'Standard' Kettle element - 38mm or 40mm . . . . I have a 40mm and all the kettle elements I have measured are 38mm (DOH!) And standard 1/2" BSP nipple diameter is 20mm . . . more or less . . . 19mm if you want to thread it . . . If you punch 20mm you will need to tickle it out a fraction with a file so its a snug fit.