Screws · DIN 7991
DIN 7991 Allen CSK Screw
Accurate Fastners manufactures and supplies DIN 7991 Allen CSK Screws in stainless steel grades SS 202, SS 304 and SS 316, cold forged to metric dimensions ranging from M4 to M10 and lengths from 10 mm to 80 mm.
- Standard
- DIN 7991
- Grades
- 304 / 316 / 202
- Size
- M4 – M10
- Length
- 10 mm – 80 mm
Overview
What is the DIN 7991 Allen CSK Screw?
DIN 7991 is a German standard that specifies the dimensional and mechanical requirements for hexagon socket countersunk head screws with a flat (82°) countersunk profile and an internal Allen drive. The standard is widely referenced in European and global engineering practice, making components produced to it broadly interchangeable wherever the same specification is adopted. Accurate Fastners produces these screws through cold forging, a process that work-hardens the material and delivers consistent head geometry.
The defining design of the DIN 7991 Allen CSK Screw combines two purposeful features: a countersunk, flat-topped head that sits flush with or below the mating surface when installed, and a hexagon socket recess that accepts a standard Allen key or hex bit. This combination allows the fastener to be driven with reasonable torque without projecting above the assembly surface, which may be important where protruding heads would interfere with adjacent components or the intended finish of the assembly.
Cold forging, the production process used for this range, shapes the screw blank under high pressure at or near room temperature rather than by cutting or casting. This typically results in a more uniform grain structure, improved surface finish and tighter dimensional consistency compared with hot-worked alternatives. The metric thread on each screw conforms to standard pitch conventions, supporting straightforward engagement with standard metric nuts, threaded inserts or tapped holes across the M4–M10 diameter range covered by this product.
In Depth
Product description
Construction & Geometry
Each screw features a countersunk flat head whose conical underside seats into a matching chamfered recess in the mating part, drawing flush as the fastener is tightened. The hexagon socket is machined centrally into the head, accepting a standard metric Allen key. Diameter coverage runs from M4 to M10 and length coverage from 10 mm to 80 mm, providing a range of grip lengths suitable for varying material thicknesses.
The fully metric thread engages standard-pitch nuts or tapped holes without adaptation. Cold forging produces a shank and thread form with close dimensional tolerances and a smooth surface finish that supports consistent thread engagement. The countersunk head profile follows DIN 7991 geometry, meaning the head angle and socket dimensions are consistent across the range and interchangeable with tooling and mating components designed to that standard.
Functional Behaviour in an Assembly
When installed into a properly countersunk hole, the screw head draws progressively into the chamfer as torque is applied, clamping the joint while the top face of the head remains level with the surrounding surface. This flush-mount behaviour may be useful in assemblies where a smooth outer surface is required or where overhanging heads could create a snagging or clearance problem. The Allen drive allows torque application in confined spaces where a spanner or Phillips head tool may not fit.
The hexagon socket also resists cam-out more effectively than some slotted or cross-head drives under high torque, which can help maintain drive engagement during assembly. Because the head exerts a wedging action against the countersunk hole rather than a bearing action on a flat surface, the clamping geometry differs from that of a pan or button head screw, and the quality of the countersink in the mating part influences overall joint performance.
How It Differs from Similar Fasteners
Compared with a standard hexagon socket cap screw (DIN 912), the DIN 7991 variant uses a countersunk rather than a cylindrical head, trading the higher bearing area of a cap head for a flush-mount profile. Against a Phillips or slotted countersunk screw, the Allen socket drive generally offers better torque transfer and is less prone to drive damage under power tools, though it requires an Allen key rather than a common screwdriver.
ISO 10642 is a closely related international standard specifying a hexagon socket countersunk head screw of similar geometry; the two are often cross-referenced, though buyers should confirm dimensional interchangeability for their specific application. Compared with a truss or button head screw, the countersunk type requires pre-drilling a matching chamfered recess in the mating material, adding a preparation step but achieving a profile that sits at or below the surface plane.
Material & Grade
Material and grade information
- SS 304
- SS 316
- SS 202
This range is available in three stainless steel grades: SS 202, SS 304 and SS 316. SS 304, an austenitic grade containing approximately 18% chromium and 8% nickel, is among the most commonly used stainless steels in general industrial fastener applications and offers good corrosion resistance in many indoor and mildly corrosive environments. SS 202 substitutes a portion of nickel with manganese and nitrogen, making it a cost-effective alternative where the operating environment is less demanding.
SS 316 adds molybdenum to the chromium-nickel composition of SS 304, which typically improves resistance to chloride-containing environments and certain process chemicals. It is often selected for food processing, pharmaceutical machinery and marine-adjacent applications where enhanced corrosion resistance may be needed. Grade selection should be based on the actual service environment, chemical exposure and relevant engineering requirements; Accurate Fastners' sales team can advise on grade suitability for a given enquiry.
Grades shown are those documented for this product in the Accurate Fastners brochure. General material notes are guidance only; confirm grade selection for your environment.
Technical Data
Technical specifications
| Product type | Screw – DIN 7991 Allen CSK Screw |
|---|---|
| Material | Stainless steel |
| Material grade | SS 304, SS 316, SS 202 |
| Applicable standard | DIN 7991 |
| Nominal diameter / size | M4 – M10 |
| Length | 10 mm – 80 mm |
| Thread type | Metric |
| Head / form | Countersunk (flat) head |
| Drive / tightening | Hexagon socket (Allen) |
| Manufacturing process | Cold forged |
Sizes
Available sizes
Documented size range
M4 – M10
Documented length range
10 mm – 80 mm
Individual sizes within the documented range, thread pitches and lengths are confirmed at quotation stage.
Applications
Industrial applications
Water Purification Equipment
Flush-head fasteners may suit RO water purifier assemblies where smooth internal surfaces are preferred and protruding screw heads could obstruct flow paths or create cleaning difficulties in compact housings.
Solar Energy Structures
Allen CSK screws in SS 304 or SS 316 may be used in solar panel mounting frames and junction boxes where a flush profile reduces wind-loading interference and stainless material offers outdoor durability.
LED Lighting Assemblies
The compact M4–M6 end of the range can suit LED driver enclosures and fixture bodies where a flush countersunk head maintains a clean finish and the Allen drive aids assembly in tight spaces.
Food & Beverage Machinery
SS 316 grade screws may be considered for food-contact or wash-down environments in food and beverage processing equipment, where flush surfaces simplify cleaning and hygiene maintenance.
Pharmaceutical Machinery
In pharmaceutical equipment fabrication, flush-mounted fasteners can support smoother surface profiles on panels and enclosures, with SS 316 often preferred for its resistance to cleaning agents and process chemicals.
Featured for: Food & Beverage, Pharmaceutical Machinery
Features & Selection
Features and selection considerations
Design features
DIN 7991 Standard Compliance
Dimensional and geometric conformance to DIN 7991 means these screws are interchangeable with mating components and tooling designed to the same specification across a range of engineering applications.
Cold Forged Construction
Cold forging work-hardens the stainless steel blank, typically producing a more uniform grain structure, consistent head geometry and reliable dimensional accuracy compared with cut or cast alternatives.
Flush Countersunk Head Profile
The flat countersunk head seats into a matching chamfer so the screw face sits level with or below the mating surface, eliminating protrusion where a smooth exterior is needed.
Hexagon Socket Drive
The internal Allen socket allows torque application with a compact hex key, suits confined assembly spaces and reduces cam-out compared with slotted or cross-head drives under power tool use.
Broad Size Range
Coverage from M4 to M10 diameter and 10 mm to 80 mm length provides flexibility across thin-panel to thicker structural joints without changing fastener type or supplier.
Before you order
Diameter & Length
Select diameter based on tapped hole or nut size in the mating part. Choose length so the engaged thread meets the minimum required for the joint, accounting for countersink depth and material stack thickness.
Material Grade
Match the grade to the service environment: SS 202 for mild indoor use, SS 304 for general corrosion resistance and SS 316 where chlorides, chemicals or wash-down conditions are present.
Mating Part Preparation
DIN 7991 screws require a correctly angled countersunk recess in the mating material. Confirm the countersink angle and diameter match the screw's head geometry before specifying or ordering.
Quantity & Documentation
For bulk or project orders, provide quantity, diameter, length and grade when enquiring. Request material test certificates or dimensional inspection records through the sales team if project documentation is required.
FAQs
Frequently asked questions
What does DIN 7991 specify?
DIN 7991 is a German dimensional standard for hexagon socket countersunk flat head screws. It defines head angle, socket size and overall proportions for each diameter, ensuring interchangeability with tooling and mating components produced to the same standard.
Which stainless steel grades are available for this screw?
Accurate Fastners supplies the DIN 7991 Allen CSK Screw in SS 202, SS 304 and SS 316. Each grade offers a different balance of corrosion resistance and cost; the sales team can advise on which grade suits your operating environment.
What sizes are available?
The range covers M4 to M10 in diameter and 10 mm to 80 mm in length. If your project requires a specific combination within this range, please confirm availability and lead time with the sales team at the time of enquiry.
Is this screw suitable for outdoor or corrosive environments?
Stainless steel grades offer varying levels of corrosion resistance. SS 316 is commonly selected for outdoor, marine-adjacent or chemical-exposure environments. Suitability depends on specific conditions; we recommend discussing your application with our technical sales team before specifying.
What complementary fasteners are typically used with this screw?
Depending on the joint, DIN 7991 screws may be paired with metric hex nuts, nylon insert nuts, flat washers or threaded inserts in the mating part. Accurate Fastners also supplies a broad range of nuts, washers and anchors that can be quoted alongside.
How can I request a quotation?
Quotations can be requested by completing the enquiry form on our website, emailing export@accuratefastners.com or reaching us via WhatsApp. Please include diameter, length, grade, quantity and any documentation requirements to receive an accurate and prompt response.
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