How to size your tap hose connector correctly!

The history of the British Standard Pipe (BSP) thread dates back to the 19th century. In 1841, Sir Joseph Whitworth introduced the Whitworth thread, a uniform threading system that addressed compatibility issues caused by different types of screw threads. The Whitworth thread became widely adopted internationally for joining low carbon steel pipes.

The confusion with sizing threads for taps is that the imperial inch fractional notation refers not to the actual measured diameter of the threaded fitting. BSP thread sizes are related to the internal diameter or bore of the pipe. The BSP size refers to the nominal bore of the pipe, and it is important to note that the thread diameter measurement needs to be adjusted by subtracting approximately 1/4" (6mm) to determine the nominal pipe size.

Tap Body Stamped Size

Tap bodies normally have a size stamped onto them like 1/2" or 3/4" these normally relate to the thread size on the inlet side of the tap (the back plate the tap body screws into) NOT the outlet.

 For example, it is very common to have a 1/2" tap that has a 3/4" outlet. The only way to be sure of the outlet size is from the supplier's literature or by physically measuring the outlet.

Outside Tap Outlet Size

The most common standard tap thread for UK garden taps is 3/4 BSP. However, some older taps may have a 1/2, or even 5/8 BSP thread.  Larger taps normally have a 1 BSP outlet, but there is a 3/4BSP inlet tap with a 7/8 BSP outlet called a “farm tap” or “farmers tap” this catches a lot of people out so don’t assume and check your measurements.  The above diagram shows the major thread diameters as stated in the international standard, it will be very difficult to practically measure to that accuracy so rounding up is necessary.

Measuring across the threaded portion of your tap outlet is the easiest way to get the correct fitting size. Be-careful to get within 1mm resolution as some of the sizes are quite close to each.  other. The image to the left shows a measurement of 22.5mm that makes this old tap a 5/8 BSP.

Purchasing the correct hose connector

Measuring across the threaded portion of your tap outlet is the easiest way to get the correct fitting size. Be-careful to get within 1mm resolution as some of the sizes are quite close to each.  other. The image to the left shows a measurement of 22.5mm that makes this old tap a 5/8 BSP.

If purchasing a connector from Robox Design please refer to the descriptions and text to obtain the correct size or contact us if you need any help