Ceramic 7 Volt Glow Plug CZ167 / 91063 by NGKAs opposed to a metal rod glow plug, the heating coil of a ceramic glow plug has an especially high melting point. It is also sheathed in silicon nitrite, an extremely rugged ceramic material. The combination of the heating coil & ceramic sheath enable higher temperatures & extremely short preheating times due to excellent thermal conductivity. Also, ceramic glow plugs have a more compact design. This is important, because there is very little free space in todays engines.