Countersunk Bolts Manufacturer in India | ISO 10642 CSK Socket Screws

Overview

ISO 10642 CSK Socket Screws & DIN 7991 Countersunk Bolts: Manufacturer & Distributor

A countersunk bolt has a cone-shaped head that sinks into a matching countersink. The head ends flush with the surface, leaving nothing to catch, snag or interfere with moving parts. The cone also centres the part on the bolt.

Exotic Alloy manufactures countersunk bolts in Mumbai, India, for machine builders, tool rooms, automation, marine and architectural fabricators. We supply across India and distribute to Italy, Europe, the Gulf and Africa. The stainless A2-70 screws in the photo are cold headed with the hex socket formed in the head. 8.8, 10.9 and 12.9 alloy steel versions are hardened and tempered.

Three things to know

The length includes the head. Metric heads are 90° and inch heads 82°. And ISO 10642 heads are taller than the DIN 7991 heads they replaced, so cut the countersink for the standard you fit.

ISO 10642 / DIN 7991 M3 – M20 82° inch flat heads A2-70 / A4-70 8.8 / 10.9 / 12.9

Countersunk bolt at a glance

Head: 90° cone (metric), 82° (inch)
Length: Overall, including the head
Drive: Hex socket; M8 takes a 5 mm key
Load: Reduced loadability (ISO 10642)
Standards: ISO 10642, DIN 7991, ASME B18.3
Related: Allen bolts, socket head bolts, carriage bolts

ISO 10642 M8 × 30, drawn to scale. The 17.92 mm head diameter is a theoretical sharp edge (dashed); the real head is at least 15.24 mm across. The head height k = 4.96 is exactly (17.92 − 8) / 2, because the head is a 90° cone.

Specification

Countersunk Bolt Specification

The range we manufacture and supply.

StandardsISO 10642 (DIN EN ISO 10642); DIN 7991; ASME B18.3 flat countersunk head; ISO 14581; ISO 2009; ISO 7046-1; DIN 605 / DIN 608

Size rangeM3 to M20 (DIN 7991 to M24); No. 4 to 1″ inch; lengths per standard, special lengths to order

Head angle90° metric; 82° inch; 60° for DIN 7991 M22 and M24

ThreadMetric coarse 6g, fine pitch to order; UNC / UNF; threaded to the head on short lengths

GradesA2-70, A4-70 (ISO 3506-1); 8.8, 10.9, 12.9 (ISO 898-1); ASTM F835 / F879 inch

FinishPassivated stainless; black oxide; zinc flake; zinc plated (8.8 / 10.9)

DrivesHex socket; hexalobular (Torx type); slot; cross recess; tamper-resistant pin hex or pin hexalobular on request

MarkingMaterial and class on the head (e.g. A2-70) plus maker’s mark

CertificationMill certificate; EN 10204 3.1; PMI for stainless

PackingBoxes of 100 / 200 / 500, bulk cartons, labelled kits

Types

Types of Countersunk Bolts We Manufacture

The same flush head with different drives, and one countersunk bolt with no drive at all.

Drive and head profile of six countersunk types (schematic). The square neck bolt has a smooth head and is tightened from the nut, like a carriage bolt. The inch flat head has a steeper 82° cone.

Hex Socket Countersunk Bolts

ISO 10642 / DIN 7991 · M3 – M20

The common Allen countersunk screw for machines, tooling, slides and covers, in stainless or alloy steel.

Hexalobular Countersunk Screws

ISO 14581

Six-lobe drive for higher torque without cam-out, suited to assembly lines and small sizes.

Slotted & Cross Recess

ISO 2009 · ISO 7046-1

Machine screws for panels, hinges, fittings and electrical work.

Countersunk Square Neck Bolts

DIN 605 (M6–M10) · DIN 608 (M10–M12)

Long or short square neck under a flat countersunk head, for timber, conveyor and agricultural parts.

Dimensions

Countersunk Bolt Dimensions: ISO 10642, DIN 7991 & ASME B18.3

Size	Pitch P	d_k theoretical sharp, max	d_k actual min	k head height, max	s hex key	t socket depth, min	b thread, ref.
M3	0.5	6.72	5.54	1.86	2	1.1	18
M4	0.7	8.96	7.53	2.48	2.5	1.4	20
M5	0.8	11.2	9.43	3.1	3	1.75	22
M6	1	13.44	11.34	3.72	4	2.2	24
M8	1.25	17.92	15.24	4.96	5	2.9	28
M10	1.5	22.4	19.22	6.2	6	3.5	32
M12	1.75	26.88	23.12	7.44	8	4.3	36
M14	2	30.8	26.52	8.4	10	4.5	42
M16	2	33.6	29.01	8.8	10	4.8	48
M20	2.5	40.32	35.04	10.16	12	5.6	60

DIN EN ISO 10642:2020, dimensions in mm. 90° head. The theoretical diameter is where the cone meets the top face if the edge were sharp; size countersinks to it. A 2026 edition of ISO 10642 has since been published.

Size	d_k max / min	k max	Head angle	s hex key	t	b l ≤ 125 / ≤ 200 / > 200
M3	6 / 5.7	1.7	90°	2	1.2	12 / – / –
M4	8 / 7.64	2.3	90°	2.5	1.8	14 / – / –
M5	10 / 9.64	2.8	90°	3	2.3	16 / – / –
M6	12 / 11.57	3.3	90°	4	2.5	18 / 24 / –
M8	16 / 15.57	4.4	90°	5	3.5	22 / 28 / –
M10	20 / 19.48	5.5	90°	6	4.4	26 / 32 / 45
M12	24 / 23.48	6.5	90°	8	4.6	30 / 36 / 49
M14	27 / 26.48	7	90°	10	4.8	34 / 40 / 53
M16	30 / 29.48	7.5	90°	10	5.3	38 / 44 / 57
M18	33 / 32.38	8	90°	12	5.5	42 / 48 / 61
M20	36 / 35.38	8.5	90°	12	5.9	46 / 52 / 65
M22	36 / 35.38	13.1	60°	14	8.8	50 / 56 / 69
M24	39 / 38.38	14	60°	14	10.3	54 / 60 / 73

DIN 7991:1986, dimensions in mm. DIN 7991 has been replaced by ISO 10642 but is still widely ordered. Note the 60° heads on M22 and M24.

Size	Threads / in UNC / UNF	A head dia. theoretical sharp, max	H head height max	Hex key	Check (A − D) / 2 tan 41°
No. 4	40 / 48	0.255	0.083	1/16″	0.082
No. 6	32 / 40	0.307	0.097	5/64″	0.097
No. 8	32 / 36	0.359	0.112	3/32″	0.112
No. 10	24 / 32	0.411	0.127	1/8″	0.127
1/4″	20 / 28	0.531	0.161	5/32″	0.162
5/16″	18 / 24	0.656	0.198	3/16″	0.198
3/8″	16 / 24	0.781	0.234	7/32″	0.234
7/16″	14 / 20	0.844	0.234	1/4″	0.234
1/2″	13 / 20	0.938	0.251	5/16″	0.252
5/8″	11 / 18	1.188	0.324	3/8″	0.324
3/4″	10 / 16	1.438	0.396	1/2″	0.396
1″	8 / 12	1.938	0.540	5/8″	0.540

ASME B18.3 hex socket flat countersunk head cap screws, inches, 82° head. The last column is our check: every head height matches the cone geometry to within 0.001 in.

ISO vs DIN

ISO 10642 vs DIN 7991: Will the Head Sit Flush?

The keys and angles match, but the head heights don’t.

M10 heads in a countersink cut for ISO 10642 (section at 10 × scale, theoretical head form). Both 90° cones meet the 10 mm shank at the same depth, so the head top is set by k alone.

Size	k ISO 10642 mm	k DIN 7991 mm	DIN head in an ISO-flush countersink below surface	ISO head in a DIN-flush countersink above surface	Same hex key?
M3	1.86	1.7	0.16 mm	0.16 mm	Yes, 2 mm
M4	2.48	2.3	0.18 mm	0.18 mm	Yes, 2.5 mm
M5	3.1	2.8	0.30 mm	0.30 mm	Yes, 3 mm
M6	3.72	3.3	0.42 mm	0.42 mm	Yes, 4 mm
M8	4.96	4.4	0.56 mm	0.56 mm	Yes, 5 mm
M10	6.2	5.5	0.70 mm	0.70 mm	Yes, 6 mm
M12	7.44	6.5	0.94 mm	0.94 mm	Yes, 8 mm
M14	8.4	7	1.40 mm	1.40 mm	Yes, 10 mm
M16	8.8	7.5	1.30 mm	1.30 mm	Yes, 10 mm
M20	10.16	8.5	1.66 mm	1.66 mm	Yes, 12 mm

Our calculation from maximum head heights; real heads vary within their tolerances. In stainless or visible work, ask for the standard your drawings use.

Countersinks

Countersink Size Chart & Common Mistakes

Four M8 joints in section, to scale. In panel 2 the 4 mm plate is thinner than the 4.46 mm countersink depth. Panel 3 uses a countersink 3 mm too small in diameter.

Countersink chart for ISO 10642 (flush head)

Size	Countersink dia. at surface = d_k theoretical, mm	Clearance hole ISO 273 medium, mm	Countersink depth to the hole, mm	Thinnest plate without knife edge mm	Tool angle
M3	6.72	3.4	1.66	> 1.7	90°
M4	8.96	4.5	2.23	> 2.2	90°
M5	11.2	5.5	2.85	> 2.9	90°
M6	13.44	6.6	3.42	> 3.4	90°
M8	17.92	9	4.46	> 4.5	90°
M10	22.4	11	5.70	> 5.7	90°
M12	26.88	13.5	6.69	> 6.7	90°
M14	30.8	15.5	7.65	> 7.7	90°
M16	33.6	17.5	8.05	> 8.1	90°
M20	40.32	22	9.16	> 9.2	90°

Our calculation: depth = (countersink diameter − clearance hole) / 2 for a 90° countersink. Cut slightly larger (about 0.2 to 0.5 mm) if the head must not stand proud at its upper tolerance.

Do

Match the countersink angle to the head: 90° metric, 82° inch
Size the countersink to the theoretical head diameter
Keep the plate thicker than the countersink depth
Use thread locker or A4 stainless on vibrating or outdoor joints

Don't

Put an inch flat head in a 90° countersink, or the reverse
Use several countersunk bolts in a row with loose hole positions: each cone pulls the part to its centre
Tighten countersunk bolts to socket cap screw torques
Countersink thin sheet: use a dimpled hole or a pan head instead

Calculator

Countersink Diameter & Depth Calculator

Choose the screw, the clearance hole and the plate. We work out the countersink diameter and depth for a flush or recessed head, and warn you about knife edges.

Screw & plate

Standard

Size

Hole, mm

Plate, mm

Recess, mm

The hole defaults to ISO 273 medium (metric) or D + 1/32″ (inch). Recess is how far the head top should sit below the surface (0 = flush).

Countersink

17.92

mm countersink diameter at 90°

Countersink depth:4.46 mm

Straight hole left below:5.54 mm

Hex key:5 mm

Grades & materials

Countersunk Bolt Grades & Materials

Grade	Standard	Tensile R_m min, MPa	Yield R_p0.2 min, MPa	Hardness	Use
A2-70	ISO 3506-1	700	450	—	General stainless, indoor and mild outdoor
A4-70	ISO 3506-1	700	450	—	Marine, food, chemical and coastal
8.8	ISO 898-1	800	640	22–32 HRC	General machinery
10.9	ISO 898-1	1040	940	32–39 HRC	Machine tools, fixtures
12.9	ISO 898-1	1220	1100	39–44 HRC	Tooling and dies, black oxide
Alloy steel (inch)	ASTM F835	Per ASTM F835 for socket button and flat countersunk heads			Inch flat head socket screws
Stainless (inch)	ASTM F879	Per ASTM F879			Inch stainless flat head screws

These are the material properties of the class. Because of the reduced loadability of the countersunk head, a complete ISO 10642 screw is not expected to reach the full ISO 898-1 tensile load.

Stainless A2

304 · A2-70

The grade in the photo, marked A2-70 on the head.

304 304L

Stainless A4

316 · A4-70

Chloride and chemical resistance for marine and food plants.

316 316L

Alloy steel

10.9 · 12.9

Cr-Mo steels hardened and tempered, black oxide or zinc flake.

4140 4137

Special alloys

To order

Titanium and duplex countersunk bolts for weight or corrosion.

Ti Gr 5 2205

Manufacturing

How Our Countersunk Bolts Are Made

1

Wire

Stainless or alloy steel wire, certified by heat.

2

Heading

Cold heading forms the 90° or 82° cone and the drive in one piece.

3

Thread rolling

Threads rolled up to the head on short lengths.

4

Heat treatment

8.8, 10.9 and 12.9 hardened and tempered; stainless is not hardened.

5

Finishing

Passivation, black oxide or zinc flake.

6

Inspection

Head angle, height and socket gauged on every lot.

Applications

Where Our Countersunk Bolts Are Used

Machine slides & guides

Flush heads on linear rails, wear strips and sliding surfaces.

Tooling & fixtures

Locating plates, jaws and inserts where nothing may stand proud.

Marine & boat fittings

A4 stainless countersunk bolts on decks, cleats and rails.

Architectural metalwork

Balustrades, glass clamps, signage and cladding with clean, flush faces.

Conveyors & food machines

Smooth, cleanable surfaces with stainless countersunk screws.

Vehicles & aerospace ground equipment

Flush panels and floors; titanium where weight counts.

Quality & testing

Countersunk Bolt Inspection & Certification

Check	Method	Applied to
Head angle & height	Countersink gauge and height gauge	Every lot
Head diameter	Callipers against d_k min	Every lot
Hex socket	Go / no-go socket gauges	Every lot
Thread	Ring gauges	Every lot
Hardness / tensile	ISO 898-1 / ISO 3506-1	Every heat-treatment or material lot
Material	PMI spectrometer	Stainless lots

Every shipment includes

Mill certificate with heat number
Dimensional report including head height
Lot number on every box

On request

EN 10204 3.1 certificate
Tensile test report
Salt spray test for coated lots

FAQ

Countersunk Bolt Questions, Answered

A countersunk bolt has a cone-shaped head that sits in a matching countersink, so the head ends flush with or below the surface. The most common type is the hex socket countersunk screw to ISO 10642 (formerly DIN 7991). Metric countersunk heads are 90 degrees and inch flat heads are 82 degrees.

A countersunk bolt is measured from the top of the head to the end of the thread, so the head is included in the length. This differs from hex, socket cap and button head bolts, which are measured from under the head. An M8 x 30 countersunk bolt is 30 mm long overall.

ISO 10642 replaced DIN 7991. Both use the same hex keys from M3 to M20 and 90 degree heads, but ISO 10642 heads are taller: an M10 head is 6.2 mm high to ISO and 5.5 mm to DIN. In a countersink cut for an ISO head, a DIN screw therefore sits about 0.7 mm lower. DIN 7991 M22 and M24 heads are 60 degrees.

Metric countersunk screws to ISO 10642, ISO 2009, ISO 7046 and DIN 7991 up to M20 have 90 degree heads. Inch flat head screws, including ASME B18.3 socket screws, have 82 degree heads. Always cut the countersink with a tool of the same angle as the head.

For an ISO 10642 M8 head to sit flush, cut a 90 degree countersink 17.92 mm in diameter at the surface, the theoretical head diameter. With a 9 mm clearance hole the countersink is 4.46 mm deep, so the plate must be thicker than that to avoid a knife edge. The countersink calculator on this page works this out for every size.

No. ISO 10642 describes countersunk socket screws as having reduced loadability: the cone head can fail before the thread reaches the full load of the class marked on it. Use socket head cap screws or hex bolts where high tension is needed, and countersunk bolts where a flush surface matters.

Metric countersunk socket screws are made in A2-70 and A4-70 stainless steel and in class 8.8, 10.9 and 12.9 alloy steel. Inch flat head socket screws are made to ASTM F835 in alloy steel and ASTM F879 in stainless steel.

ISO 10642 countersunk bolts take a 2 mm key for M3, 2.5 mm for M4, 3 mm for M5, 4 mm for M6, 5 mm for M8, 6 mm for M10, 8 mm for M12, 10 mm for M14 and M16, and 12 mm for M20. These keys are smaller than those for socket cap screws of the same thread.