Countersunk Bolt DIN 7991 | Marine Flat Head Bolt for Flush Surface Installation
DIN 7991 Countersunk Bolt (CSK Bolt)
TheDIN 7991 countersunk bolt (also called flat head socket screw,CSK bolt, or countersunk cap screw) features an 82° conical head that sits flush or below the workpiece surface when installed into a countersunk hole. The hexagon socket (Allen key drive) enables high torque transmission with a clean exterior profile — essential for marine applications where exposed bolt heads create drag, interference or safety hazards.
SINOFASTENER supplies CSK bolts in carbon steel Class 8.8/10.9/12.9 and stainless steel A2-70/A4-70/A4-80 (316). Surface options: plain black oxide, white zinc, yellow zinc, hot-dip galvanized (HDG min 45μm), passivated SS, and PTFE/Xylan coated. Thread range: M3–M24.
Technical Specifications
| Parameter | Specification |
|---|---|
| Standard | DIN 7991 / ISO 10642 / GB/T 70.3 / JIS B1176 |
| Thread Range | M3 – M24 (metric coarse pitch standard, fine pitch optional) |
| Head Angle | 82° ± 1° included cone angle |
| Drive Type | Hexagon socket (Allen key / Hex key drive) |
| Material Grades | Carbon Steel 8.8 / 10.9 / 12.9 | SS A2-70 (304) / A4-70 (316) / A4-80 (316 High-Strength) |
| Surface Finish | Plain, White Zn, Yellow Zn, HDG (min 45μm), Passivated SS, PTFE/Xylan Coated |
| Certification | EN 10204 3.1 mill certificate + optional ABS/BV/LR/DNV third-party inspection |
Size Reference (DIN 7991)
| Thread | Head Dia dk (mm) | Head Height k (mm) | Key Size s (mm) | Torque* Grade 10.9 (Nm) |
|---|---|---|---|---|
| M4 | 7.4 | 2.0 | 2 | 3.5 |
| M5 | 9.4 | 2.5 | 2.5 | 7 |
| M6 | 11.7 | 3.0 | 3 | 12 |
| M8 | 15.5 | 4.0 | 4 | 30 |
| M10 | 19.5 | 5.0 | 5 | 59 |
| M12 | 23.3 | 6.0 | 6 | 105 |
| M16 | 31.0 | 8.0 | 8 | 255 |
| M20 | 38.5 | 10.0 | 10 | 500 |
| M24 | 46.5 | 12.0 | 12 | 865 |
*Torque reference for dry assembly into steel tapped hole. Adjust per actual friction condition and VDI 2230.
Material Selection Guide
| Grade | Rm (MPa) | Re (MPa) | Best Application |
|---|---|---|---|
| 8.8 + Zinc Plated | ≥800 | ≥640 | Hull interior structure, engine room, dry compartments |
| 10.9 + HDG | ≥1000 | ≥900 | Hull framing, deck substructure, high-load structural joints |
| 12.9 Black Oxide | ≥1200 | ≥1080 | Critical load-bearing connections, machinery foundations |
| A4-70 SS (316) | ≥700 | ≥450 | Deck hardware, external fittings, splash zone, BWTS components |
| A4-80 SS (316 High-Strength) | ≥800 | ≥600 | High-load seawater-exposed joints, offshore platform deck fittings |
| PTFE/Xylan Coated | Per base grade | Per base grade | BWTS piping flanges, chemical tank internals, extreme corrosion zones |
Marine Applications of CSK Bolts
| Application Area | Usage | Recommended Material |
|---|---|---|
| Hull Plating | Outer plate attachments, stringer/frame connections | 10.9 HDG / A4-80 SS |
| Deck Plates | Manhole covers, floor plates, walkway surfacing | A4-70/A4-80 SS / 10.9 HDG |
| Hatch Covers | Pontoon-type and single-pull covers, weather-tight surfaces | A4-80 SS / PTFE coated 10.9 |
| Bulkheads | Watertight/oiltight bulkhead stiffeners, partitions | A4-70 SS / 8.8 Zn-plated |
| Machinery Foundation | Engine bedplate, generator base mounting | 10.9 / 12.9 + locking compound |
| Pipe Support Brackets | Seawater pipe clamp brackets, duct flanges (flush mount required) | A4-80 SS / PTFE coated |
Why Choose CSK Bolts for Marine Use?
- Flush Surface: Eliminates protruding heads on hull plating, decks, and hatch covers — critical for hydrodynamic efficiency, safety and equipment clearance.
- Tamper-Resistant: Recessed Allen drive provides security for access panels and passenger vessel fittings.
- High Clamping Force: Socket drive transmits high torque comparable to SHCS for vibration-resistant joints.
- Clean Paintability: Seamless paint/coating coverage over fastened area without raised edges that trap moisture.
SINOFASTENER manufactures DIN 7991 countersunk bolts from M3 to M24 with custom lengths up to 200mm. Full EN 10204 3.1 traceability. Custom materials available: Duplex 2205, Inconel 625, Monel K400. Contact us for quotation.
Contact SINOFASTENER for Quotation
📧 Email: sales@sinofastener.com
🌐 Website: www.sinofastener.com
📍 Ningbo Zhonglian Fastener Manufacture Co., Ltd. — SINOFASTENER
