BSP Thread Size Guides - BSP sizes in mm | Finding the right thread size
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BSP Thread Size Guides

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Use our tables and free downloads to ensure you're ordering the correct parts when replacing bushings and parts on old radiators.

British Standard Pipe: The Plumbing Standard

British Standard Pipe is the standard thread used on plumbing fittings across most of the world (but not in North America). Below, you'll find tables showing BSP sizes in mm; tables detailing the difference between BSP and the American standard NPT; an explanation of straight and tapered thread types; and a handy thread size ruler and template to print out and use at home.

- Download our BSP Thread Size Template here -

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.

BSP Sizes In Millimetres

Nominal pipe size (BSP) Threads per inch Average thread diameter, mm
18" 29 9.1
14" 19 12.3
38" 19 15.8
12" 14 19.8
34" 14 25.3
1" 11 31.8
1-14" 11 40.4
1-12 11 52.3
2" 11 58.1
2-12" 11 73.7

- Looking for replacement radiator parts? You'll find them here. -

G & R Thread Types

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.

Right first time

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.

Diagram: Adding a plug to a cast iron radiator

- Download our BSP Thread Size Template here -

The difference between BSP and NPT

NPT - National Pipe Thread - is the US standard. Most of the rest of the world uses BSP - British Standard Pipe.

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.

Nominal pipe size NPT BSP
Threads per inch Threads per inch
18" 27 28
14" 18 19
38" 18 19
12" 14 14
34" 14 14
1" 11-12 11
1-14" 11-12 11
1-12 11-12 11
2" 11-12 11
2-12" 8 11

The threads we use in our radiators

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.

View our complete range of replacement parts for DIY radiator refurbishment.

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