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).

IS 6639

: 1972

High strength structural bolts with large width across flats

CurrentFrequently UsedSpecificationBIMStructural Engineering · Fasteners, Bolts and Anchors

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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.

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)

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.