What is the primary application of black bolts?

They are predominantly used for light structures, secondary structural members, and applications with primarily static loads.

What does property class 4.6 mean for these bolts?

It indicates a nominal ultimate tensile strength of 400 MPa and a yield stress of 240 MPa (60% of ultimate tensile strength).

Can these bolts be used for slip-critical connections?

No, black bolts are bearing-type fasteners. For slip-critical (friction-grip) connections, HSFG bolts (IS 3757) must be used.

IS 2155

: 1982

Black bolts, nuts and lock nuts (diameter 6 to 39 mm)

CurrentFrequently UsedSpecificationBIMStructural Engineering · Fasteners, Bolts and Anchors

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This standard specifies the dimensions, mechanical properties, and material requirements for black bolts, nuts, and lock nuts in the diameter range of 6 mm to 39 mm. It is widely used in the fabrication and erection of light structural steelwork and general engineering applications.

Specifies requirements for black hexagon head bolts, nuts and lock nuts of diameters 6 to 39 mm.

Quick Reference Values

diameter range6 mm to 39 mm

typical bolt property class4.6

typical nut property class4

thread typeISO Metric (Coarse Pitch)

Practical Notes

Black bolts (also known as ordinary or rough bolts) have larger hole clearances and tolerances compared to precision or high-strength bolts.

They are strictly recommended for static loads and secondary connections, and should be avoided in structures subjected to vibrations, impact, or fatigue.

Always ensure that the property class of the nut matches or exceeds the property class of the corresponding bolt to prevent thread stripping.