ISO 7991
Hexagon Socket Countersunk Head Cap Screws
Flush-Fit High-Strength Screws for Precision & Aesthetic Assemblies
Manufacturer & Exporter: Super Industries India
1. What is ISO 7991?
ISO 7991 defines hexagon socket countersunk head cap screws, designed to sit flush with or below the surface of the assembled parts.
These screws combine:
- A countersunk (flat) head
- An internal hexagon drive
- High mechanical strength
ISO 7991 is widely used where space constraints, smooth surfaces, and strength are equally important.
Super Industries
Global Manufacturer of Heavy & Custom Engineered Fasteners & Flanges
2. Engineering Purpose of ISO 7991 Screws
ISO 7991 screws are selected when:
- Protruding bolt heads are unacceptable
- Aerodynamics or smooth surfaces are required
- Parts must slide or rotate over the fastener area
- High preload is still necessary
They are common in precision-machined components.
3. Head Geometry & Countersink Angle
- Head type: Flat countersunk
- Drive: Internal hex
- Standard countersink angle: 90°
The 90° angle ensures:
- Even load distribution
- Flush seating
- Reduced risk of surface deformation
4. Dimensional Range
Typical ISO 7991 sizes:
- Thread sizes: M2 to M24
- Lengths: 6 mm to 200 mm
- Metric coarse threads as standard
Countersunk depth must be machined accurately to maintain strength.
5. Thread System & Tolerances
- ISO metric threads
- External thread tolerance: 6g
- Compatible with ISO 965 internal threads
Proper thread engagement is critical due to reduced head height.
Super Industries
Global Manufacturer of Heavy & Custom Engineered Fasteners & Flanges
6. Strength Grades & Mechanical Properties
ISO 7991 screws are commonly produced in:
| Property Class | Tensile Strength |
|---|---|
| 8.8 | 800 MPa |
| 10.9 | 1000 MPa |
| 12.9 | 1200 MPa |
Super Industries India specializes in 10.9 & 12.9 grades for industrial and export applications.
7. Material Options
- Alloy steel (most common)
- Carbon steel
- Stainless steel (A2-70, A4-80)
- Special alloys on request
Material selection affects corrosion resistance and preload capability.
8. Heat Treatment Considerations
High-strength ISO 7991 screws undergo:
- Quenching & tempering
- Controlled hardness testing
- Microstructural verification
Typical hardness (12.9): 44–48 HRC
9. Surface Finishes
Available finishes include:
- Black oxide (standard)
- Zinc plated
- Phosphate coated
- Stainless steel natural finish
- Custom coatings for export compliance
Countersunk screws are rarely hot-dip galvanized due to tolerance concerns.
10. Load & Stress Behavior
Because the head is countersunk:
- Tensile load capacity is slightly reduced compared to cylindrical heads
- Proper seating is essential to avoid stress concentration
- Best suited for tension-dominant joints, not heavy shear
11. ISO 7991 vs ISO 4762 (Engineering Choice)
| Feature | ISO 7991 | ISO 4762 |
|---|---|---|
| Head | Countersunk | Cylindrical |
| Surface | Flush | Protruding |
| Strength | Slightly lower | Higher |
| Appearance | Clean & smooth | Industrial |
12. Installation Guidelines
✔ Machine accurate countersink
✔ Use correct Allen key size
✔ Avoid over-tightening
✔ Ensure full head seating
Improper countersinking can lead to premature failure.
13. Washer Usage
- Washers are not used
- Load is transferred through the countersunk head
- Surface material must be strong enough to support seating
14. Common Engineering Applications
- Machine tool components
- Automotive assemblies
- Robotics & automation
- Aerospace subassemblies
- Sliding plates & covers
- Precision enclosures
15. Advantages of ISO 7991
✔ Flush finish
✔ Internal drive for high torque
✔ Clean aesthetics
✔ Suitable for moving interfaces
16. Limitations to Consider
❌ Not ideal for soft materials
❌ Reduced head strength compared to socket head cap screws
❌ Requires precise machining
17. Export Supply by Super Industries India
We offer:
- Batch-controlled production
- Grade & heat traceability
- Export-standard packaging
- OEM and project supply support
18. Custom Manufacturing Capabilities
- Special lengths
- Fine-thread variants
- Special coatings
- Stainless & alloy variants
19. Industries Served
- Automotive & EV
- Industrial machinery
- Defense & aerospace components
- Precision engineering OEMs
20. Conclusion
ISO 7991 hexagon socket countersunk head cap screws are the ideal solution when flush installation, precision, and strength are all required.
Super Industries India manufactures ISO 7991 screws to meet international ISO standards and global export expectations.