ANSI B18.3 Hex Socket Head Shoulder Screws
ANSI/ASME B18.3 hex socket head shoulder screws in inch sizes — threaded end #4 to 1-1/2 under an unthreaded precision shoulder from 1/8 to 2 in — with the collected 14-size table: shoulder diameter bands, head geometry, socket size and thread data.
Last updated: 2026-11
Specifications
A shoulder screw is two fasteners in one: a precision-ground unthreaded shoulder that acts as a bearing pin, bushing or pivot, and a threaded end that clamps the mating part down against it. That split is the whole point — the shoulder carries sliding or rotating loads with its close-tolerance diameter, while the thread underneath only has to hold the joint snug, so the moving part never rides on thread crests. The collected B18.3 table runs 14 sizes from a 1/8 in shoulder up to 2 in and tabulates the three-diameter system per size: the shoulder diameter d s with max/min limits (0.123/0.121 in at the 1/8 size — a few tenths of a thousandth under nominal, which is what makes it a running fit), the head diameter d k and height k, the hex socket nominal size s with its socket depth t, and then the thread-end data: thread size (#4 up to 1-1/2), nominal diameter, threads per inch, and the thread length b. Note the deliberate step-down: the threaded end is always 2–4 sizes smaller than the shoulder, so a 1/4 in shoulder screw carries a 10-24 thread — specify the shoulder as the working diameter and let the thread follow the standard, not the other way round. For the metric counterpart, ISO 7379 shoulder screws are covered separately on this site.
| Standard | ANSI/ASME B18.3 — hex socket head shoulder screws (inch series) |
|---|---|
| Series | Shoulder diameters 1/8 – 2 in (14 sizes tabulated) |
| Tabulated geometry | Per shoulder size: shoulder d s (max / min), head d k (max / min), head height k (max / min), socket nominal size s, socket depth t (min), thread size d, nominal diameter, threads per inch PP, thread length b |
| Shoulder at 1/8 | d s 0.123 / 0.121 in; head d k 0.25 in max; head height k 0.125 in max; socket s 0.078 in; thread #4-40 UNC, b 0.157 in |
| Shoulder at 1/4 | d s 0.248 / 0.246 in; head d k 0.375 in max; head height k 0.188 in max; socket s 0.125 in; thread 1/4-20 UNC, b 0.375 in |
| Shoulder at 1/2 | d s 0.498 / 0.496 in; head d k 0.75 in max; head height k 0.312 in max; socket s 0.25 in; thread 3/8-16 UNC, b 0.625 in |
| Shoulder at 1 | d s 0.998 / 0.996 in; head d k 1.312 in max; head height k 0.625 in max; socket s 0.5 in; thread 3/4-10 UNC, b 1 in |
| Materials / strength | Not tabulated by the collected specification — per RFQ (alloy, hardness and finish per drawing callout) |
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 Hex Socket Head Shoulder 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 threaded end smaller than the shoulder, and does that matter for design?
It matters more than any other single number on the page. The shoulder is the functional diameter — it locates the part, takes the pivot or sliding load, and its ground limits (read the 1/2 in row: 0.498/0.496 in) hold the clearance to a few tenths of a thousandth so the mating part does not rock. The thread only needs to pull the joint together, so the standard steps it down — a 1/2 in shoulder carries a 3/8-16 thread — leaving more shoulder length usable and keeping the thread from interfering with a close-fitting bore. The design trap is specifying by thread: a "3/8 shoulder screw" can mean several shoulder diameters across standards, whereas the shoulder diameter plus usable length pins the part exactly. Quote the shoulder, the shoulder length and the thread length separately, and check replacements against all three.
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