Patent ID: 8115371

Claim:
A spark plug comprising: a rod-shaped center electrode; a substantially cylindrical insulating member which has an axial hole extending along a direction of an axis of the center electrode and holds the center electrode within the axial hole while allowing a front end portion of the center electrode to be exposed; a substantially cylindrical metal shell which is provided around an outer circumference of the insulating member; and a ground electrode which is joined to a front end face of the metal shell and which forms a spark gap with a front end portion of the center electrode, wherein a front end portion of the insulating member projects 2 mm or more from the front end face of the metal shell, and a volume of a portion of the insulating member which lies within a range from a front end to a position lying 1 mm towards a rear end of the insulating member from the front end is 11 mm 3 or smaller, and wherein when assuming in a section of the spark plug which passes through the axis that: a corner portion where a front end face of the insulating member and a side surface of the axial hole intersect is referred to as a position PA, a position on the center electrode where a straight-line distance from the position PA to the center electrode within the axial hole becomes shortest is referred to as a position PB, a position where the insulating element first contacts the metal shell from the front end face of the insulating member along a surface of the insulating member is referred to as a position PC, and a position on the insulating member where when a straight line BC which connects the position PB with the position PC is displaced parallel towards an outside of the axis, the straight line BC contacts the surface of the insulating member is referred to as a position PD, a parallel displacement amount E by which the straight line BC is displaced parallel so as to contact the position PD is 0.75 mm or more.