British Standard Pipe is the standard thread used on plumbing fittings across most of the world (but not in North America).
The nominal size refers not to the outside of the pipe, nor even to the outside of the thread, but to its internal bore - the inside of the tube where the fluid flows. It makes sense when sizing pipes for flow rate but it makes things confusing when measuring for replacement parts.
Below is a summary of the common BSP thread sizes in used with cast iron radiators and their corresponding actual outside diameters in millimetres. The actual outside diameter is what you can measure when looking at a valve tail, a threaded nipple, reducing bush or an opening in the radiator.
pipe size (BSP)
|Threads per inch||Average thread
G is the international standard shorthand for BSPP - BSP Parallel threads. R is the standard for BSPT - BSP Tapered. Parallel (G) threads are used where the seal is made on a mating face, like inside a cast iron radiator with the aid of a gasket. Tapered fittings achieve a tight thread-to-thread seal with the help of a little sealant such as PTFE tape or hemp and Boss White.
We've made a handy PDF that you can print out at home to use as a gauge when measuring threads. Download our BSP Thread Size Template, print it onto A4 paper at 100% scale and follow the instructions. It includes circles that are sized for various BSP threads as well as a handy cut-off tape measure that wraps around the pipe if the ends aren't accessible.
NPT has flattened peaks and troughs where BSP is rounded. NPT has a thread angle of 60º where BSP is 55º.
At certain sizes, NPT and BSP share the same number of threads per inch - at ½" and ¾" - but at all other sizes the two are quite different. The table below lists the TPI for both NPT and BSP at sizes commonly used with domestic radiators.
|Threads per inch||Threads per inch|
In brief: products for sale in the UK and Europe ship with BSP threads; products that we export to North America are fitted with NPT threads.
Our radiators are delivered with factory-installed ½" BSP bushes as standard.
Our valves use self-sealing BSP tails and 15mm compression fittings.