Socket
Countersunk
Head Screws
Flat 90° conical head Allen drive fastener in Grade 10.9 & 12.9 high tensile and SS 304 / 316. Manufactured to DIN 7991 · ISO 10642 · IS 6761. Metric M3–M16 — sits flush with the parent surface for a completely clean, snag-free finish. Ready stock Bengaluru.
Grade
10.9 · 12.9 · SS 304 · SS 316
Standard
DIN 7991 · ISO 10642 · IS 6761
Size Range
M3 – M16
What is a Socket Countersunk Head Cap Screw?
A socket countersunk head cap screw — also called a CSK Allen screw or flat head socket screw — is a precision fastener with a flat, 90° conical head and an internal hexagonal (Allen) socket drive. When fully tightened into a countersunk hole, the head sits flush with or below the surface of the parent material, leaving a completely clean, snag-free finish with no protrusion.
RM Traders stocks socket CSK screws in Grade 10.9 and 12.9 high tensile alloy steel and in SS 304 / SS 316 stainless steel. Metric sizes M3 to M16 in a wide range of lengths — manufactured to DIN 7991 · ISO 10642 · IS 6761.
High Tensile
Grade 10.9
1040 MPa tensile. High-load precision assemblies and machine tooling. Black Oxide standard; Zinc on request per ISO 4042.
High Tensile
Grade 12.9
1220 MPa tensile. Maximum strength for die tooling and precision fixtures. Black Oxide recommended — electroplating not advised.
Corrosion Resistant
SS 304 / 316
Austenitic stainless. SS 304 for general use; SS 316 with molybdenum for chloride and marine environments. M3–M16.
Standards & Specifications
- DIN 7991 / ISO 10642Primary standard for socket countersunk head cap screws. DIN 7991 (German) and ISO 10642 (international) specify identical 90° head angle, head diameter (dk), head height (k), socket size (s), and thread tolerances. Fully interchangeable. Product Grade A per ISO 4759-1.
- IS 6761Bureau of Indian Standards equivalent. Specifies the same 90° countersunk geometry. Property classes 10.9 and 12.9 per IS 1367. Threads per IS 4218 / ISO 261. RM Traders stocks conform to this standard.
- Head Angle90° included angle — the metric standard globally. Not interchangeable with 82° imperial countersunk screws (ASME B18.3). Always match the screw head angle to the countersink angle in your workpiece.
- ISO 898-1 / IS 1367Mechanical properties for property classes 10.9 and 12.9. Grade 12.9 hardness: 39–44 HRC. Thread tolerance class 6g per ISO 965-2.
Dimensional
Tolerances
Per DIN 7991 / ISO 10642 — Product Grade A. All dimensions in mm.
| d | p (mm) | Head dia dk (mm) | A/F — s (mm) | k max (mm) | |||
|---|---|---|---|---|---|---|---|
| nom/max | min | nom | max | min | |||
| M3 | 0.50 | 6.00 | 5.70 | 2 | 2.10 | 2.02 | 1.7 |
| M4 | 0.70 | 8.00 | 7.64 | 2.5 | 2.60 | 2.52 | 2.3 |
| M5 | 0.80 | 10.00 | 9.64 | 3 | 3.10 | 3.02 | 2.8 |
| M6 | 1.00 | 12.00 | 11.57 | 4 | 4.12 | 4.02 | 3.3 |
| M8 | 1.25 | 16.00 | 15.57 | 5 | 5.14 | 5.02 | 4.4 |
| M10 | 1.50 | 20.00 | 19.48 | 6 | 6.14 | 6.02 | 5.5 |
| M12 | 1.75 | 24.00 | 23.48 | 8 | 8.175 | 8.025 | 6.5 |
| M14 | 2.00 | 27.00 | 26.48 | 10 | 10.175 | 10.025 | 7.0 |
| M16 | 2.00 | 30.00 | 29.48 | 10 | 10.175 | 10.025 | 7.5 |
dk = head diameter (nominal = maximum for countersunk heads) · k = countersunk head height · s = across flat (A/F) hex socket size · p = thread pitch. Head angle: 90° included. All sizes fully threaded.
Sizes Available
Standard stocked metric range in High tensile (Grade 10.9 & 12.9) and SS 304 / SS 316. Sizes outside these ranges available on enquiry with MOQ.
HT Grade 10.9 & 12.9 — Metric (M3 – M16)
| Diameter | Available Lengths (mm) |
|---|---|
| M3 | 6 |
| M4 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50 |
| M5 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 55 |
| M6 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 |
| M8 | 16, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 90, 100 |
| M10 | 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 90, 100, 110, 120 |
| M12 | 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 90, 100, 110, 120, 130 |
| M14 | 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 130 |
| M16 | 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 130 |
SS 304 (A2) — Metric (M3 – M16)
| Diameter | Available Lengths (mm) |
|---|---|
| M3 | 6, 8, 10, 12, 16, 20, 25, 30 |
| M4 | 6, 8, 10, 12, 16, 20, 25, 30, 35, 40 |
| M5 | 6, 8, 10, 12, 16, 20, 25, 30, 35, 40, 50 |
| M6 | 10, 12, 16, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100 |
| M8 | 16, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100 |
| M10 | 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100 |
| M12 | 25, 30, 35, 40, 50, 60, 70, 80, 90, 100 |
| M16 | 35, 40, 50, 60, 70, 80, 90, 100 |
SS 316 (A4) available in the same diameter range as SS 304 above — same length availability. Contact us for pricing and lead time.
HT surface finish: Manganese coating / Black Oxide standard per IS 1367. Zinc plating available on Grade 10.9 on customer request. Electroplating of Grade 12.9 requires post-plate de-embrittlement bake per ISO 4042 — Black Oxide recommended for 12.9 grade.
HT surface finish: Manganese coating / Black Oxide standard per IS 1367. Zinc plating available on Grade 10.9 on customer request. Electroplating of Grade 12.9 requires post-plate de-embrittlement bake per ISO 4042 — Black Oxide recommended for 12.9 grade.
Applications
Machine Guarding & Covers
CSK screws assemble machine covers and access panels where a flush surface is mandatory — protruding heads on rotating machinery create serious safety hazards and interfere with guards.
Electronics & PCB Enclosures
Flush-fit CSK screws are standard on electronics chassis, rack-mount server enclosures, and PCB standoffs where head protrusion would block adjacent boards or cable routing.
Aerospace & Precision Tooling
Jigs, fixtures, and aerospace inspection tooling specify Grade 12.9 CSK screws for flush assembly — the flat head eliminates interference with workpiece surfaces on precision CMM fixtures.
Furniture & Cabinetry
Hinges, drawer runners, and structural joints in commercial furniture use SS 304 or HT CSK screws for concealed fixing — the flush head can be covered with a cap or left visible as a clean detail.
Marine & Food Equipment
SS 316 CSK screws are used on marine hatches, food-processing panels, and pharmaceutical trays where a fully flush, crevice-minimising surface is required for hygiene and corrosion resistance.
Automotive Body Panels
Interior trim, underbody covers, and structural brackets specify CSK screws for a flush, aerodynamically clean surface — particularly in areas exposed to airflow or where panel alignment is critical.
Frequently Asked
Questions
What is a socket countersunk head cap screw?
A socket countersunk head cap screw (also called a CSK Allen screw or flat head socket screw) is a fastener with a flat 90° conical head driven by an internal hexagonal (Allen) socket. Manufactured to DIN 7991 / ISO 10642 / IS 6761, when tightened into a properly countersunk hole the head sits flush with or below the surface of the parent material — leaving a completely flat, snag-free finish with no protrusion. RM Traders stocks these in Grade 10.9 and 12.9 high tensile and SS 304 / SS 316 stainless, in metric M3–M16.
What is the head angle of a socket countersunk screw, and does it matter?
Socket countersunk head cap screws per DIN 7991 / ISO 10642 have a 90° included head angle — the global metric standard. This matches standard countersinking drill bits and spot-facing tools. An 82° head angle is used in some imperial/American standards (ASME B18.3) and is not interchangeable with 90° metric screws. Using the wrong angle causes the head to rock in the countersink, reducing clamping force and damaging the bore. Always confirm the countersink angle in your workpiece matches the screw head angle before specifying.
What is the difference between DIN 7991 and ISO 10642?
DIN 7991 is the German standard for socket countersunk head cap screws. ISO 10642 is the international equivalent — both specify the same 90° head angle, identical head diameter (dk), head height (k), and socket size (s). A DIN 7991 and ISO 10642 screw of the same size are fully interchangeable. In India, IS 6761 is the Bureau of Indian Standards equivalent. RM Traders stocks to all three standards simultaneously.
What grade are socket countersunk head cap screws available in?
RM Traders stocks socket CSK screws in Grade 10.9 (1040 MPa — high-load machine and tooling) and Grade 12.9 (1220 MPa — maximum strength, die and precision tooling), plus SS 304 (A2-70, 700 MPa) and SS 316 (A4-80, 800 MPa) stainless steel. Note: the countersunk head geometry places the clamping load differently from a cylindrical head — avoid using CSK screws in primary shear-critical joints without engineering verification.
Can socket countersunk head cap screws be electroplated?
For Grade 10.9, standard electroplating (zinc BZP, nickel) is acceptable on customer request. For Grade 12.9, electroplating carries a serious hydrogen embrittlement risk — hydrogen generated during acid pickling diffuses into the high-strength steel lattice and can cause delayed brittle fracture. ISO 4042 mandates a post-plating de-embrittlement bake at 200–230°C for a minimum of 8 hours within 4 hours of plating for 12.9 grade. Many specifications prohibit electroplating Grade 12.9 entirely. The safe standard finish for Grade 12.9 is Black Oxide (Manganese Phosphate). For corrosion-resistant coatings on Grade 12.9, specify mechanical zinc (Dacromet / Geomet) — hydrogen-free and safe.
What Allen key size fits each socket countersunk head cap screw diameter?
Hex socket (Allen key) sizes per DIN 7991 / ISO 10642: M3 = 2 mm · M4 = 2.5 mm · M5 = 3 mm · M6 = 4 mm · M8 = 5 mm · M10 = 6 mm · M12 = 8 mm · M14 = 10 mm · M16 = 10 mm. The socket in a countersunk head is shallower than in a socket head cap screw of the same diameter. Always ensure the Allen key is fully seated before applying torque — a partially inserted or worn key on smaller diameters (M3–M6) will round the socket, making removal extremely difficult. For Grade 12.9, use a quality T-handle or torque wrench bit.
Are socket countersunk head cap screws available in stainless steel?
Yes. RM Traders stocks socket CSK screws in SS 304 (A2) and SS 316 (A4) in metric M3–M16 with a wide range of lengths. SS 304 suits general indoor, mildly corrosive, and outdoor environments. SS 316 adds molybdenum for superior resistance to chlorides, salt water, and acids — the preferred grade for marine fittings, food-processing lines, and pharmaceutical equipment where a flush, crevice-minimising surface is critical. Stainless CSK screws are rated A2-70 / A4-80 (700–800 MPa) — significantly lower clamping strength than Grade 12.9 alloy steel.
What is the difference between a countersunk and a button head screw?
Both are Allen-drive screws for low-profile assemblies, but they work differently. A socket countersunk head screw (DIN 7991 / ISO 10642) has a flat 90° conical head that seats inside a countersunk hole — the head is flush with or below the surface, leaving a completely flat finish. It requires a countersink in the workpiece. A socket button head cap screw (ISO 7380-1) has a low domed head that sits on the surface — no countersink needed, but the dome protrudes slightly. Use countersunk screws when absolute surface flatness is required. Use button heads when the material is too thin or soft for countersinking, or when a slightly raised but snag-free dome is acceptable.
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