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BS-21 British Standard Pipe Taper

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 a socket.

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 gauge design.

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 10226 counterpart.

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 accurate to the best of my knowledge as of 12/1/2008.
If you find error or deisre to make comment contact Wayne at:gageguy@gagecrib.com

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