Patent ID: 7432641

Claim:
A spark plug comprising an insulator having a bore extending in an axial direction and holding a center electrode for generating spark discharge in a front end portion of the axial bore and a terminal electrically connected to the center electrode, at a rear end portion of the axial bore, wherein the axial bore of the insulator comprises a large diameter portion continuous with an opening located at a rear end of the axial bore and a small diameter portion continuous with a front end of the large diameter portion and smaller in diameter than the large diameter portion, and wherein an axial length A in mm of the insulator, an outside diameter B in mm of a rear trunk portion located rearward of a portion of the insulator having a maximum outside diameter, a diameter C in mm of the large diameter portion, an axial length D in mm of the large diameter portion, and a diameter E in mm of the small diameter portion satisfy the following inequalities: A≧80 E≦3.4 0.01(17.527+0.141 A− 0.285 B− 6.124 C− 0.14 D+ 1.105 E )<0.07.