ANSI B18.3 Flat Socket Head Cap Screws
ANSI/ASME B18.3 flat socket head cap screws, #0 to 1-1/2 in — the 82° countersunk inch hex socket screw for flush or sub-flush seating — with the collected 23-size table: head diameter and reference height, socket size, minimum full-thread length and head angle.
Last updated: 2026-11
Specifications
The flat socket head cap screw is the countersunk member of the B18.3 hex socket family: an 82° conical head that seats flush or marginally below flush in a countersunk hole, driven by a hex socket so the flush result carries real torque without a slot or recess that cams out. It is the head for rotating parts it must not proud-stand on, gauging surfaces, and any assembly where the screw is the topmost feature. The collected table runs 23 sizes from #0 to 1-1/2 in and tabulates per size: nominal diameter with UNC/UNF/UNEF threads per inch, shank d s limits, head diameter d k with max/min bands, the head height k given as a reference value (countersunk heads locate by cone, not by height), hex socket nominal size s with minimum depth t, the minimum full-thread length b that scales with diameter, and the head angle α held to ±2° around 82°. The angle matters: an 82° head dropped into a 90° metric countersink seats on the wrong cone and loses both flushness and bearing — inch and metric countersunk hardware do not interchange. ISO 10642 / DIN 7991 (90°) metric flat socket heads are covered separately on this site.
| Standard | ANSI/ASME B18.3 — flat socket head cap screws, 82° countersunk head (inch series) |
|---|---|
| Series | #0 – 1-1/2 in (23 sizes tabulated); UNC/UNF, UNEF on selected sizes |
| Tabulated geometry | Per size: nominal diameter d, threads per inch by series, shank d s (max / min), head d k (max / min), head height k (Ref), socket s, socket depth t (min), full thread b (min), head angle α 82° ±2° |
| At 1/4 | UNC 20 / UNF 28 / UNEF 32; head d k 0.531 / 0.480 in; height k 0.161 in Ref; socket s 5/32 in, depth t min 0.111 in; thread b min 1 in; angle 82° ±2° |
| At 1/2 | UNC 13 / UNF 20 / UNEF 28; head d k 0.938 / 0.872 in; height k 0.251 in Ref; socket s 5/16 in, depth t min 0.172 in; thread b min 1.5 in; angle 82° ±2° |
| At 1 | UNC 8 / UNF 12 / UNEF 20; head d k 1.938 / 1.841 in; height k 0.540 in Ref; socket s 5/8 in, depth t min 0.297 in; thread b min 2.5 in; angle 82° ±2° |
| Materials / strength | Not tabulated by the collected specification — per RFQ (property class / hardness and finish per drawing callout) |
| Nominal Thread | UNC / UNF (TPI) | Head Dia. (A) Sharp Max / Abs Min | Head Height Ref (H) | Hex Key (J) | Key Engagement Min (T) |
|---|---|---|---|---|---|
| #4 | 40 / 48 | 0.255 / 0.218 | 0.083 | 1/16 | 0.055 |
| #5 | 40 / 44 | 0.281 / 0.240 | 0.090 | 5/64 | 0.061 |
| #6 | 32 / 40 | 0.307 / 0.263 | 0.097 | 5/64 | 0.066 |
| #8 | 32 / 36 | 0.359 / 0.311 | 0.112 | 3/32 | 0.076 |
| #10 | 24 / 32 | 0.411 / 0.359 | 0.127 | 1/8 | 0.087 |
| 1/4 in | 20 / 28 | 0.531 / 0.480 | 0.161 | 5/32 | 0.111 |
| 5/16 in | 18 / 24 | 0.656 / 0.600 | 0.198 | 3/16 | 0.135 |
| 3/8 in | 16 / 24 | 0.781 / 0.720 | 0.234 | 7/32 | 0.159 |
| 1/2 in | 13 / 20 | 0.938 / 0.872 | 0.251 | 5/16 | 0.172 |
| 5/8 in | 11 / 18 | 1.188 / 1.112 | 0.324 | 3/8 | 0.220 |
| 3/4 in | 10 / 16 | 1.438 / 1.355 | 0.396 | 1/2 | 0.220 |
| Size | Body Dia. (D) Max | Body Dia. (D) Min | Head Dia. (A) Sharp Max | Head Dia. (A) Abs Min | Head Height (H) Ref | Protrusion Gage (G) Max | Protrusion Gage (G) Min | Protrusion (P) Ref | Protrusion (P) Max | Hex Socket (J) Nom | Key Engagement (T) Min | Fillet (F) Max | Tensile (lbs min) | Shear UNRC (lbs min) | Shear UNRF (lbs min) | Torque UNRC (in.-lbs) | Torque UNRF (in.-lbs) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 4 | 0.1120 | 0.1075 | 0.255 | 0.218 | 0.083 | 0.172 | 0.174 | 0.049 | 0.036 | 1/16 | 0.055 | 0.136 | 900 | - | 940 | 8. | - |
| 5 | 0.1250 | 0.1202 | 0.281 | 0.240 | 0.090 | 0.196 | 0.195 | 0.051 | 0.037 | 5/64 | 0.061 | 0.153 | 1,185 | - | 1,180 | 12. | - |
| 6 | 0.1380 | 0.1329 | 0.307 | 0.263 | 0.097 | 0.220 | 0.219 | 0.052 | 0.037 | 5/64 | 0.066 | 0.168 | 1,350 | - | 1,440 | 15. | - |
| 8 | 0.1640 | 0.1585 | 0.359 | 0.311 | 0.112 | 0.267 | 0.266 | 0.055 | 0.039 | 3/32 | 0.076 | 0.194 | 2,085 | - | 2,030 | 30. | - |
| 10 | 0.1900 | 0.1840 | 0.411 | 0.359 | 0.127 | 0.313 | 0.312 | 0.058 | 0.041 | 1/8 | 0.087 | 0.220 | 2,610 | 2,610 | 2,720 | 40. | 45. |
| 1/4 | 0.2500 | 0.2435 | 0.531 | 0.480 | 0.161 | 0.424 | 0.423 | 0.064 | 0.043 | 5/32 | 0.111 | 0.280 | 4,750 | 4,750 | 4,710 | 100. | 110. |
| 5/16 | 0.3125 | 0.3053 | 0.656 | 0.600 | 0.198 | 0.539 | 0.538 | 0.070 | 0.047 | 3/16 | 0.135 | 0.343 | 7,800 | 7,800 | 7,360 | 200. | 220. |
| 3/8 | 0.3750 | 0.3678 | 0.781 | 0.720 | 0.234 | 0.653 | 0.652 | 0.076 | 0.050 | 7/32 | 0.159 | 0.405 | 11,600 | 11,600 | 10,600 | 350. | 400. |
| 7/16 | 0.4375 | 0.4294 | 0.844 | 0.781 | 0.234 | 0.690 | 0.689 | 0.092 | 0.063 | 1/4 | 0.159 | 0.468 | 15,900 | 15,900 | 14,400 | 560. | - |
| 1/2 | 0.5000 | 0.4919 | 0.938 | 0.872 | 0.251 | 0.739 | 0.738 | 0.119 | 0.087 | 5/16 | 0.172 | 0.530 | 21,200 | 21,200 | 18,850 | 850. | 1,000. |
| 5/8 | 0.6250 | 0.616 | 1.188 | 1.112 | 0.324 | 0.962 | 0.961 | 0.135 | 0.096 | 3/8 | 0.220 | 0.655 | 33,800 | 33,800 | 29,450 | 1,700 | - |
| 3/4 | 0.7500 | 0.7406 | 1.438 | 1.355 | 0.396 | 1.186 | 1.185 | 0.150 | 0.105 | 1/2 | 0.220 | 0.780 | 50,000 | 50,000 | 42,400 | 3,000 | - |
- Row 1 (4): The source merges its header across three rowspan rows; the 17 printed values are kept in exact source order (column count per row verified constant across the ladder). Strength/torque pairing: tensile, UNRC/UNRF shear, UNRC/UNRF torque — "-" where the source prints none.
| Thread Length Group | Up to 1 in. | Over 1 in. to 2-1/2 in. | Over 2-1/2 in. to 6 in. |
|---|---|---|---|
| 0 thru 3/8 (Inclusive) | -.03 | -.04 | -.06 |
| 7/16 thru 3/4 (Inclusive) | -.03 | -.06 | -.08 |
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 Flat 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
Why is the head height listed only as a reference, and why does the 82° angle matter so much?
A countersunk head does not locate the way a cap head does. Its position is set by the cone engaging the countersink, so the height above the surface falls out of the cone fit rather than being a controlled feature — which is why the table marks k as Reference. The controlled feature is the angle, held to 82° ±2°, and that is where interchange ends: inch-series countersunk holes are cut 82°, metric (ISO 10642 / DIN 7991) hardware uses 90°. Seat an 82° head in a 90° countersink and the cone touches only near its largest diameter — the head sits slightly proud, rocks under load, and the bearing circle that should spread the clamping force shrinks to a ring. So the practical rule is absolute: match the head angle to the cutter that made the seat, and treat head height as the consequence, not the check. Verify replacements by angle and head diameter d k together.
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