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IS 6639:1972 is the Indian Standard (BIS) for high strength structural bolts with large width across flats. This standard specifies the requirements for high-strength structural bolts with a large width across flats, in sizes from M16 to M36. It covers the dimensions, material, mechanical properties, and marking for the bolts (property class 8.8) and their associated nuts and hardened washers. These fasteners are intended for use in high-stress friction-grip or bearing-type connections in steel structures.
Specifies requirements for high strength structural bolts with large width across flats in diameters from 16 mm to 36 mm.
Overview
Status
Current
Usage level
Frequently Used
Domain
Structural Engineering — Fasteners, Bolts and Anchors
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! The 'large width across flats' provides a larger bearing area under the head, but requires larger installation clearances and tools compared to standard structural bolts (IS 3757).
! Ensure these bolts are used as a complete assembly with corresponding high-strength nuts (e.g., IS 6623) and hardened washers (e.g., IS 6610) to achieve the specified performance.
! This standard has been reaffirmed multiple times (e.g., 2003, 2021), indicating its continued relevance, although designers should check for the latest practices in steel connection design.
Consolidated list per BIS. For the text of each amendment, refer to the BIS portal link above.
steelhigh strength steelfasteners
International Equivalents
Similar International Standards
ISO 14399-3:2015ISO (International Organization for Standardization), International
HighCurrent
High-strength structural bolting assemblies for preloading — Part 3: System HR — Hexagon bolt and nut assemblies
Specifies modern requirements for high-strength structural hexagon bolt/nut assemblies (System HR), the direct successor concept.
ASTM F3125 / F3125M - 22ASTM International, USA
HighCurrent
Standard Specification for High Strength Structural Bolts and Assemblies, Steel and Alloy Steel, Heat Treated, Inch and Metric Dimensions
Consolidated US standard for high-strength structural bolts, covering grades equivalent to 8.8 (A325) and 10.9 (A490).
EN 14399-3:2015CEN (European Committee for Standardization), Europe
HighCurrent
High-strength structural bolting assemblies for preloading - Part 3: System HR - Hexagon bolt and nut assemblies
The European adoption of ISO 14399-3, defining the harmonized requirements for structural bolts across Europe.
BS 4395-1:1969BSI (British Standards Institution), UK
HighWithdrawn
Specification for high strength friction grip bolts and associated nuts and washers for structural engineering. General grade.
A historical contemporary to IS 6639, it also specified 'large series' heads for general grade (similar to 8.8) bolts.
Key Differences
≠IS 6639:1972 is a withdrawn standard, while modern equivalents like ISO 14399 and ASTM F3125 are current and regularly updated.
≠IS 6639 specified a 'large width across flats' (e.g., 34 mm for M20) which is larger than the modern ISO 14399-3 standard (32 mm for M20).
≠Modern standards like ISO 14399 mandate that the bolt, nut, and washer(s) are supplied as a matched 'assembly' from a single manufacturer and tested as a system. IS 6639 focused on the properties of individual components.
≠Current international standards include more comprehensive testing requirements for suitability in preloaded connections, such as the 'k-factor' test (torque coefficient), which were not detailed in IS 6639.
Key Similarities
≈All standards are intended for the same application: high-strength bolting in structural steel connections, especially for slip-resistant (friction) joints.
≈The fundamental material requirements are similar, specifying quenched and tempered medium carbon or alloy steels to achieve high tensile strength and ductility.
≈The mechanical properties are based on internationally recognized strength grades, with IS 6639 grade being equivalent to Property Class 8.8 found in the ISO standards.
≈All standards mandate clear head markings to identify the manufacturer and the bolt's strength grade or property class.
Parameter Comparison
Parameter
IS Value
International
Source
Status
Withdrawn
Current
ISO 14399-3:2015
Primary Strength Grade
8.8
8.8 and 10.9 (System HR)
ISO 14399-3:2015
Tensile Strength (Min, Grade 8.8)
80 kgf/mm² (~785 MPa)
800 MPa
ISO 14399-3:2015
Yield Strength (Min, Grade 8.8)
64 kgf/mm² (~628 MPa)
640 MPa
ISO 14399-3:2015
Width Across Flats (M20 Bolt)
34 mm
32 mm
ISO 14399-3:2015
Head Height (M20 Bolt)
13.0 mm
12.5 mm
ISO 14399-3:2015
Supply Concept
Individual components (bolt, nut, washer)
Matched assembly from a single lot and manufacturer
ISO 14399-1:2015
⚠ Verify details from original standards before use
Key Values5
Quick Reference Values
Bolt Property Class8.8
Nut Property Class8
Washer Hardness Range38 to 45 HRC
Covered Bolt SizesM16 to M36
Thread SeriesCoarse pitch series as per IS 4218 (Part 3)
What are the property classes specified for bolts and nuts in IS 6639?+
The bolts are specified as property class 8.8 and the corresponding nuts are of property class 8 (Clause 4.2 and 5.1).
What is the main difference between IS 6639 and IS 3757 bolts?+
IS 6639 specifies bolts with a larger width across flats (wrench size) for the same nominal diameter compared to IS 3757, providing a larger bearing surface under the head.
What are the standard sizes covered by this code?+
The standard covers nominal sizes from M16 to M36 (Clause 1.1).
What kind of washers should be used with these bolts?+
Hardened and tempered circular washers with hardness between 38 and 45 HRC are specified (Clause 6.1).