# Countersunk screws

Countersunk head and seat geometry, length conventions, drive and thread distinctions, and source-specific limits on fit and loadability.

Reviewed: 2026-10-30

Tags: components, fasteners, assembly

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](https://www.accu.co.uk/pozi-countersunk-screws/988538-SPK-M3-16-CS-BL), [Accu inch socket SKU](https://www.accu.co.uk/imperial-socket-countersunk-screws/260724-SSK-1-4-20-3-8-A2-R360).

## 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](https://www.accu.co.uk/p/444-different-screw-head-types-an-engineers-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](https://www.bossard.com/us-en/eshop/screws-and-bolts-with-internal-drive/hex-socket-flat-countersunk-head-screws-fully-threaded/p/2105/).

## Head forms and variants

| Form | Distinguishing geometry or designation |
| --- | --- |
| Flat countersunk | Flat exposed top with a conical seating portion; flush seating is the intended geometry. |
| Raised countersunk | Countersunk seating portion with a raised, domed top; the dome remains above the nominal seating plane. |
| Cap head | A projecting cylindrical head, distinct from a flat countersunk form. |
| Button head | A low, rounded projecting head; a low profile alone does not make it countersunk. |

The cap, button and flat-head descriptions follow
[Accu’s head-form guide](https://www.accu.co.uk/p/444-different-screw-head-types-an-engineers-guide).
The raised-head distinction and its length datum are described in
[Bolts.co.uk’s measurement guide](https://www.bolts.co.uk/guides-and-tips/screws/how-is-the-length-of-a-screw-measured/).
“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](https://www.iso.org/standard/90795.html)

**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](https://www.iso.org/standard/78695.html)

## 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:

| Supplier example | Head and thread data | Scope of the evidence |
| --- | --- | --- |
| Accu SPK-M3-16-CS-BL | 90° countersunk angle; M3 thread, 0.5 mm pitch; 16 mm length; PZ1 drive; steel class 4.8 with black zinc finish | Cataloged to DIN 965; one metric Pozi product. |
| Accu SSK-1/4-20-3/8-A2-R360 | 82° countersunk angle, supplier tolerance ±2°; 1/4-20 UNC thread; 3/8 inch overall length; 5/32 inch hex socket; A2 stainless with thread-locking finish | Cataloged to ASME B18.3 without an edition; one inch-series product. |

Sources: [Accu metric Pozi SKU](https://www.accu.co.uk/pozi-countersunk-screws/988538-SPK-M3-16-CS-BL)
and [Accu inch socket SKU](https://www.accu.co.uk/imperial-socket-countersunk-screws/260724-SSK-1-4-20-3-8-A2-R360).
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](https://www.bolts.co.uk/guides-and-tips/screws/how-is-the-length-of-a-screw-measured/)

**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](https://www.iso.org/standard/90795.html)

## 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](https://www.iso.org/standard/90795.html)

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](https://www.accu.co.uk/imperial-socket-countersunk-screws/260724-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](https://www.accu.co.uk/pozi-countersunk-screws/988538-SPK-M3-16-CS-BL)
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](../index.md).