and any finish: for example 3/8-16 × 1-1/4 Socket Head Cap Screw, Alloy Steel, Black Oxide. For a locking version, ask for a nylon insert (nylon pellet) SHCS, which carries a nylon locking element in the threads so the screw resists loosening under vibration.