| 摘要 |
To provide a spark plug which can secure favorable gas-tightness and can reliably prevent, for example, breakage of an insulator. A spark plug (1) includes a ceramic insulator (2) having a diameter-decreasing portion (14), and a metallic shell (3) which is provided around the ceramic insulator (2) and which has a protrusion (21) projecting inwardly in a radial direction. The protrusion (21) has a diameter-decreasing portion (21A), and the diameter-decreasing portion (21A) seats on the engagement portion (14) via an annular plate packing (22). In or on a longitudinal cross section including an axial line CL1, a relation of ¸s > ¸p is satisfied, wherein ¸p represents an acute angle (°) between a straight line orthogonal to the axial line CL1 and the contour of the engagement portion (14), and ¸s represents an acute angle (°) between a straight line orthogonal to the axial line CL1 and the contour of the diameter-decreasing portion (21A). In the aforementioned cross section, a relation of Hvo > Hvi is satisfied, wherein Hvo represents the Vickers hardness (Hv) of the plate packing (22) at the midpoint CP1 of a first line segment SL1, and Hvi represents the Vickers hardness (Hv) of the plate packing (22) at the midpoint CP2 of a second line segment (SL2). |
