Patent ID: 8878426

Claim:
An ignition plug comprising: a center electrode; a tubular insulator into which the center electrode is inserted; and a tubular metallic shell into which the insulator is inserted, said tubular metallic shell including a crimp portion having a crimped rear end portion, a male screw portion for mounting, a seat portion located on the crimp portion side of the male screw portion and expanding radially, and a bent portion located on the crimp portion side of the seat portion, wherein the bent portion has a largest outer diameter portion which is the largest in outer diameter measured from a center axis of the metallic shell, a first smallest outer diameter portion which is the smallest in the outer diameter in a portion of the bent portion located on the crimp portion side of the largest outer diameter portion, and a second smallest outer diameter portion which is the smallest in the outer diameter in a portion of the bent portion located on the seat portion side of the largest outer diameter portion; and wherein, in the bent portion on a cross section containing the center axis, 0.01≦(L 2 +L 3 −L 1 )/(L 2 +L 3 )≦0.3 is satisfied, where: L 1 is a distance between (i) a point in the first smallest outer diameter portion which is the farthest from the center axis and (ii) a point in the second smallest outer diameter portion which is the farthest from the center axis, L 2 is a distance between (i) the point in the first smallest outer diameter portion which is the farthest from the center axis and (ii) a point in the largest outer diameter portion which is the farthest from the center axis, and L 3 is the distance between the point in the second smallest outer diameter portion which is the farthest from the center axis and the point in the largest outer diameter portion which is the farthest from the center axis.