Unless you are meaning the fill plug.
That is 1/2" BSP. Plugs are cheap and available from any supplier for a few dollars. ERR4686B.
https://www.roversnorth.com/ProductDesc.aspx?code=PLC997
Solid one: https://www.ebay.com/itm/NEW-BRASS-...-RRC-RADIATOR-FILL-PLUG-ERR4686-/252928555327
http://new.lrcat.com/#!/1228/13789/16869/1322/17233
Is this for a Madman sensor?