Co-operating organizations

The Mechanical Engineering Industry Standards Committee, under whose supervision this British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations:

The Government departments and scientific and industrial organizations marked with an asterisk in the above list, together with the following were directly represented on the Committee entrusted with the preparation of this standard:

This British Standard, having been approved by the Mechanical Engineering Industry Standards Committee and endorsed by the Chairman of the Engineering Divisional Council, was published under the authority of the General Council on 17 September 1963

First published September 1951

First revision September 1963

The following BSI references relate to the work on this standard:

Committee reference MEE/60

Draft for comment AB(MEE) 4808

Foreword

This standard makes reference to the following British Standards:

BS 18, Methods for tensile testing of metals.

BS 131, Methods for notched bar tests — Part 1: The Izod impact test on metals.

BS 240, Methods of Brinell hardness test — Part 1: Testing of metals.

BS 860, Table of approximate comparison of hardness scales.

BS 919, Screw gauge limits and tolerances — Part 1: Gauges for screw threads of Unified form.

BS 969, Plain limit gauges: Limits and tolerances.

BS 970, Specification for wrought steels for mechanical and allied engineering purposes — Part 1: General inspection and testing procedures and specific requirement for carbon, carbon manganese, alloy and stainless steels.

BS 1574, Split cotter pins.

BS 1580, Unified screw threads — Parts 1 and 2: Diameters ¼ in and larger.

BS 3111, Specification for steel wire for cold forged fasteners and similar components — Part 1: Carbon and low alloy steel.

When BS 1768 was first issued in 1951, its publication marked an important stage in the development of the Unified screw thread system established in accordance with the agreement reached by the United Kingdom with the United States and Canada at the Ottawa Conference of 1945 and ratified by the Declaration of Accord signed by representatives of those three countries in November, 1948.

It was recognized at that time that effective adoption of the system depended very largely on the provision of commonly used types and sizes of fasteners, the general features of which would be unified as well as the threads. As a first step, two standards for hexagon bolts, screws and nuts were published: BS 1768 for the Unified precision normal series, and BS 1769 for the Unified black heavy series.

BS 1768 was restricted at that time to sizes up to and including 1 inch but the many requests received for an extension of the standard have long indicated that this range was inadequate for the needs of industry and this edition accordingly provides for sizes up to and including 2 inches.

It should be mentioned that this extension of the Unified range is also in accordance with agreement reached with the U.S.A. and Canada at a further ABC Conference held in Ottawa in June, 1960, and that an ISO Recommendation now in course of preparation for hexagon bolts, screws and nuts will include the range of Unified normal hexagons for nominal sizes up to 1½ inches.

In revising the standard, the opportunity has been taken to deal more fully with material and manufacture. In addition to specifying materials for cold forged bolts, screws and nuts and offering guidance in the selection of materials for those produced by other methods, the standard now includes full details of the tests to be employed for determining the mechanical properties of the fasteners. Reference is also made to the control of decarburization and rolling laps.

Special attention is drawn to an important feature of this revision, namely the replacement of Grade R bolts which had a tensile range of 45 to 55 tonf/in² with Grade S bolts having a minimum tensile strength of 50 tonf/in² which is more nearly in line with both American and continental practice.

It is recognized that a transition period will necessarily ensue during which stock s of both grades will be held by manufacturers and either may be offered when bolts in this tensile range are ordered.

Washers have been excluded from this standard as they are now covered by BS 3410, “Metal washers for general engineering purposes.”

A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunity from legal obligations.

Summary of pages

This document comprises a front cover, an inside front cover, pages i to iv, pages 1 to 34, an inside back cover and a back cover.

This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.

1 Scope

This British Standard covers the Normal series of Unified precision hexagon bolts, screws and nuts having Unified screw threads in a range of nominal sizes from ¼ in to 2 in inclusive. The dimensional requirements of this standard apply both to ferrous and non-ferrous bolts, screws and nuts. Mechanical properties are given only in respect of steel bolts, screws and nuts, and the finishes in which they are normally supplied are described in Clause 3.

NOTE It is considered that the range of nominal sizes included in this standard is adequate for most of the applications for which this series is likely to be employed, but for the convenience of users requiring larger sizes, the formulae from which the proportions for bolt heads and nuts are derived are given in Appendix F.

2 Material and manufacture

3 Finish

4 Mechanical properties of steel bolts and screws

5 Tensile strength and proof stress of bolts and screws

6 Grading and mechanical properties of steel nuts

7 Proof load for nuts

8 Dimensions

9 Length of bolts and screws

10 Ends of bolts and screws

11 Screw threads

12 Length of thread

13 Nuts, squareness of thread to face

14 Chamfering, washer facing and countersinking

15 Split-pin holes an slots

16 Marking

17 Inspection and testing

BS 1768:1963

Specification for Unified precision hexagon bolts, screws, & nuts (UNC & UNF threads). Normal series

Current

Published:

17 Sep 1963