Patent ID: 6617769
Filing Date: 2003-09-09
Classification: H01T

Abstract:
A spark plug adapted to be mounted in a plug hole formed in a cylinder head, said spark plug comprising:a central electrode extending in an axial direction thereof; an insulator having a tip end portion which surrounds said central electrode; and a main metal shell provided around said insulator, said main metal shell having a flange portion with a plug bearing surface at a tip end of said flange portion, and said main metal shell having a top end portion which extends in an axial direction thereof from said plug bearing surface to a tip end side of the main metal shell, wherein the top end portion of said main metal shell is formed into a substantially cylindrical shape having an outer circumferential face on which no external thread is provided, and the outer diameter of the top end portion satisfying a following relation, &phgr;Dâˆ’&phgr;d&lE;0.15 â€ƒwhere &phgr;d (unit: mm) defines the outer diameter of said top end portion of said main metal shell, and &phgr;D (unit: mm) denotes the inner diameter of a top end counterpart of said plug hole where is in confront with said top end portion.