| 摘要 |
An object is to effectively enhance the heat conduction performances of an insulator and a center electrode to thereby suppress overheating of the insulator, etc. while preventing breakage of a tube portion more reliably. An ignition plug 1 includes a tubular ceramic insulator 2 and a metallic shell 3 having a protrusion 21 protruding radially inward. The ceramic insulator 2 has an engagement portion 14 which is engaged with a receiving surface 21A of the protrusion 21 and an intermediate trunk portion 12 extending rearward from the rear end of the engagement portion 14. The metallic shell 3 has a screw portion 15 located on the radially outer side of the protrusion 21 and having a screw diameter of 10 mm or less, and a tube portion 17 located on the radially outer side of the intermediate trunk portion 12. A ‰¤ 1.70 and B ‰¥ 1.20 are satisfied, where A is the thickness (mm) of the metallic shell 3 along a direction which passes through the center CP of the receiving surface 21A and is orthogonal to the axial line CL1 on a cross section including the axial line CL1, and B is the minimum thickness (mm) of the metallic shell 3 at the tube portion 17 along the direction orthogonal to the axial line CL1. |
