Hexagon Head Bolts – Fine Thread Series (DIN 960 / DIN 961)
For critical assemblies subjected to high vibration—such as automotive powertrains, drive shafts, and precision machinery—standard coarse threads often fail to maintain preload due to their steeper helix angle. Sunhyings Fine Thread Hex Bolts are engineered to solve this specific kinetic challenge. With a shallower pitch and a larger minor diameter, these bolts provide a greater tensile stress area (As) and superior self-locking capabilities. They allow for finer torque adjustments and higher clamping forces in hard materials, eliminating the risk of loosening where safety factors are non-negotiable.
- Superior resistance to vibrational loosening.
- Higher tensile strength (vs. coarse).
- Precise torque and preload adjustment.
- ISO 8765 / DIN 960 compliance.
- Grade 10.9 / 12.9 Alloy Steel.
- Reduced risk of self-loosening.
Technical Specifications
Standards
DIN 960 (Partial Thread), DIN 961 (Full Thread), ISO 8765, ISO 8676, ANSI B18.2.1 (UNF)
Material Grades
Carbon Steel: Class 8.8, 10.9 (SCM435), 12.9 (4140 Alloy)
Stainless: A2-70, A4-80 (Usually requires anti-seize)
Tolerance Class
Product Grade A (Precision fit recommended)
Thread Type
Metric Fine (MF), UNF (Unified Fine)
Diameter Range
M8×1.0 – M64×4.0
Surface Finish
Black Oxide, Zinc Plated (Yellow/Clear), Zinc-Nickel, Phosphated
Certifications
ISO 9001:2015, IATF 16949 (for Automotive), EN 10204 3.1
Vibration Resistance (Self-Locking Physics):
The primary engineering justification for specifying Fine Thread is the Helix Angle. Fine threads have a smaller incline compared to coarse threads. Under transverse vibration, this lower angle creates higher friction at the thread interface, significantly reducing the tendency of the bolt to “back off” or loosen. This eliminates the need for additional locking mechanisms (like patches or wiring) in many automotive chassis applications.
Maximized Tensile Strength:
Fine threads utilize a shallower groove depth, which leaves a larger core diameter (minor diameter) of the bolt shank intact. This results in a larger Tensile Stress Area (As). For example, an M12 coarse bolt has a stress area of 84.3 mm², while an M12×1.25 fine bolt increases this to 92.1 mm². This allows engineers to achieve higher clamp loads without increasing the bolt diameter.
Precision Adjustment:
In applications requiring exact alignment or calibration (such as adjusting valves or sensor positioning), the smaller pitch allows for finer axial travel per rotation. This provides granular control over tensioning that coarse threads cannot achieve.
Common Metric Fine Pitches (DIN 961 / ISO 8676)
| Thread Size (d×P) | Pitch (P) | Head Width (s) | Head Height (k) | Tensile Stress Area (As) |
| M8 × 1.0 | 1.00 mm | 13.00 mm | 5.30 mm | 39.2 $mm^2$ |
| M10 × 1.0 | 1.00 mm | 17.00 mm | 6.40 mm | 64.5 $mm^2$ |
| M10 × 1.25 | 1.25 mm | 17.00 mm | 6.40 mm | 61.2 $mm^2$ |
| M12 × 1.25 | 1.25 mm | 19.00 mm | 7.50 mm | 92.1 $mm^2$ |
| M12 × 1.5 | 1.50 mm | 19.00 mm | 7.50 mm | 88.1 $mm^2$ |
| M16 × 1.5 | 1.50 mm | 24.00 mm | 10.00 mm | 167.0 $mm^2$ |
Avoid Cross-Threading: Fine threads are less tolerant of misalignment than coarse threads. Ensure the bolt is orthogonal to the mating surface before engaging power tools. We recommend starting the first 3 threads by hand to prevent thread deformation.
Cleanliness is Critical: Due to the tight thread clearances, fine threads are highly susceptible to fouling from paint, rust, or plating debris. Ensure tapped holes are cleaned with a thread chaser if necessary. Contamination can artificially spike torque readings without achieving the target preload.
Torque Spec Sensitivity: Because fine threads produce higher preload for the same input torque (due to the shallow helix), strictly adhere to specified torque values. Using a coarse-thread torque chart on a fine-thread bolt can lead to plastic deformation or yield failure of the fastener.
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FAQ
Why use fine thread bolts instead of coarse thread?
Fine thread bolts are used because they are stronger in tension (due to a larger stress area) and resist loosening from vibration better than coarse threads. They are standard in automotive and aerospace applications where high strength and precision are required.
How do I measure the pitch of a fine thread bolt?
Use a thread pitch gauge to measure the distance between thread peaks. For metric fine bolts, this is measured in millimeters (e.g., 1.25mm). For Imperial (UNF), count the number of threads per inch (TPI). Visual inspection is unreliable due to the subtle differences.
Are fine thread bolts easier to strip?
Yes, fine threads are more prone to stripping, especially when assembling into soft metals like aluminum or cast iron, or if debris is present. They require careful alignment and are best suited for steel-to-steel connections.
What is the standard for Metric Fine Hex Bolts?
The primary standards are DIN 960 (partially threaded) and DIN 961 (fully threaded). The ISO equivalents are ISO 8765 and ISO 8676, respectively.
Can I use a fine thread bolt in a coarse thread nut?
No, fine and coarse threads are not compatible. Attempting to force them together will destroy the threads immediately (cross-threading) and result in a failed connection. Always verify the pitch (e.g., x1.25 vs x1.75) before assembly.