Background to Garden Tap Threads

Screwing a tap into a fitting or screwing a hose connector onto a tap is a simple affair. But these screw sections (known as threads) have a considerable amount of engineering within them. Threads come in different sizes and forms, but the majority of UK plumbing systems are based on the BSP system

The BSP (British Standard Pipe) threads are based on the Whitworth thread style, which was developed by Sir Joseph Whitworth in the 19th century. Whitworth threads have a 55° angle and are designed for high precision and durability, making them ideal for industrial applications, especially in pipe fitting.

Over the years BSP thread forms have been optimised because of a broader effort to standardise pipe fittings and threads in the UK and internationally.

BSP threads are commonly used for plumbing and pipe fittings in the UK and many other countries. There are two main types:

1. BSPT (Tapered Threads): These threads get narrower along their length, which helps create a tight seal when pipes are screwed together. They are often used in systems that need to be leak-proof, like gas or high-pressure pipes.
2. BSPP (Parallel Threads): These threads are straight and don’t naturally form a seal on their own. Instead, seals like O-rings or gaskets are used to prevent leaks. BSPP threads are usually used in systems that don’t need to be as tightly sealed, like water pipes.

The simple garden tap is based on the parallel system and usually requires a rubber washer to seal the connector or PTFE tape/sealant to seal the gaps on the inlet threads.

Old Garden Taps

The vast majority of modern garden taps are based on the 3/4BSP thread size. Plastic, brass and stainless steel connectors are widely available for this tap size.

However, on the older taps (there are a significant number still in use) homeowners struggle to purchase the correct connector size. 

Confusion with BSP sizes

The problem with the BSP designation is it refers to the bore of the pipe, not the external measured diameter of the fitting. So 1”BSP does not measure one inch - its about a ¼” bigger.

With older taps lots of people will buy a 3/4BSP (too big) and 1/2BSP (too small) fitting expecting one to fit. 

If you measure across the thread of a 1/2BSP thread it will be around 21mm or 0.825in. If you measure the diameter across the 3/4BSP thread it will measure around 26mm or 1.041.  With an older tap it is likely you will need a fitting in between those two sizes – say hello to 5/8BSP.

5/8BSP is around the 23mm or 0.902in in diameter.  This seems to have been the size of choice for early brass founders when they made garden taps.

Connecting a hose to your old garden tap

RoBox offer a 5/8BSP brass connector that is built to last. If you have an old garden tap and want to connect a hose fitting to it, check out our 5/8BSP connector here. This product listing has all the product information and photographs to help you make an informed decision. Please also refer to our help video if you need any further information.