BS 3692

BS 3692:2001

ISO Metric Precision Hexagon Nuts Specification

This standard specifies mechanical and physical properties of nuts with coarse thread and fine pitch thread made of carbon steel and alloy steel with ISO metric threads in diameters from M1.6 to M68 inclusive.

The dimensional requirements of this standard also apply to non-ferrous and stainless steel nuts.

This standard covers nuts which

  • have threads and diameters in accordance with BS 3643-1
  • have threads made to tolerances of medium class fit 6H/6g as specified in BS 3643-2
  • able to mate with bolts, screws and studs with property classes according to ISO 898-1

This standard does not cover nuts which are used for special applications such as

  • Requiring weldability
  • Corrosion resistance
  • Ability to withstand temperatures below –50°C or above 300°C
  • Nuts with an effective height of less than 0.6D
  • Nuts with across flats or diameters of less than 1.45D

The use of free-cutting steels at temperatures above 250°C is not recommended.

Strength Grade Designation 4 5 6 8 10 12  
Proof load stress N/mm2 400 500 600 800 1000 1200 *
Brinell Hardness (HB) max 302 302 302 302 353 375 All nuts
Rockwell Hardness (HRC) max 30 30 30 30 36 39 All nuts
Vickers Hardness (HB) max 310 310 310 310 370 395 All nuts

* All nuts except those exempted by agreement between purchaser and manufacturer. Nuts with a proof load in excess of 500kN may be exempt from proof load testing. Such nuts shall meet the minimum hardness as agreed between the purchaser and the manufacturer.


Chemical Composition Table


Chemical Composition

% by mass

Property Class









4, 5 and 6 0.50 - 0.110 0.150
8 0.58 0.30 0.060 0.150
10 and 12 0.58 0.45 0.048 0.058

Free cutting steel may be used by agreement between purchaser and manufacturer. In such cases, the following maximum contents are permissable:

P (Phosphorous) max 0.12%

S (Sulphur) max 0.34%

Pb (Lead) max 0.35%

Alloying elements may be added if necessary to develop the mechanical properties of the nuts.