Patent ID: 8841826

Claim:
A spark plug comprising: a tubular insulator having an axial bore extending through the insulator in a direction of an axis; a center electrode inserted into a forward end portion of the axial bore; a tubular metallic shell provided externally of an outer circumference of the insulator; a ground electrode disposed at a forward end portion of the metallic shell; and a noble metal tip joined to a forward end portion of the center electrode, a gap being formed between said metal tip and the ground electrode; wherein the center electrode has a shoulder portion at a forward end portion of the center electrode, the shoulder portion tapering forward with respect to the direction of the axis; the noble metal tip is jointed to the center electrode via a fusion zone provided at least partially at a rear end portion of the noble metal tip; and a shortest distance between the fusion zone and a forward end surface of the noble metal tip is 0.8 mm to 1.2 mm inclusive as measured on an outer side surface of the noble metal tip along the axis, wherein; an outside diameter of the fusion zone as measured at a forward end of the fusion zone is smaller than that as measured at a rear end of the fusion zone, and relational expressions θ 1 >θ 2 and (θ 1 −θ 2 )≦50° are satisfied, where θ 1 is an acute angle formed by a straight line L 1 and a straight line L 2 defined below, satisfying a relational expression θ 1 ≦72°, and θ 2 is an acute angle formed by a straight line L 3 and a straight line L 4 defined below, as viewed on a section which contains the axis and on which θ 2 is maximized, where the straight line L 1 is, as viewed on a section which contains the axis, an axially forwardly extended straight line of one of two outlines of the shoulder portion, the two outlines being located on opposite sides with respect to the axis, the straight line L 2 is, as viewed on the section which contains the axis, an axially forwardly extended straight line of the other of the two outlines of the shoulder portion, the two outlines being located on opposite sides with respect to the axis, the straight line L 3 is, as viewed on the section which contains the axis, a straight line which passes through opposite ends of one of two outlines formed on an externally exposed surface of the fusion zone, the one outline being located on one side with respect to the axis, and the straight line L 4 is, as viewed on the section which contains the axis, a straight line which passes through opposite ends of the other of the two outlines formed on an externally exposed surface of the fusion zone, the other outline being located on the other side with respect to the axis.