How to know what reseating tool size

[Clumsybob]

I had a look at earlier questions but I could not see the answer.

My bathroom sink tap takes 3/8" washers. If I want to use a reseating tool where do I measure from?

Most on sale mention 1/2 and 3/4 inches. None mention 3/8 except one site that implied this was an unusual old size. These are bog standard Wickes taps.

Is this the size of the valve, or the hole where the tap body goes?

 

[oldbuffer]

1. If its a basin tap, almost certain to be 1/2". (Same for kitchen sinks)
2. Bath taps are generally 3/4".
3. The sizes (which are 1/2 or 3/4" British Standard Pipe thread) refer to the nominal bore of the pipes (pre-metrication) which supplied the fittings. They bear no obvious relationship to any normally measured sizes.
4. The sizes on the tap reseating tool refer to the threads where the outside part of the tool screws into the body of the tap.
5. They usually come with two or more reseating cutters. Use the 1/2" diameter one, or smaller, so long as it covers the outside diameter of the inner hollow brass piece shown in your .....5002.jpg photo.
6. Go carefully and try and take off an absolute minimum to clean up the seating. If you take off too much the tap could end up as scrap.

 

[Clumsybob]

Good call Oldbuffer. Thank you. It's annoying that you almost have to know the answer to ask a coherent question. Like many people I inherited the plumbing when I bought the house. The bath outlet pipe was a home made fitting of perspex and araldite