ASME B18.6.3 UNC Slotted Round Head Machine Screws
Slotted round head machine screws per ASME B18.6.3, UNC thread — #1 through 3/4 in basic diameter in steel or stainless steel. The traditional domed inch-series head: a half-sphere over a flat bearing face, driven by a flat blade.
Last updated: 2026-11
Specifications
The round head is the oldest machine screw head of the inch series: a domed half-sphere sitting on a flat bearing face, taller and narrower than a pan head, driven by a single slot. It remains the callout in legacy inch equipment, electrical hardware and restoration work — drawings that predate the pan-head adoption specify it, and replacement parts keep the designation. The collected ASME B18.6.3 table runs from #1 (0.0730 in basic diameter) through 3/4 in on UNC threads per inch: 40 TPI at #4, 24 at #10, 20 at 1/4. At #10 the head measures 0.359 in maximum diameter by 0.137 in maximum height, with a 0.060–0.050 in slot 0.087–0.065 in deep. Compared with the metric line, the geometry carries over — the DIN 7985 pan head and DIN 963 countersunk head are the metric counterparts most drawings pair with it. For driven assembly with powered drivers, the cross-recessed inch heads serve the same range; see the ASME B18.6.3 pan head page for the slotted alternative with a lower, wider dome.
| Standard | ASME B18.6.3 — slotted round head machine screws, UNC (unified coarse) thread |
|---|---|
| Size range | #1 (0.0730 in) – 3/4 in (0.7500 in) basic diameter, UNC threads per inch |
| Dimensions at #4 (#4-40) | dk (head diameter) max 0.211 in (min 0.193 in); k (head height) max 0.086 in (min 0.075 in); slot width n 0.039–0.031 in; slot depth t 0.058–0.044 in |
| Dimensions at #10 (#10-24) | dk max 0.359 in (min 0.334 in); k max 0.137 in (min 0.123 in); slot width n 0.060–0.050 in; slot depth t 0.087–0.065 in |
| Dimensions at 1/4 (1/4-20) | dk max 0.472 in (min 0.443 in); k max 0.175 in (min 0.160 in); slot width n 0.075–0.064 in; slot depth t 0.109–0.082 in |
| Materials | Steel, stainless steel — grade and finish confirmed at quotation (tenant supply range) |
Materials
- Steel
- Stainless steel
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 (ASME B18.6.3 UNC Slotted Round Head Machine 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
Round head or pan head — the two slotted domes, how do they differ?
Both are domed slotted heads, but the proportions differ: the round head is a taller half-sphere on a smaller seat, while the pan head is lower and wider with a flat top and a larger bearing circle — compare the collected tables, where #10 measures 0.359 in across by 0.137 in high as a round head against 0.373 in across by 0.110 in high as a pan head. Practically, the pan head gives more bearing and driver engagement in the same envelope, which is why modern drawings moved to it; the round head survives where legacy tooling, appearance or the original callout governs. Match the head form already in the assembly — the two are not interchangeable without a seating check.
Is ASME B18.6.3 the same as ANSI B18.6.3?
Yes — it is the same machine screw standard under its current publisher. Older drawings and catalogue pages print ANSI B18.6.3; the designation in force is ASME B18.6.3, and the dimension tables are the lineage those drawings were built to. Order against the designation your documentation prints and state gauge size, threads per inch and length explicitly.
What is the MOQ and lead time for these inch machine screws?
Minimum order quantity is 10,000 pieces per item. Stocked sizes ship in 3–7 days; non-stock gauge sizes, 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.
What should an RFQ for these inch machine screws state?
State the standard designation (ASME B18.6.3 — often still called ANSI B18.6.3), gauge size and threads per inch (for example #4-40, #10-24, 1/4-20 UNC), length and head style, material and grade (steel or stainless; state the stainless grade such as 18-8/A2 or 316/A4 if specified), finish (plain, zinc plated, or other), required certificates such as a mill test report per EN 10204 3.1, and quantity per size — plus destination country.
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