What are BSP connections?
British Standard Pipe (BSP) is a family of standard screw thread types. This standard has been adopted in the UK and many other countries for interconnecting plumbing fittings.
An external (male) thread fits into an internal (female) thread.
Many of the radiator valve keys that have been described here are for use with radiator parts that have BSP fittings.
The parts include blanking and bleed plugs, valves and valve tails.
British Standard Pipe diameters range from 1/16 inch to 6 inches, but 1/2 inch is normal for radiator fittings on modern central heating systems. Some systems may have 3/4 inch or 1 inch fittings.
Confusingly, the pipes that take the hot water to and from the radiators are usually measured in millimetres. They are commonly 15mm or less.
This means that, while a radiator valve may be connected to the radiator with a 1/2 inch BSP fitting, its connection to the pipework is usually measured in millimetres.