Patent ID: 8963407
Filing Date: 2015-02-24
Classification: H01T

Abstract:
1. A spark plug comprising: an insulator having a center electrode which is formed by plastic working, said center electrode having; a seal portion which fixes the center electrode within the axial hole, wherein, 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, and when a value (L′/C) obtained by dividing an axial length L′ of the projection portion along an axis by the average diameter C of the leg portion is 1 or greater, an upsetting ratio (L−L′)/L×100(%) is 13% or less, where the upsetting ratio being the ratio of the difference (L−L′) between a pre-formation projection portion length L and the axial length L′ of the projection portion to the pre-formation projection portion length L, and the pre-formation projection portion length L being a value (V/D) obtained by dividing a volume V of the projection portion by a cross-sectional area D of the leg portion.