The blank plug and chain comes as a UK standard 64mm (2.5") and will fit instantaneous fire fittings such as dry riser landing valves, standpipes and hose outlets. This stops dirt and other objects getting into the valve or pipework. In hi-rise buildings, rubbish is common stuffed down dry riser valves and, in a fire situation could be catastrophic.

The four versions are red plastic (most popular), black rubber, light alloy and gunmetal for marine use. If the plug is to retain water pressure, you should use the alloy or gunmetal versions.