Patent ID: 6849995

Claim:
A spark plug comprising a rodlike center electrode ( 3 ), a rodlike insulator ( 2 ) surrounding said center electrode ( 3 ) and having a protrusion ( 2 e ) at a central portion thereof, a metallic shell ( 1 ) assuming an open-ended, tubular shape and surrounding said insulator ( 2 ), and a ground electrode ( 4 ), a first end of said ground electrode ( 4 ) being joined to said metallic shell ( 1 ) and a second end of said ground electrode ( 4 ) facing said center electrode ( 3 ) to thereby define a spark discharge gap (g), and characterized in that: an insulator insertion hole ( 40 ) into which said protrusion ( 2 e ) of said insulator ( 2 ) is inserted is formed in said metallic shell ( 1 ) while extending in a direction of an axis (O); when a side toward said spark discharge gap (g) with respect to the direction of said axis (O) is taken as a front aide, a rear end portion of said metallic shell ( 1 ) is hot-crimped toward said insulator ( 2 ) to thereby form a curved, crimped portion ( 1 d ); two protrusions ( 1 e and 1 g ) and a thin-walled portion ( 1 h ) are formed on an outer surface of said metallic shell ( 1 ) such that said thin-walled portion ( 1 h ) is located between said two protrusions ( 1 e and 1 g ), the thin-walled portion ( 1 h ) is thinner than said two protrusions ( 1 e and 1 g ), and assumes a section whose inner and outer surfaces are swollen in a radially convex condition with respect to said axis (O) and such that one of said protrusions ( 1 e and 1 g ) is formed to be located adjacent to and on the front side of said crimped portion ( 1 d ); and an inside diameter of said insulator insertion hole ( 40 ) of said metallic shell ( 1 ) is 8-12 mm as measured at a position ( 1 i ) where an inner wall surface of said insulator insertion hole ( 40 ) transitions to an inner wall surface of said crimped portion ( 1 d ) with respect to the direction of said axis (O) of said metallic shell ( 1 ); and a cross-sectional area S of said metallic shell ( 1 ) as measured when said metallic shell ( 1 ) is cut at said position ( 1 i ) by a plane perpendicular to said axis (O), and a carbon content of a steel material used to form said metallic shell ( 1 ) satisfies either of the following conditions A and B: condition A: 15≦S<5 mm 2 and a carbon content of 0.20%-0.45% by weight; and condition B: 25≦S<35 mm 2 and a carbon content of 0.15%-0.45% by weight.