Patent ID: 6653768
Filing Date: 2003-11-25
Classification: H01T

Abstract:
A spark plug having a center electrode (3), an insulator (2) provided on the outer side of the center electrode (3), a cylindrical main metal member (1) provided on the outer side of the insulator (2), and an earth electrode (4) which is provided so that the earth electrode is combined at one end portion thereof with the main metal member (1) and opposed at the other end portion thereof to a free end of the center electrode (3), and which forms a spark discharge gap g having a width &agr; between the earth electrode and center electrode (3),said spark plug having a front side at which the spark discharge gap g is positioned with respect to an axial direction O of the insulator (2) with the other side being a rear side, characterized in that: the insulator (2), a diameter of a front end portion (2i) of which is reduced by a circumferentially extending stepped portion thereof provided as an insulator-side locking portion (2h), is inserted into the main metal member (1) from a rear opening thereof, the insulator-side locking portion (2h) is engaged with a metal member-side locking portion (1c) projecting from an inner circumferential surface of the main metal member (1) with a clearance-forming outer circumferential surface (2k) of the portion (2i) positioned ahead of the locking portion (2h) of the insulator (2) opposed to a clearance-forming inner circumferential surface (52) so as to form in a locking position a clearance of a predetermined amount therebetween, and an amount &bgr; of a clearance in a locking position is expressed by equation (1): &bgr;=(D1âˆ’d1)/2â€ƒâ€ƒ(1) wherein d1 represents an outer diameter of the clearance-forming outer circumferential surface (2k); and D1 represents an inner diameter of the clearance-forming inner circumferential surface 52, where &bgr; is not higher than 0.4 mm but not less than 0.05 mm.