To provide a spark plug for an internal combustion engine excellent in smoldering resistance and durability.
The spark plug 1 comprises an insulator 2 with a center through-hole 21, a center electrode 3 retained by the center through-hole 21, a metal fitting 4 retaining the insulator 2, a first grounding electrode 51 forming a first gap A between the tip of the center electrode 3, and the first grounding electrode and a second grounding electrode 52 forming a second gap G between the side face of the center electrode 3 the second grounding electrode. With the first gap as A, the shortest distance between the second grounding electrode 52 and the insulator 2 as B, a distance in an axial direction between the corner 522 of the grounding electrode 2 and the tip face of the insulator 2 as C, and the tip thickness of the insulator 2 as T, provided the axial direction distance C is + in the case the corner 522 of the second grounding electrode 52 is further protruded toward a tip end side from the tip face 22 of the insulator 2, following relations are satisfied: 0.4mm≤A≤1.0mm, 0.3mm≤T≤0.8mm, 0.2mm≤B≤0.9mm, -1.0mm≤C≤0.5mm.
OKABE SHINICHI
NIPPON SOKEN
Tamiwa Iwakura
Takahashi Shoki