Engineering Guide · 5 min read
Flat Countersunk Screws: ASME B18.3, ISO 10642 and DIN 7991
Match flat countersunk screw heads to the seat: included angle, flushness, overall length and reduced loadability across ASME B18.3, ISO 10642 and DIN 7991.
The short answer: a flat countersunk screw uses a conical underside to seat in a matching countersink, allowing its flat top to finish flush when the head and mating recess are correctly specified. For hex-socket products, check the inch requirements in ASME B18.3 or the metric requirements in ISO 10642. Match the head angle, diameter and length reference before ordering: a flat top and a matching thread are not enough.[1][2]
This article belongs to the Bolt and Screw Head Types Guide. It focuses on flat countersunk heads with a hexagon socket. Cross-recess and six-lobe drives need their own product references.
Identify the cone beneath the flat top
The defining feature is the tapered bearing surface underneath the head. It bears on the matching cone in the mating part. The internal hexagon only identifies the drive. A low button head has a flat under-head seat and remains above the surface; a countersunk head needs a shaped recess to achieve a flush result.
The flat head screw drawing and inch dimension tables cover the site's stated inch product. Check head angle, head diameter and socket details against the specified edition and the mating drawing.
Use the flush face when the assembly needs it
A flush fit depends on both the screw and the mating countersink. An access plate beneath a sliding component, or a cover that must present a smooth face, can benefit from a correctly seated countersunk head. The recess consumes material around the hole, so the drawing must provide enough thickness and support for that seat. On thin sheet, confirm a designed countersink or formed seating feature rather than simply cutting deeper until the head disappears.
A cylindrical counterbore is a different recess. It provides a flat floor for a head with a flat under-head bearing surface, such as a cylindrical socket head cap screw. Keep the specified seat geometry when comparing alternatives.
Inspect the seated condition as well as the loose screw: the head's position relative to the finished surface, the contact around the cone and alignment with the screw axis all matter. ISO 10642:2026 specifically calls attention to alignment between the countersunk head and its bearing surface.[2]
Match the head angle and measure length from the right surface
Common inch and metric examples use different nominal included angles. Accu publishes an ASME B18.3 inch socket countersunk product at 82° and an ISO 10642 metric product at 90°.[3][4] Those examples explain why the mating countersink must be checked; use the exact angle and tolerance on the required product and assembly drawings.
| Check | What the drawing needs to establish |
|---|---|
| Included head angle | The full angle across the cone, with its tolerance; a nominal 82° head and a nominal 90° seat are different geometries. |
| Head diameter and countersink size | The head limits and the recess diameter/depth that achieve the required flush or recessed condition. |
| Nominal length | For this flat countersunk form, overall length includes the head, unlike an under-head length on a conventional hex or cylindrical socket screw.[3][4] |
| Remaining engagement | The thread available after passing through the clamped part and its countersink, including any thread runout or blind-hole restriction. |
A mismatched cone can contact near an edge instead of across the intended seating surface. Pulling it down harder changes neither angle. Resolve the head and recess dimensions before approving the assembly torque.
Keep ASME, ISO and legacy DIN requirements distinct
| Product route | Reference and scope | Selection consequence |
|---|---|---|
| Inch socket flat countersunk form | ASME B18.3-2026, within the covered inch socket-screw forms.[1] | State the flat countersunk form, thread and edition, then verify its head and socket requirements. |
| Metric hexagon socket countersunk head | ISO 10642:2026; steel and stainless steel, coarse threads M2–M20, product grade A, with reduced loadability due to head design.[2] | Apply the finished-product load requirements as well as the dimensional requirements. |
| Legacy metric designation | DIN 7991; check the exact edition and the DIN EN ISO 10642 replacement history.[5] | Obtain the permitted replacement and compare head dimensions, socket and seating requirements. |
ISO's product grade A concerns product accuracy. A steel property-class designation concerns mechanical properties; it does not remove a head-design limit. ISO 10642:2026 expressly identifies reduced ultimate tensile load and the relevant leading-zero marking.[2] Use assembly instructions appropriate to that screw. Bossard's reduced-loadability guidance treats the listed countersunk products separately from conventional full-loadability screws.[6]
A DIN-to-ISO replacement also deserves a seating check. DIN Media's historical record links DIN 7991 to the DIN EN ISO 10642 route; Keller & Kalmbach's current product information explicitly notes changed ISO head dimensions and limited interchangeability.[5][7] A replacement screw that enters the old thread may still sit differently in the old countersink.
Put the mating countersink on the inquiry
Start with a head-and-standard description such as hexagon socket countersunk head screw — ISO 10642:2026 — M6 × 20. That identifies a dimensional specification, not a complete material or load selection. Attach the mating recess detail when flushness matters.
- Product: flat countersunk head, hexagon socket, exact standard and edition.
- Head and recess: included angle and tolerance, head limits, countersink diameter/depth and allowed flush or recessed position.
- Thread and length: diameter, pitch or TPI, fit, overall length including the head, thread length and needed engagement.
- Material and supply: material, mechanical properties and head-load requirements, finish, quantity and inspection documents.
Use How to Specify a Bolt for the full order format and Fastener Grades Explained to interpret the separate mechanical-property designations.
References
- ASME. ASME B18.3-2026. Inch socket-screw public listing and scope.
- ISO. ISO 10642:2026. Public scope, material and thread range, reduced-loadability and countersink-alignment notes.
- Accu. SSK-5/16-18-1-A2 — inch socket countersunk screw. Supplier example with ASME B18.3 designation, nominal 82° angle and overall-length reference; product data, not a standard-edition equivalence claim.
- Accu. SSK-M3-14-A2-BL — metric socket countersunk data sheet. Supplier ISO 10642 example, nominal 90° angle and overall length.
- DIN Media. DIN 7991:1986-01 — historical replacement record. Withdrawn DIN edition; lists DIN EN ISO 10642:1998-02 / 2004-06 successors.
- Bossard. Preload and tightening torques (2025), printed page 66. Guidance for the listed reduced-loadability head-and-drive forms; no generic torque table reproduced.
- Keller & Kalmbach. ISO 10642 M6 × 12 A2, article 5210642612, PDF page 2. Supplier warning on changed ISO head dimensions and limited DIN/ISO interchangeability.