ANSI B18.3 Button Socket Head Cap Screws
ANSI/ASME B18.3 button socket head cap screws, #0 to 3/4 in — the low-profile domed-head inch hex socket screw — with the collected 16-size table: head diameter and height bands, socket size, and the maximum standard length per size.
Last updated: 2026-11
Specifications
The button socket head cap screw is the inch-series answer to "hex socket strength without hex head height": a domed cylindrical head roughly half the height of a standard socket head cap screw, driven by a hex socket, with a low, smooth profile that clears moving parts, slides under guards, and presents no snag edge on operator-facing equipment. It is the standard head for robotics and automation assemblies, jigs, die sets and consumer-adjacent machinery where the screw is visible and handled. The collected B18.3 table runs 17 sizes from #0 to 3/4 in and tabulates per size: nominal diameter with UNC/UNF/UNEF threads per inch (UNEF from 1/4 up on selected sizes), head diameter d k and height k with max/min limits, the under-head diameter d a, hex socket nominal size s with minimum depth t, and the maximum standard length L tabulated per size — 0.5 in at the small sizes stepping to 2.5 in at 3/4. Note where the series stops: 3/4 is the top of the collected button-head range, so larger joints step up to standard socket head cap screws. For the metric counterpart, ISO 7380-1 button heads are covered separately on this site.
| Standard | ANSI/ASME B18.3 — button socket head cap screws (inch series) |
|---|---|
| Series | #0 – 3/4 in (16 sizes tabulated); UNC/UNF, UNEF on selected sizes |
| Tabulated geometry | Per size: nominal diameter d, threads per inch by series, head d k (max / min), head height k (max / min), under-head d a (max / min), socket s, socket depth t (min), maximum standard length L |
| At 1/4 | UNC 20 / UNF 28 / UNEF 32; head d k 0.437 / 0.419 in; height k 0.132 / 0.122 in; socket s 0.156 in, depth t min 0.087 in; max standard length L 1 in |
| At 3/8 | UNC 16 / UNF 24 / UNEF 32; head d k 0.656 / 0.636 in; height k 0.199 / 0.185 in; socket s 0.219 in, depth t min 0.122 in; max standard length L 1.25 in |
| At 3/4 | UNC 10 / UNF 16 / UNEF 20; head d k 1.125 / 1.105 in; height k 0.375 / 0.355 in; socket s 0.437 in, depth t min 0.245 in; max standard length L 2.5 in |
| Materials / strength | Not tabulated by the collected specification — per RFQ (property class / hardness and finish per drawing callout) |
| Size | Head Dia. (A) Max | Head Dia. (A) Min | Head Height (H) Max | Head Height (H) Min | Head Side Height (S) Ref | Hex Socket (J) Nom | Key Engagement (T) Min | Fillet Transition (F) Max | Max Standard Length (LG) | Max Standard Length (LB) | Tensile Test Load (lb) | Single Shear (lbs min) | Seating Torque Coarse (in.-lbs) | Seating Torque Fine (in.-lbs) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 4 | 0.213 | 0.201 | 0.059 | 0.051 | 0.015 | 1/16 | 0.035 | 0.132 | 0.50 | 0.50 | 840 | 950 | 7.0 | 8 |
| 6 | 0.262 | 0.250 | 0.073 | 0.063 | 0.015 | 5/64 | 0.044 | 0.158 | 0.63 | 0.63 | 1,260 | 1,400 | 13 | 15 |
| 8 | 0.312 | 0.298 | 0.087 | 0.077 | 0.015 | 3/32 | 0.052 | 0.194 | 0.75 | 0.75 | 1,940 | 2,000 | 25 | 26 |
| 10 | 0.361 | 0.347 | 0.101 | 0.091 | 0.020 | 1/8 | 0.070 | 0.220 | 1.00 | 1.00 | 2,440 | 2,700 | 45 | 48 |
| 1/4 | 0.437 | 0.419 | 0.132 | 0.122 | 0.031 | 5/32 | 0.087 | 0.290 | 1.00 | 1.00 | 4,430 | 4,700 | 95 | 110 |
| 5/16 | 0.547 | 0.527 | 0.166 | 0.152 | 0.031 | 3/16 | 0.105 | 0.353 | 1.00 | 1.00 | 7,300 | 7,300 | 190 | 210 |
| 3/8 | 0.656 | 0.636 | 0.199 | 0.185 | 0.031 | 7/32 | 0.122 | 0.415 | 1.25 | 1.25 | 10,800 | 10,600 | 300 | 300 |
| 1/2 | 0.875 | 0.851 | 0.265 | 0.245 | 0.046 | 5/16 | 0.175 | 0.560 | 2.00 | 2.00 | 19,800 | 18,800 | 900 | 960 |
| 5/8 | 1.000 | 0.970 | 0.331 | 0.311 | 0.062 | 3/8 | 0.210 | 0.685 | 2.00 | 2.00 | 31,500 | 29,400 | 1,700 | 1,900 |
| Nominal Thread | UNC / UNF (TPI) | Head Dia. (A) Max / Min | Head Height (H) Max / Min | Hex Key (J) | Key Engagement Min (T) |
|---|---|---|---|---|---|
| #2 | 56 / 64 | 0.164 / 0.154 | 0.046 / 0.038 | 0.050 | 0.028 |
| #3 | 48 / 56 | 0.188 / 0.176 | 0.052 / 0.044 | 1/16 | 0.035 |
| #4 | 40 / 48 | 0.213 / 0.201 | 0.059 / 0.051 | 1/16 | 0.035 |
| #6 | 32 / 40 | 0.262 / 0.250 | 0.073 / 0.063 | 5/64 | 0.044 |
| #8 | 32 / 36 | 0.312 / 0.298 | 0.087 / 0.077 | 3/32 | 0.052 |
| #10 | 24 / 32 | 0.361 / 0.347 | 0.101 / 0.091 | 1/8 | 0.070 |
| 1/4 in | 20 / 28 | 0.437 / 0.419 | 0.132 / 0.122 | 5/32 | 0.087 |
| 5/16 in | 18 / 24 | 0.547 / 0.527 | 0.166 / 0.152 | 3/16 | 0.105 |
| 3/8 in | 16 / 24 | 0.656 / 0.636 | 0.199 / 0.185 | 7/32 | 0.122 |
| 1/2 in | 13 / 20 | 0.875 / 0.851 | 0.265 / 0.245 | 5/16 | 0.175 |
| 5/8 in | 11 / 18 | 1.000 / 0.970 | 0.331 / 0.311 | 3/8 | 0.210 |
| Nom. size | Alloy UNRC ultimate (lbs.) | Alloy UNRF ultimate (lbs.) | Alloy single shear, body (lbs. min.) | Alloy seating torque UNRC (in.-lbs.) | Alloy seating torque UNRF (in.-lbs.) | Stainless UNRC ultimate (lbs.) | Stainless UNRF ultimate (lbs.) | Stainless single shear, body (lbs. min.) | Stainless seating torque UNRC (in.-lbs.) | Stainless seating torque UNRF (in.-lbs.) |
|---|---|---|---|---|---|---|---|---|---|---|
| #1 | 390 | 390 | 402 | 2.5 | 2.5 | 237 | 250 | 137 | 1.7 | 1.8 |
| #2 | 555 | 555 | 556 | 4.5 | 4.5 | 333 | 355 | 191 | 2.8 | 3.0 |
| #3 | 725 | 725 | 739 | 7 | 7 | 438 | 471 | 253 | 4.3 | 4.6 |
| #4 | 1,040 | 1,040 | 946 | 8 | 8 | 544 | 595 | 325 | 6.0 | 6.6 |
| #5 | 1,260 | 1,310 | 1,180 | 12 | 13 | 716 | 747 | 403 | 8.9 | 9.3 |
| #6 | 1,440 | 1,620 | 1,440 | 15 | 17 | 818 | 913 | 491 | 11 | 12 |
| #8 | 2,220 | 2,240 | 2,030 | 30 | 31 | 1,260 | 1,327 | 693 | 20 | 21 |
| #10 | 2,780 | 3,180 | 2,770 | 40 | 45 | 1,575 | 1,800 | 931 | 30 | 34 |
| 1/4 | 5,070 | 5,790 | 4,710 | 100 | 110 | 2,862 | 3,276 | 1,610 | 71 | 81 |
| 5/16 | 8,350 | 9,250 | 7,360 | 200 | 220 | 4,716 | 5,220 | 2,520 | 123 | 136 |
| 3/8 | 12,400 | 14,000 | 10,600 | 350 | 400 | 6,975 | 7,900 | 3,620 | 218 | 247 |
| 7/16 | 16,900 | 18,900 | 14,400 | 560 | 625 | 9,570 | 10,680 | 4,930 | 349 | 388 |
| 1/2 | 22,800 | 25,600 | 18,850 | 850 | 1,000 | 12,770 | 14,390 | 6,440 | 532 | 600 |
| 9/16 | 28,900 | 32,300 | 23,900 | 1,200 | 1,360 | 16,300 | 18,300 | 8,150 | 767 | 856 |
| 5/8 | 36,000 | 40,800 | 29,450 | 1,700 | 1,900 | 20,300 | 23,000 | 10,100 | 1,060 | 1,200 |
| 3/4 | 53,200 | 59,300 | 42,400 | 3,000 | 3,200 | 30,100 | 33,600 | 14,500 | 1,880 | 2,100 |
| 7/8 | 73,500 | 81,000 | 57,700 | 5,000 | 5,400 | 41,500 | 45,800 | 19,700 | 3,030 | 3,340 |
| 1 | 96,300 | 106,000 | 75,400 | 7,200 | 7,600 | 54,500 | 59,700 | 25,800 | 4,550 | 5,000 |
Materials
- Material, hardness and finish specified per RFQ — the collected specification table covers geometry only
Ordering & documentation
Documents
Shipments are backed by Mill Test Certificate (MTC), Dimensional Report, Salt Spray Test Report on request, with full batch traceability from raw material to finished product. Quality management follows the owner-confirmed compliance list: ISO 9001:2015, RoHS, REACH. Exact document sets are confirmed on the order.
Quotation
- The fastener family (ANSI B18.3 Button Socket Head Cap Screws) plus the standard designation from your drawing — DIN / ISO / JIS / ASTM / ANSI, or a drawing for custom parts.
- Size (diameter and length), thread type (metric or inch), property class or grade, and coating or finish.
- Quantity per size and your target delivery date.
- Destination country, and any required documents (for example a mill test certificate) so the quote includes them.
Sample & supply
Minimum order quantity is 10,000 pieces. Stock items run 3–7 days. Quantities or variants outside the standard range are confirmed at quotation.
Storage & shipping
Parts are packed for export and shipped factory-direct to buyers in Europe, Asia, the Americas and the Middle East. Store fasteners dry and protected until installation, and keep coated parts in their packaging until use; specific packing and preservation requirements can be stated on the order and confirmed at quotation.
Frequently asked questions
What does the tabulated "maximum standard length" column tell the buyer?
It marks where each size stops being a stock-standard part and becomes a made-to-order one. The button head is forged, so very long lengths are limited by what the head-forming process produces economically — the collected table prints that ceiling per size: 1 in maximum standard length at 1/4, 1.25 in at 3/8, 2.5 in at 3/4. A requirement past that column is still manufacturable, but it moves into special-tooling territory with its own schedule and price. So the column is a design-economics boundary, not a capability boundary: if the joint genuinely needs a long button head, either accept the special-order route or re-check whether a standard-length screw plus a through-clearance hole with a nut on the far side does the same job off the shelf. Check the column before finalising drawings on long grips — it is the one row that changes quotes most.
What is the MOQ and lead time for these screws?
Minimum order quantity is 10,000 pieces per item. Stocked sizes ship in 3–7 days; non-stock diameters, lengths or finishes are quoted with a confirmed schedule. Common sizes in this designation are widely stocked — confirm the exact schedule against quantity and size mix at quotation.
Related knowledge
- Thread-Forming vs Thread-Cutting vs Thread-Rolling Self-Tapping Screws: What the Evidence Settles on Base Material
- Screw Family Selection by Base Material: Wood, Machine and Self-Tapping Threads, Engagement and Coating
- Hex Bolts vs Socket Head Cap Screws: What the Evidence Settles on Strength, Clearance and Cost
Request a Quote
Get factory-direct pricing from the manufacturer.
What to include in your inquiry
- ✓ Standard and property class (e.g. DIN 912, ISO 4762, class 8.8) — or a drawing for custom parts.
- ✓ Sizes, lengths and thread type (metric or inch).
- ✓ Quantity per size and your target delivery date.
- ✓ Destination country, plus any coating or finish requirement.
- ✓ Any documents you need with the shipment (e.g. mill test certificate) so the quote includes them.
- ✓ Your inquiry goes straight to the factory team by email and gets a reply within 24 hours.
- ✓ Factory-direct pricing
- ✓ Standard and custom specifications
- ✓ Factory-direct export to buyers in Europe, Asia, the Americas and the Middle East
- ✓ Full fastener family support