| 摘要 |
<p>A spark plug (1) includes a tubular metallic shell (3) extending in the direction of an axis (CL1) and having a tool engagement portion (19) formed through extrusion. The tool engagement portion (19) has a 12-point shape which is a sectional shape taken orthogonally to the axis (CL1) and has a plurality of protrusions and recesses provided alternately. As viewed on a section taken orthogonally to the axis (CL1), when D (mm) represents the diameter of a circle which passes radially outermost positions on the protrusions, and d (mm) represents the diameter of a circle which passes radially innermost positions on the recesses, the relational expression 0.45 ‰¤ (D - d)/2 ‰¤ 0.75 is satisfied. By virtue of this, the spark plug whose tool engagement portion has a 12-point shape can provide more reliable restraint of slippage of a tool at the time of mounting and enables the tool engagement portion to be reliably formed so as to assume a desired shape.</p> |
