British Standard Pipe Threads
BSP threads are fairly uncommon in the workshop but widely used in the home where most U.K. and European plumbing fittings use these threads. The threads are sized not on diameter but on the nominal bore of the pipe to which they are applied. Just to be really odd a ½" pipe doesn′t necessarily have a ½" bore because wall thickness varies with the pressure rating of the pipe. The table below is for tapered threads which are most often used when fitting say a valve with a female thread to a length of steel pipe. Parallel threads are normally found on fittings like ballcocks or taps.
British Standard Pipe Threads (BSP) are used throughout the EU but may no longer be referred to as BSP. The current standard for these threads is BS 21 (taper threads) although this has been generally replaced by BS EN 10226-1. and BS EN ISO 0228-1. BSP-R designates a taper thread and BSP-G designates a parallel thread. The thread is a standard Whitworth form with 55° angle with rounded crests and roots.
Mostly applicable to taper thread assemblies:-
- Gauge Diameter - Major diameter of Thread (Int and Ext) at gauge plane.
- Major Cone - Imaginary cone on taper thread which touches the crest of all external threads or the roots of all internal threads.
- Gauge plane - Plane on taper threads, which is perpendicular to the thread axis, at which the major cone has gauge diameter.
- Gauge length - On external threads the distance from the gauge plane to the small end of the thread.
- Complete thread - The thread which is full formed at both crest and root.
- Washout thread - Thread which is not full formed at the root.
- Useful thread - All thread excluding washout thread.
- Assembly Length - (Fitting Allowance) Useful thread beyond the gauge plane on an external thread required to provide an assembly with an internal thread at the upper limit of tolerance. It includes the wrenching allowance.
- Wrenching Length - (Wrenching Allowance) Useful thread length provided to accommodate the relative movement when wrenching from the hand tight position to the fully tightened position.
- Accommodation length - Distance from the face of the internal thread to the first obstruction which the externally threaded component will encounter on tightening