A spark plug (20) includes an insulator seat angle (αi) of 35° to 50° and an increased insulator thickness (ti) in selected areas around the insulator seat (28). The insulator seat angle (αi) is greater than or equal to a boundary value provided by the equation: 90°−a cos [1−(R1−R2)÷(R4+R5)], and preferably not greater than 150% of the boundary value. The radii (R1, R2, R3, R4, R5) can be adjusted to maximize R4 while maintaining an acceptable R2. A gasket is compressed between the insulator (22) and shell (58), and the inner gasket thickness (tg2) is greater than or equal to 70% of the outer gasket thickness (tg1).