There have been a number of articals in ME about making spark plugs, One used machinable ceramic, another, might be similar to Grey's teflon one, araldite I think was mentioned. An interesting one consisted of a metal body with a bit of glass tube melted into it, the tube came from inside an incandesant light bulb, the exposed insulator on top was a little ceramic insulator that you sometimes find on heating element lead in wires. Ian S C