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Flush Valve, Syphon and Inlet Valve Thread Diameter Size Guide

In the UK there are four common thread diameters for flush valves, lever flush syphons, and inlet float valves; so it can be a little confusing choosing the correct size for your toilet cistern.

The best way to know the diameter of your thread is by physically taking a measurement of it. We'd always recommend doing this if you're not 100% sure.

The two main thread diameters of flush valves and lever flush syphons is 1 1/2" and 2". Lever flush syphons are almost always 1 1/2", flush valves can be a little more tricky.

Most modern toilet cisterns (especially those from European manufacturers) tend to have 2" outlets on the bottom, older cisterns tend to have 1 1/2" outlets, this is not always the case though, so it's important to measure the outlet before purchasing a replacement flush valve.

The one exception where this isn't as important is if you're just replacing the top section of the flush valve. Some flush valves include both a 1 1/2" and a 2" cradle (also sometimes called a "base" or "strainer"), typically the valve seat is the same size on both of these. In these cases the top section of the valve should be compatible with either cradle providing it is the same model.

Flush pipes are also manufactured to suit both of these sizes. Standard flush pipes are equal at both ends (1 1/2" to 1 1/2"), the European / modern size flush pipes are usually different sizes at both ends; 2" at the flush valve end which then reduces down to 1 1/2" at the toilet pan end.

When it comes to inlet valves the two main thread diameters is 1/2" and 3/8". 1/2" is the British standard thread size, so most inlet valves tend to be 1/2". 3/8" is almost always reserved for cisterns and inlet valves from European manufacturers, such as Geberit, Villeroy & Boch, Grohe, and Roca. As with the flush valves the sizes do vary, so it's important to check first.

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