BS 21: 1985 Pipe Threads for Tubes and Fittings Where Pressure-Tight
Joints are Made on the Threads
This British Standard specifies requirements for the following
Whitworth pipe screw threads. Thread sizes from 1/16 inch to 6 inch
inclusive are covered by this standard and requirements for Whitworth
pipe screw thread forms, Whitworth pipe screw thread dimensions
and Whitworth pipe screw thread tolerances are given, together with
the designation of each type of Whitworth pipe screw thread.
a) Jointing screw threads, which are Whitworth pipe screw threads
for joints made pressure-tight by the mating of the screw threads
and are taper external screw threads, taper internal screw threads
or parallel internal screw threads.
b) Longscrew screw threads, which are parallel pipe screw threads
used for longscrews (connectors) specified in BS 1387 where a pressure-tight
joint is achieved by the compression of a soft material on to the
surface of the external screw thread by tightening a backnut against
BS 21: 1985 was the original Whitworth pipe screw thread standard,
but changes are inevitable.
The first change was that the Whitworth pipe screw thread form
was redefined in ISO 7-1:1994. The
Whitworth pipe screw thread design is technically equivalent between
the two standards. The Whitworth pipe screw thread gages were redefined
in ISO 7-2: 2000. While the Whitworth
pipe screw thread gages measure the same product Whitworth pipe
screw thread, the design ot the Whitworth pipe screw thread gages
has deviated from the original BS 21 Whitworth pipe screw thread
The next change was that the screw thread was again redefined in
EN 10226-1: 2004 and EN
10226-2: 2006.The pipe thread design is now technically equivalent
between four screw thread standards. The screw thread gages were
redefined in EN 10226-3: 2005 to
be technically equivalent to ISO 7-2:
2000, but again the screw thread gages do not 100% match the original
BS 21: 1985.
Recently, under agreement with the Europan Community to harmonize
European Country standards with the European Community standards,
the British Standards Institute established BS EN
10226-1:2004 and are in the process of reviewing BS EN
10226-2 and BS EN 10226-3.
All of these will be identical to their EN
Today, the status of BS 21:1985 is in flux. The intension of the
Europan Community is to completely replace BS 21: 1985. In an effort
to comply, in 2007 the British Standards Institute actually rescinded
BS 21: 1985, but there was an outcry from the users who did not
want to give up the original style BS 21: 1985 gauges. The consensus
of the screw thread gage users and manufacturers is that the ISO
7-2: 2000, and thus the EN 10226-3:
2005, screw thread gauges are inferior in design and function to
the BS 21: 1985 gages. As a result, BS 21: 1985 was partially reinstated.
This data is provided for general information only. The intention is to provide accurate information; regardless; errors may exist in the supplied information. If accuracy is critical, base your final decisions on the data provided in the root document: BS-21; which is a copyrighted document. To purchase copies visit an Authorized Reseller.
Original Posting: 12/1/2008
Last Revision: 1/12/2016
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