Spark plug

Abstract

A spark plug according to this invention includes an elongate center electrode, an insulator enclosing the center electrode, a metallic shell having open opposite ends and enclosing the insulator, the metallic shell having a male-threaded portion formed on a front-side outer circumferential surface of the metallic shell, and a tool engagement portion formed on the outer circumferential surface of the metallic shell at a rear side with respect to the male-threaded portion, the tool engagement portion projecting circumferentially outwardly, and a ground electrode connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode. The size of the tool engagement portion is reduced such that |A-E| is not greater than 1.5 mm, where A is an outside dimension of the tool engagement portion, and E is an effective diameter of a male-threaded portion of the metallic shell. Also, the effective diameter E of the male-threaded portion of the metallic shell and the diameter D2 of an intermediate-bore portion of the metallic shell are determined such that the relationship 0.4≦(D2/E)2≦0.6 is satisfied. Therefore, even when the outside diameter of the insulator decreases in association with a reduction in the size of the tool engagement portion, the wall thickness of the male-threaded portion of the metallic shell falls within an appropriate range, and a forging punch is less susceptible to breakage and is less likely to cause a working defect during forging of the metallic shell.

Dependencies are inferred from the claim text.