| 摘要 |
An object is to provide a spark plug in which a center electrode is fixed within an axial hole in a good condition. A spark plug of the present invention includes an insulator which has an axial hole extending in the direction of an axis, a first inner circumferential surface which forms a forward end portion of the axial hole, a second inner circumferential surface which forms a rear end portion of the axial hole and which has a diameter greater than that of the first inner circumferential surface, and a ledge portion which connects the first inner circumferential surface and the second inner circumferential surface; a center electrode which is formed by plastic working and which has a large-diameter portion supported by the ledge portion, a projection portion projecting rearward from the large-diameter portion, and a circular columnar leg portion which is located adjacent to the large-diameter portion and projects into a space surrounded by the first inner circumferential surface; and a seal portion which fixes the center electrode within the axial hole. When C < A, A-C ‰¤ B-A, where A represents a diameter (mm) of an imaginary cylinder having the minimum diameter required for surrounding the projection portion, B represents a maximum diameter (mm) of the large-diameter portion, and C represents an average diameter (mm) of the leg portion present in the space surrounded by the first inner circumferential surface. |
