Spark plug with a stepped insulator and an inner constriction in the housing

A spark plug comprising a central axial electrode, an insulator which covers said axial electrode except for its tip, and a housing which surrounds said insulator with an open tubular gap left therebetween and which at its tip portion furnishes at least one ground electrode which opposes the tip of said axial electrode, said housing being formed with a constriction on its inner surface which approaches said insulator at a portion near its tip portion with an open annular gap being left therebetween, wherein said insulator is formed with a first portion nearer to its root, a second portion nearer to its tip which is of generally smaller radius than said first portion, and a stepped portion between said first and second portions, and wherein the constriction has an annular inside surface which opposes both said second portion and said stepped portion, said constriction inside surface and the surfaces of said insulator at said second portion and said stepped portion being shaped to define an open annular gap therebetween which has a substantially constant width throughout its length, said open annular gap angling sharply radially outwardly from its portion which follows said second portion to its portion which follows said stepped portion of the insulator, said first portion of said insulator being formed with an annular bulge which extends in continuity with said stepped portion.