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Home » Zhejiang Zhongrui Auto Parts Co., Ltd. » Articles
Comparing DIN 6921 and GB/T 16674 Hex Flange Bolts-Dimensions-Substitutability-Applications, current inventory

By Zhejiang Zhongrui Auto Parts Co., Ltd.

05/22/2025

Flanged hexagon bolts are critical fasteners widely used across numerous industries, providing a larger bearing surface under the head compared to standard hexagon bolts, thus eliminating the need for a separate washer in many applications. Two common standards governing these fasteners are the German standard DIN 6921 and the Chinese national standard GB/T 16674. While both define bolts of a similar type, understanding their specific requirements is essential for proper application and sourcing.

Overview of the Standards
  • DIN 6921: Hexagon bolts with flange: This standard specifies the characteristics for hexagon bolts with metric threads and a circular flange under the head. It is a long-established German standard that has seen widespread use internationally, particularly in European engineering contexts.
  • GB/T 16674: Hexagon bolts with flange: This is the Chinese national standard for hexagon bolts with flange. Like many modern GB standards, GB/T 16674 is often harmonized with international standards, aiming for compatibility with global practices, frequently referencing ISO standards (though specific harmonization levels can vary). It is the primary standard for this type of bolt within China.

Dimensional Parameter ComparisonWhile both standards describe the same type of bolt, there can be subtle differences in specified dimensions and tolerances. Key dimensions to consider include:
  1. Thread Size and Pitch: Both standards cover standard metric threads (e.g., M5, M6, M8, M10, etc.) with corresponding pitches. For common sizes, the nominal thread dimensions are generally identical, adhering to basic metric thread standards (like ISO 262).
  2. Across Flats (Wrench Size): The dimension across the hexagon flats, which determines the required wrench size, is typically standardized for given thread sizes and is often the same between the two standards, reflecting international agreements (like potentially ISO 4162 or similar).
  3. Head Height: The overall height of the bolt head (excluding the flange) can be similar between DIN 6921 and GB/T 16674 for corresponding sizes, but variations in tolerances or specific height values for certain ranges might exist.
  4. Flange Diameter: The diameter of the integrated flange is a critical dimension affecting the bearing surface area. While often comparable, there could be slight differences in the specified flange diameters or tolerances between the two standards, particularly for larger bolt sizes.
  5. Flange Thickness and Shape: Differences can sometimes be found in the minimum/maximum flange thickness or the exact profile of the flange edge (e.g., chamfering or radius).
  6. Bolt Length: Both standards cover a range of standard bolt lengths for each thread size. The available standard lengths might vary slightly between the two standards' specifications.
  7. Tolerances: Even if nominal dimensions are the same, tolerances (allowable variations) for dimensions like head height, flange diameter, or thread minor diameter can differ, impacting interchangeability in precision applications.

In summary regarding dimensions: For many common sizes and in non-critical applications, the major dimensions (thread, across flats) are often very similar or identical. However, critical applications require careful verification of all specific dimensions, tolerances, and material specifications as listed in the respective, current standard documents.

Substitutability AssessmentCan a DIN 6921 bolt be substituted for a GB/T 16674 bolt, and vice versa?
  • Potential for Substitutability: Due to the fundamental similarity in design and often close correspondence in key nominal dimensions (especially thread size and across flats), DIN 6921 and GB/T 16674 bolts are often considered substitutable for many general-purpose or non-critical fastening applications. If the bore hole accommodates the thread, the across flats matches the tool, and the flange provides sufficient bearing area for the joint's requirements, substitution might be acceptable.
  • Limitations and Risks:
    • Tolerance Stack-up: Slight differences in tolerances can lead to issues in assemblies with tight clearances or critical fits.
    • Load Distribution: Variations in flange diameter or thickness might affect the distribution of clamping force, which is crucial in high-stress or vibration-prone applications.
    • Material Grades and Strength Classes: While both standards cover various strength classes (e.g., 8.8, 10.9), the specific material compositions, testing requirements, or marking conventions might have subtle differences as defined by their respective governing standards.
    • Application Criticality: For applications where failure could result in significant safety risks, structural compromise, or high costs (e.g., automotive safety systems, critical machinery joints), adhering strictly to the specified standard (DIN 6921 or GB/T 16674) is paramount. Substitution is not recommended without thorough engineering validation and approval.

Conclusion on Substitutability: While frequently possible in less demanding roles due to dimensional similarities, substitution is not a guaranteed one-to-one replacement. Always assess the specific requirements of the application and, for critical uses, consult the official standards or seek expert engineering advice before substituting one for the other.
Typical Application ScenariosBoth DIN 6921 and GB/T 16674 flanged hexagon bolts are versatile fasteners used in a wide array of applications where a secure joint with an integrated bearing surface is needed. Common scenarios include:

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