BS 21:1985

Foreword
Committees responsible
Specification
1 Scope
2 Definitions
3 Symbols
4 Basic forms of pipe threads
5 Jointing threads
6 Longscrew threads
Appendices
A Recommended gauging systems for jointing threads
B Methods of verification of jointing thread dimensions
    and form using recommended gauging systems described
    in appendix A
C Special parallel external threads for gas appliances
    where pressure-tight seals are made on machined faces
Tables
1 Lengths for dimension A in figure 4
2 Basic dimensions and limits of size
3 Dimensions of taper full-form screw plug and ring
    gauges for system A
4 Dimensions of taper full-form screw and taper plain
    plug and ring gauges for system B
5 Manufacturing tolerances for gauges for systems A and
    B
6 Special parallel external threads for gas appliances
    where pressure-tight seals are made on machined faces
Figures
1 Terms relating to pipe threads
2 Basic Whitworth form
3 Basic form of taper pipe thread
4 Typical designs illustrating internal taper or
    parallel pipe threads complying with 5.1
5 System A screw gauges assembled respectively with
    threads of maximum and minimum sizes
6 System B screw gauges assembled respectively with
    threads of maximum and minimum sizes
7 Taper full-form screw plug gauge (system A)
8 Taper full-form screw ring gauge (system A)
9 Taper full-form screw plug gauge (system B)
10 Taper full-form screw ring gauge (system B)
11 Taper plain plug gauge (system B)
12 Taper plain ring gauge (system B)

Range of threads from $9A to 6, together with thread forms, dimensions and tolerances, designations; includes appendices for recommended gauging systems and methods of use of gauges. Requirements for jointing threads for taper external threads, for assembly with either tapes or parallel internal threads and for longscrew threads for longscrews specified in BS 1387.