Abstract: A ceramic insulator structure for use in a spark plug is manufactured by mixing granular or powdery particles of AlN, Si3N.sub.4 or Al.sub.2 O.sub.3 with an organic binder and an auxiliary sintering agent, the average spherical diameter of the particles making up the resulting ceramic mixture being in the range 20-100 microns. The ceramic mixture is then pressed in a mold into a desired shape and sintered at a temperature of 1600.degree.-2000.degree. C.

Abstract: A spark plug structure comprising; a cylindrical metallic shell; a joint type insulator having a center bore, and including a front half piece and a rear half piece, each made of a tubular aluminum nitride (AlN), and the front and rear half pieces being joined at their respective end by means of a glass sealant, and encased into the metallic shell; a center electrode placed into the center bore of the insulator; an elongated terminal placed into the rear half piece of the insulator; an electrically conductive glass provided to seal respective spaces appeared between the center electrode, insulator and the terminal; the front half piece having an elongated projection, the length of which is more than 2.0 mm, and the rear half piece having a recess the depth of which is more than 2.0 mm, the front and rear half pieces being jointed at the projection and the recess by means of an annular glass sealant which has thickness of less than 2.0 mm and length of more than 2.0 mm.

Abstract: A spark plug where parts of a center electrode on both sides of an outer electrode are projected toward the outer electrode and a groove is formed in the outer electrode by cutting or grinding the central portion of said end surface. Discharge gaps are formed between on the outer projecting surfaces and the outer electrode.

Abstract: A spark plug for an internal combustion engine with both sides of a surface, on a center electrode side, of an outer electrode and/or both side portions, on the top surface of the center electrode, form wedge-shaped projections. At least one surface having the wedge-shaped projection has no plane surface but is arcuate.

Abstract: A long distance discharge gap type spark plug comprising a center electrode, a porcelain insulator surrounding the center electrode and provided at its lower side with a firing portion made integral therewith and composed of a reduced diameter trunk wall, and a grounded electrode is disclosed. The spark plug comprises a surface gap L formed between the front end of the center electrode and the grounded electrode, a minor air gap l formed between the outer peripheral surface of the firing portion and the grounded electrode and a thickness t of the trunk wall of the firing portion and given by1.0 mm .ltoreq. L .ltoreq. 4.5 mm,0 .ltoreq. l .ltoreq. 0.6 mm and0.5 mm .ltoreq. t .ltoreq. 0.9 mm,Respectively, the surface gap L being determined within a range that satisfies the following conditionsL .ltoreq. 5 t + 2.0 mm when 0.4 mm < l .ltoreq. 0.6 mm,L .ltoreq. 5 t when 0.2 mm < l .ltoreq. 0.4 mm,L .ltoreq. 5 t - 1.5 mm when 0 < l .ltoreq. 0.2 mm, andL .ltoreq. 5 t - 2.5 mm when l = 0.