Guide to British Standard Pipe Threads

This thread type has been adopted around the world as a standard for mating external and internal threads, and is common outside the U.S.

The two types of BSP threads are BSPT (British Standard Pipe Taper) and BSPP (British Standard Pipe Parallel). BSPT threads have a diameter that increases or decreases along the length of the thread. On the other hand, BSPP threads have a constant diameter.

These two types of threads can be used to achieve two types of joints:

Joining threads joints: A taper male thread is used, in conjunction with either a taper or parallel female thread. The combination creates a pressure-tight joint.

Longscrew thread joints: In this case, parallel pipe threads are used, and the tightness of the joint is achieved by some sort of sealing ring (washer or O-ring) between the end face of the male thread and a socket or nipple face, incorporating the tightening of a backnut.

BSP threads use the Whitworth threadform. That means:

  • a symmetrical V-thread in which the angle between the flanks is 55° (measured in an axial plane)
  • truncation at top and bottom of one-sixth of this sharp V
  • threads are rounded equally at crests and roots by circular arcs ending tangentially with the flanks
  • the theoretical depth of the thread is therefore 0.64 times the nominal pitc

Pictured to the right is our GN 743.6 fluid level sight glass that is available in BSPP thread versions. You’ll find tables with all the specs for BSP thread on our Web site in the Technical Section here: British Standard Pipe Threads. If you have any other questions, feel free to contact our Technical Sales Associates at 800-877-8351.