 Whereas a conventional side electrode will (admittedly, rarely) come adrift in use and potentially cause engine damage, this is impossible with a surface discharge plug, as there is nothing to break off.Surface-discharge spark plugs have been produced by inter alia, Denso, NGK, Champion and Bosch.Most spark plugs seal to the cylinder head with a single-use hollow or folded metal washer which is crushed slightly between the flat surface of the head and that of the plug, just above the threads.