Unless you know the tap manufacturer, it gets a bit painful, as you have to measure the cartridge, the thread type, and check the spline count.
How do I know which size to order?
We sell the widest range of quarter turn and Half turn ceramic tap valves available anywhere on the web and can usually provide a replacement to match your leaking innard. We even have an extensive range of long stem valves for showers (shower flow cartridges). Our first question is “do you know the manufacturer and model of your tap” if “Yes” then we can often tell you which tap valve insert to purchase. Please Contact us for advice. But if “No” then we will need you to remove the cartridge to check the dimensions.
How do I determine the thread size?
Once you have the ceramic tap cartridge out you need to identify the thread size. In the UK 95% of taps are either BSP 1/2″ (Basin size) or 3/4″ (Bath size). The BSP “British Pipe Standard ” doesn’t refer to any physical dimension, which is very confusing and is why we display all the dimensions in mm. Our 1/2″ valves measure 18mm diameter across the bottom and 20.9mm across the thread, whereas the 3/4″ measure 24mm across the bottom and 26.4mm across the thread. We also have a range of Metric 28mm, 34mm and NPT 1/2″ valves
How Many teeth are there on the spline? (This is the stem of the valve)
The next thing to check is the number of teeth on the spline (this is the stem of the valve). It can be tricky to count the teeth by eye, we find the easiest way to count them is to take a photo looking down the stem of the valve and then to count them from the photograph. You will also need to check the diameter of the spline. Most of our valves are 7.6mm dia which is by far the most common diameter size, and fits onto the most common tap handle hole size of 8mm. However, some rarer size valves do have wider spline diamaters and some tap handle manufacturers utilize a plastic adaptor for the handle to fit onto the spline. You will need to keep any plastic spline adaptor and fit this on to the new tap cartridge. If you have broken yours then you will either need to repair it, or use resin putty to mould yourself a new adaptor piece. You can also use this solution if your valve has a wider than normal spline diamater. Please see here for further instructions on doing this How do I make my own handle adaptor piece.
– http://www.tapmagician.co.uk/TMDefault.aspx?950