1. Introduction
DIN7991 countersunk socket head cap screw, also known as flat head socket cap screw, is a precision fastener designed for applications where a flush, smooth surface is required. Its 90-degree countersunk head sits entirely below or level with the workpiece surface, eliminating protruding bolt heads and creating a clean, aerodynamic, or snag-free finish.
This type of screw is widely used in mold making, machine design, automotive components, furniture hardware, and equipment panels where surface flatness is critical. NINGBO TOPBOLT METALWORKS CO., LTD supplies DIN7991 screws in full size range from M3 to M24, with multiple strength grades and surface treatments available.
2. What Is DIN7991 Countersunk Socket Head Cap Screw?
2.1 Standard Definition
DIN7991 is the German standard for "hexagon socket countersunk head cap screws — product grade A". The standard defines the head geometry, countersunk angle, socket dimensions, thread tolerances, and mechanical properties for this fastener type. The key characteristic is the 90-degree included angle countersunk head with an internal hexagon (Allen) drive.
When properly installed, the head of a DIN7991 screw sits flush with or slightly below the surface of the workpiece, creating a smooth, flat finish. This distinguishes it from DIN912 socket head cap screws, which have a protruding cylindrical head.
2.2 Structural Features
90-degree countersunk (flat) head with internal hexagon drive
Head sits flush with workpiece surface when properly installed
Product grade A for precision dimensional tolerances
Available in full thread (standard) and partial thread options
Metric coarse thread as standard (fine thread on request)
Requires a matching 90-degree countersunk hole for proper seating
3. Dimension & Specification
3.1 Common Size Range
DIN7991 screws are available in nominal diameters from M3 to M24, with standard lengths ranging from 6mm to 200mm depending on diameter. The table below lists common sizes and key head dimensions.
Nominal Diameter (d) | Head Diameter (dk) | Head Height (k) | Socket Size (across flats) | Standard Length Range |
M3 | 5.5mm | 1.5mm | 2mm | 6–30mm |
M4 | 7.0mm | 2.0mm | 2.5mm | 8–40mm |
M5 | 8.5mm | 2.5mm | 3mm | 10–50mm |
M6 | 10.0mm | 3.0mm | 4mm | 12–60mm |
M8 | 13.0mm | 4.0mm | 5mm | 16–80mm |
M10 | 16.0mm | 5.0mm | 6mm | 20–100mm |
M12 | 18.0mm | 6.0mm | 8mm | 25–120mm |
M16 | 24.0mm | 8.0mm | 10mm | 35–160mm |
M20 | 30.0mm | 10.0mm | 12mm | 40–200mm |
M24 | 36.0mm | 12.0mm | 14mm | 50–200mm |
3.2 Countersunk Angle & Head Geometry
The DIN7991 standard specifies a 90-degree included angle for the countersunk head. This means the conical underside of the head tapers at 45 degrees from the screw axis. It is critical that the mating countersunk hole in the workpiece is also machined at 90 degrees to ensure full, even contact between the screw head and the workpiece.
A mismatch in countersunk angle (e.g., using a 120-degree countersink for a 90-degree screw) will result in the screw head seating only on its outer edge, causing reduced bearing surface, potential head deformation, and insufficient clamping force.
4. Material & Strength Grade Options
Strength Grade | Typical Material | Tensile Strength (min) | Yield Strength (min) | Common Applications |
8.8 | Medium carbon steel (C35/C45, QT) | 800 MPa | 640 MPa | General machinery, molds, fixtures |
10.9 | Alloy steel (42CrMoA, QT) | 1000 MPa | 900 MPa | High-load machinery, automotive |
12.9 | Alloy steel (42CrMoA/35CrMoA, QT) | 1200 MPa | 1080 MPa | Ultra-high load, precision molds |
A2-70 (304 SS) | Stainless steel 304 | 700 MPa | 450 MPa | Corrosion-resistant, food/medical |
A4-80 (316 SS) | Stainless steel 316 | 800 MPa | 600 MPa | Marine, chemical, coastal |
Important Note: Stainless steel DIN7991 screws have lower shear strength in the head compared to alloy steel grades. For high-torque or high-load applications, use 10.9 or 12.9 grade alloy steel screws with appropriate corrosion protection rather than stainless steel.
5. Installation Requirements
5.1 Countersunk Hole Preparation
Proper countersunk hole preparation is essential for DIN7991 screws to seat correctly and achieve full clamping force:
Drill the clearance hole for the screw shank (typically 1mm larger than nominal diameter)
Use a 90-degree countersink tool to machine the tapered seat
Countersink depth should allow the screw head to sit flush with or 0.1–0.3mm below the surface
Verify the countersink diameter matches the screw head diameter (dk) for the specific size
Deburr the hole edge to ensure clean metal-to-metal contact
5.2 Torque & Tightening Guidelines
DIN7991 screws have a lower head shear capacity compared to DIN912 button head screws due to the thinner countersunk head profile. Over-tightening can cause the head to shear off or the internal hexagon socket to strip.
Size | Grade 8.8 Torque (Nm) | Grade 10.9 Torque (Nm) | Grade 12.9 Torque (Nm) | A2-70 Torque (Nm) |
M4 | 3.5–4.5 | 4.5–5.5 | 5.5–6.5 | 2.5–3.0 |
M5 | 7–9 | 9–11 | 11–13 | 5–6 |
M6 | 12–15 | 15–18 | 18–22 | 8–10 |
M8 | 28–35 | 35–42 | 42–50 | 18–22 |
M10 | 55–65 | 65–80 | 80–95 | 35–42 |
M12 | 90–110 | 110–135 | 135–160 | 55–65 |
M16 | 220–260 | 260–320 | 320–380 | 130–155 |
Always use a properly calibrated hex key (Allen wrench) or hex bit with full engagement in the socket. For sizes M8 and above, use a ball-end or L-key with sufficient leverage to avoid cam-out and socket rounding.
6. Common Applications
Mold & die making: Securing mold plates, ejector plates, and die components where flush surfaces are required
Machine design: Equipment panels, guard rails, and machine frames requiring smooth surfaces
Automotive & motorcycle: Body panels, engine covers, brake discs, and interior trim
Furniture & cabinetry: Modern furniture hardware where visible screw heads are undesirable
Medical & food equipment: Stainless steel versions for hygienic, easy-to-clean surfaces
Aerospace & marine: Applications requiring flush, low-drag surfaces with corrosion resistance
Electronics & enclosures: Panel mounting where protruding heads would interfere with components
7. DIN7991 vs DIN912 vs DIN7984: Comparison
Feature | DIN7991 (Countersunk) | DIN912 (Cap Head) | DIN7984 (Low Head) |
Head Style | 90° countersunk flat head | Cylindrical cap head | Low cylindrical head |
Surface Profile | Flush / below surface | Protruding above surface | Low protrusion above surface |
Head Height | Lowest (0.5d approx.) | Highest (1.0d approx.) | Medium (0.65d approx.) |
Max Torque Capacity | Lower (thin head) | Highest (thick head) | Medium |
Countersunk Hole Required | Yes (90°) | No | No |
Best For | Flush surface, aesthetic | High-load, general purpose | Limited vertical space |
Typical Use | Molds, panels, furniture | Machinery, structures | Fixtures, tight spaces |
For applications requiring both high clamping force and a flush surface, consider using a heavy-duty flat washer under a DIN912 cap head screw as an alternative, provided surface protrusion is acceptable.
8. Surface Treatment Options
Surface Treatment | Corrosion Resistance | Appearance | Typical Use |
Black Oxide | Low | Matte black | Indoor machinery, molds |
Zinc Plated (Clear) | Medium | Bright silver | General purpose, indoor |
Zinc Plated (Yellow) | Medium-High | Golden yellow | Outdoor, moderate humidity |
Nickel Plated | High | Bright nickel | Decorative, furniture, automotive |
Dacromet / Geomet | Very High | Silver-gray matte | Automotive, infrastructure |
Stainless Steel (plain) | Very High | Natural silver | Corrosive, hygienic environments |
9. Frequently Asked Questions
Q1: Can DIN7991 screws be used without a countersunk hole?
No. DIN7991 screws require a properly machined 90-degree countersunk hole to seat correctly. Without a countersink, the conical head will not make full contact with the workpiece, resulting in poor clamping, potential damage, and an uneven surface.
Q2: What is the difference between DIN7991 and ISO 10642?
DIN7991 and ISO 10642 are technically equivalent standards for hexagon socket countersunk head cap screws. DIN7991 is the German national standard, while ISO 10642 is the international standard. The dimensional and mechanical specifications are essentially identical, and the fasteners are interchangeable.
Q3: Why does my DIN7991 screw head sit above the surface?
The most common causes are: (1) insufficient countersink depth, (2) incorrect countersink angle (not 90 degrees), (3) debris or burrs in the countersunk hole, or (4) using a screw with a head diameter larger than the countersink. Verify the countersink dimensions and clean the hole before reinstallation.
Q4: Can I use a washer under a DIN7991 countersunk screw?
Generally no. The purpose of a countersunk head is to seat directly into the tapered hole for a flush finish. Adding a washer would prevent proper seating and cause the head to protrude. If load distribution is a concern, use a larger diameter screw or a flat head screw with an integrated washer face design instead.
Q5: What is the maximum recommended torque for DIN7991 screws?
Torque values depend on size and strength grade. Refer to the torque table in Section 5.2. As a general rule, DIN7991 screws should be torqued to approximately 70–80% of the equivalent DIN912 cap head screw torque due to the thinner head profile. Always use a calibrated torque wrench and stop if the socket begins to round.
