Countersunk screws

Accu's SPK-M3-16-CS-BL has a 90° head, M3 × 0.5 thread and 16 mm length. Its SSK-1/4-20-3/8-A2-R360 instead lists 82° ±2°, 1/4-20 UNC and 3/8 in overall length. Check the included seat angle and the head’s length datum against the drawing. Matching an apparent head diameter alone leaves seating and loadability unresolved. Accu metric Pozi SKU, Accu inch socket SKU.

Identity and terminology

A countersunk screw has a head intended to seat in a conical recess, called a countersink, in the mating part. A flat countersunk head can finish flush with the surrounding surface when the head and recess geometry match. The head shape and the drive recess are separate attributes: a hexagon socket, cross recess or other drive does not by itself define the bearing surface. Accu’s screw-head guide distinguishes head shape from drive type and describes the flush countersunk form.

The screw’s drive recess accepts the tool. The workpiece countersink seats the head. Check each against its drawing. Bossard BN 2105.

Head forms and variants

The cap, button and flat-head descriptions follow Accu’s head-form guide. The raised-head distinction and its length datum are described in Bolts.co.uk’s measurement guide. “Flat head” here means the countersunk form, not simply any screw with a flat top. Product drawings resolve abbreviated catalog names.

ISO variants also distinguish drive and head construction. The public abstract of ISO 10642:2026, edition 4, published March 2026, covers hexagon-socket countersunk screws in steel and stainless steel, with metric coarse threads M2–M20 and product grade A. ISO identifies the 2019 edition as withdrawn. ISO 10642:2026

ISO 14581:2022, edition 2, covers the common-head-style hexalobular-socket countersunk flat form, in steel and stainless steel, metric coarse M2–M10, product grade A. Its abstract distinguishes the high-head, full-loadability form in ISO 14582 and explicitly says the products are not interchangeable because their head heights differ. A shared thread size and drive family do not establish equivalent head geometry. ISO 14581:2022

Included angle and dimensional meaning

The included head angle is the full angle across the cone in an axial section, not the half-angle between one conical flank and the screw axis. Dimensions also distinguish thread diameter, head diameter, head height, drive size, drive depth and overall length. Accu’s dimensioned product listings label those separately. Two examples show why an angle cannot be inferred from “countersunk” alone:

Sources: Accu metric Pozi SKU and Accu inch socket SKU. These are supplier specifications, not a claim that every metric screw is 90° or every inch screw is 82°. Millimetres and inches are length units; degrees describe angle. In the inch example, 20 TPI means 20 threads per inch of axial length, not a 20 mm pitch.

Geometric synthesis: for an ideal conical countersink with entry diameter D, cylindrical hole diameter d, and included angle θ, its axial depth is h = (D − d) / (2 tan(θ/2)). Thus an assumed 90° countersink with D = 10 mm and d = 5 mm has a geometric depth of 2.5 mm. This is a cone calculation, not a standardized screw dimension or machining tolerance. Real head transitions, edge radii and permitted dimensional variation are outside this idealization.

Length basis and seating

Bolts.co.uk measures flat countersunk screws including the head, from the top to the end. For raised countersunk heads, its nominal length excludes the domed portion. The same guide measures most other screws from under the head. Consequently, nominal length and total physical envelope are not synonymous across head forms. Supplier length conventions

Assembly synthesis: flushness depends on the actual head diameter and profile, countersink angle and depth, and alignment. Nominal thread size alone does not determine those quantities. At unequal cone angles, coincident axes do not create matching conical bearing surfaces. The material remaining below the seat and the available thread engagement are separate geometric questions; neither follows from overall screw length alone. ISO 10642 specifically calls attention to alignment between the head and the countersink’s bearing surface. ISO 10642 abstract, note 3

Reduced loadability and coverage limits

ISO 10642:2026 attributes reduced loadability to the combination of head dimensions and hexagon-socket penetration. Its public abstract describes a property-class marking preceded by zero and an assumed head loadability of 80% of the thread loadability. This is the standard’s stated tensile-load relationship, not an allowable assembly load, a fatigue rating or an 80% torque rule. ISO 10642 abstract, note 2

The sources establish named forms and selected dimensions, not a universal tightening torque, structural capacity or substitution rule. Joint loading, mating material, thread engagement, friction and the applicable product requirements remain outside the numerical coverage here. Material and finish labels in the examples do not establish environmental suitability or actual lot properties.

Source versions and dates

Research reviewed October 30, 2026 (PDT). The supplier evidence consists of public HTML extracts retrieved for that review. Those retrieval and review dates are separate from supplier-displayed update dates and standard editions. ISO evidence is limited to publicly accessible abstracts and edition metadata; the full standards and acceptance tables were not consulted. Supplier standard labels do not prove conformance to the latest edition. No supplier PDF or paywalled standard is used as evidence here.

For Accu SSK-1/4-20-3/8-A2-R360, one retained extract displays “Specification Last Updated” as June 14, 2026; a later extract displays October 30, 2026. A separate retrieval also displays October 30. The metric SPK-M3-16-CS-BL extract displays October 30, 2026 for that field. These are supplier-displayed calendar dates with no stated timezone, distinct from the listings’ stock-update and first-available fields. Neither listing identifies an edition of its cited product standard.

The differing inch-product dates do not establish a verified historical revision sequence, the cause of the discrepancy or a stable current source version. No conflicting technical specification values were observed in the compared extracts; the examples above retain their product-specific scope. Neither update date proves when those values changed or how fresh the origin page was when an extract was returned.

Related: Hardware and component references.