BS 21:1985 pdf download.Pipe threads for tubes and fittings where pressure-tight joints are made on the threads (metric dimensions).
2.4 complete thread that part of the thread which is fully formed at both crest and root NOTE When there is a chamfer at the start of the thread not exceeding one pitch in length, it is included in the length of complete thread.
2.5 incomplete thread that part of the thread which is fully formed at the root but truncated at the crest by its intersection with the cylindrical surface of the product
2.6 washout thread that part of the thread which is not fully formed at the root NOTE The washout thread is produced by the bevel at the start of the threading tool.
2.7 vanish cone an imaginary cone the surface of which would pass through the roots of the washout thread
2.8 major cone an imaginary cone which just touches the crests of a taper external thread or the roots of a taper internal thread
2.9 useful thread the complete thread and the incomplete thread, excluding the washout thread
2.10 total thread the complete thread, the incomplete thread and the washout thread
2.11 fitting allowance the length of useful thread beyond the gauge plane of an external thread required to provide for assembly with an internal thread at the upper limit of the tolerance
2.12 wrenching allowance the length of useful thread which is provided to accommodate the relative movement between the pipe end and the internally threaded part required for wrenching beyond the position of hand engagement
4 Basic forms of pipe threads
4.1 Parallel threads The basic form of the parallel internal pipe thread and of the parallel external longscrew thread shall be the basic Whitworth form as follows. The Whitworth thread form (see Figure 2) is that of a symmetrical V-thread in which the angle between the flanks, measured in an axial plane section, is 55°; one-sixth of this sharp V is truncated at the top and the bottom, the threads being rounded equally at crests and roots by circular arcs blending tangentially with the flanks, the theoretical depth of thread being 0.640 327 times the nominal pitch. The basic thread depth, calculated from this, is rounded off to the nearest 0.001 mm.
4.2 Taper threads The basic form of the taper pipe thread shall be as follows (see Figure 3). It is based on the Whitworth thread form and it too has an angle of 55°, the flanks making equal angles with the axis. The crests and roots are rounded off symmetrically in such a manner as to give the same basic differences between major, pitch and minor diameters as in the Whitworth thread of the same nominal pitch. The taper is 1 in 16, measured on the diameter.
BS 21 pdf download.