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Hi

I'm looking to replace the existing bathroom basin waste (and plug) with something less grotty.

I've read that all basin wastes are 32mm (1.25") diameters but where is this measurement taken? I've measured the external diameter of the threaded part of the unit and appears to be 40mm / 1.5". Is 40mm not the size for a bath or kitchen sink waste?

Should I be measuring the internal diameter of the pipe to get the standard basin waste size of 32mm?

Below is an image of the gear as it is now. Does it look like a 40mm (green line) waste going into a 32mm (red line) pipe?

If it is a 40mm waste unit going into a 32mm pipe can I just get a standard 32mm basin waste unit and a 32mm bottle trap?

Thanks

Why can't you just buy a new plug for the basin and use some elbow grease to clean the waste outlet? much cheaper.

1. A lot (most) plumbing sizes are related to the internal diameter (in inches) of the appropriate pipe. And generally to threaded steel pipe.
2. Thus you have:
2.1 1/2" BSP (British Standard Pipe) fittings. These were originally designed for 1/2" steel pipe which has a 1/2" inside diameter or bore size. The fittings typically have an outside diameter for a male thread of a fraction under 21 mm.
2.2 3/4" BSP fittings. Designed for 3/4" pipe and with an outside diameter for a male thread of slightly under 26.5 mm
3. Basin waste fittings are designed for 1 1/4" (1.25") pipe, and have a an outside diameter of just under 42 mm. Which matches reasonably closely with your green line figure.
4. There is no need to worry about the sizes when you are talking about fittings. Basins are 1 1/4", baths 1 1/2". 32 mm and 40 mm respectively with respect to pipes.
5. You do need to be a bit careful with the pipes themselves. Those designed for solvent weld jointing are a slightly different size for those designed for compression fittings. Try and stick to a single manufacturer.