| 摘要 |
Deformation of a seat portion and a protrusion is reduced in a caulking process so as to ensure good air tightness. A spark plug (1) includes an insulating insulator (2) and a metal shell (3). The metal shell (3) includes a caulking portion (19), a seat portion (16) with a tapering surface (31), a thread portion (15) with a thread size equal to or less than M12, and a protrusion (20). The insulating insulator (2) includes a lock portion (14) locked to the protrusion (20), and is secured to the metal shell (3) in a state held between the caulking portion (19) and the protrusion (20). SB/SC ‰¥ 3.5, SB/LB ‰¤ 12.0, and SC/LC ‰¤ 12.0 are satisfied in a case where: the tapering surface (31) has an area of SB (mm 2 ); a seat-portion outer peripheral surface (32) extending from a rear end of the tapering surface (31) in the seat portion (16) has a length of LB (mm); a locked surface (33) locking the lock portion (14) in the protrusion (20) has an area of SC (mm 2 ); and a protrusion inner peripheral surface (34) extending from a tip end of the locked surface (33) toward the tip end side in the protrusion (20) has a length of LC (mm). |
