ASME B18.6.3 Square Recessed Flat Countersunk Head Machine Screws
Square recessed flat countersunk head machine screws per ASME B18.6.3, #4 through 5/16 basic diameter in steel or stainless steel. The flush inch-series head with a cam-out-proof square drive — the countersunk screw for powered assembly.
Last updated: 2026-11
Specifications
This is the flush head of the square recess line: a flat countersunk head that seats into a countersunk hole leaving the recess at or just below the surface, driven by a square bit that stays engaged through full torque. The collected ASME B18.6.3 cut sheet covers #4 (0.1120 in basic diameter) through 5/16 with maximum and minimum head diameters, reference head height and full recess gaging columns. At #10 the head seats at 0.362 in maximum diameter with a 0.116 in reference head height, a 0.111 in square recess and a penetration gaging depth of 0.075/0.060 in; driver sizes run 0 through 4 across the range, stepping 2 at #8 and #10. Against the cross recessed flat countersunk head, the square recess holds the bit under side load in deep or awkward countersinks where a Phillips driver tends to slip; against the slotted countersunk head it survives powered drivers without blade walk-out. Because the recess sits below the surrounding surface once seated, penetration gaging depth — not visible head height — is the dimension that decides whether the drive still works after plating.
| Standard | ASME B18.6.3 — square recessed flat countersunk head machine screws (flush-seating countersunk head, square recess drive) |
|---|---|
| Size range | #4 (0.1120 in) – 5/16 (0.3125 in) basic screw diameter |
| Dimensions at #4 | A (head diameter) max 0.212 in / min 0.191 in; H (head height) ref 0.067 in; M (recess square) ref 0.0696 in; T (recess depth) ref 0.066 in; penetration gaging depth max 0.038 / min 0.028; driver size 0 |
| Dimensions at #10 | A max 0.362 in / min 0.333 in; H ref 0.116 in; M ref 0.111 in; T ref 0.127 in; penetration gaging depth max 0.075 / min 0.060; driver size 2 |
| Dimensions at 1/4 | A max 0.477 in / min 0.442 in; H ref 0.153 in; M ref 0.1315 in; T ref 0.158 in; penetration gaging depth max 0.095 / min 0.080; driver size 3 |
| Driver sizes | 0 (#4), 1 (#6), 2 (#8, #10), 3 (#12, 1/4), 4 (5/16) |
| 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 Square Recessed Flat Countersunk 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
What countersink angle does this head need?
The collected table carries the head diameter, height and recess gaging columns but does not print the countersink angle, so this page does not state a figure — confirm the countersink drawing at quotation and match the angle of the mating countersink you already cut. The practical rule that does carry over from the collected data: the head is sized by its maximum diameter (0.362 in at #10, for example) and the flushness by the reference head height, so the countersink must leave the full recess depth visible to the driver.
Square recessed or cross recessed countersunk — which for my assembly?
The head forms are equivalent flush countersunk geometries with different drives. The square recess keeps the bit engaged under side load and in deep countersinks where a Phillips driver tends to slip out before full torque; the Phillips is the choice where generic cross-recess tooling must drive every fastener on the line. If drawings mix both, keep the drive called out per drawing — the recesses are not interchangeable and filling a square recess with a Phillips bit rounds both.
What is the MOQ and lead time for these square recess machine screws?
Minimum order quantity is 10,000 pieces per item. Stocked sizes ship in 3–7 days; non-stock 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 square recess machine screws state?
State the standard designation (ASME B18.6.3 — often still called ANSI B18.6.3) and drive type (square recessed; combination slotted + square recessed where both drives are wanted), nominal screw size and thread series with threads per inch, 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