1-1/4 inch BSP alloy backnut.
This is the normal size for a bathroom wash hand basin waste.
BSP (British Standard Pipe Thread) sizes don’t match up with what you would measure using a ruler. Have a look at the BSPT link at the bottom of this page for more information about how plumbing threads are measured.
You can save money on this item when you purchase several at a time. The table below shows the minimum quantities required to benefit from this offer, and how much they'll cost.
||Price per unit