Spark plug and method of manufacturing the same

A spark plug comprising:

• an insulator having a center through-hole formed therein;
• a center electrode held in the center through-hole;
• a metallic shell holding the insulator, the metallic shell having a male-threaded portion formed on an outer circumferential surface of a front end portion of the metallic shell, a tightening portion formed on an outer circumferential surface of the metallic shell, a seat portion located between the male-threaded portion and the tightening portion and having a diameter greater than that of the male-threaded portion, and a curved portion extending between the tightening portion and the seat portion, the metallic shell being formed such that a distance between two opposed parallel surfaces of the tightening portion is not greater than 14 mm, the curved portion being buckled by pressing the tightening portion and the seat portion toward each other while heating the curved portion;
• a cushion material charged into a cylindrical space defined by an outer surface of the insulator and an inner surface of the metallic shell to thereby form a cushion-material charged portion having an axial length L of from 0.5 mm to 10.0 mm inclusive (0.5 mm≦L≦10.0 mm) and a thickness M of from 0.5 mm to 1.3 mm inclusive (0.5 mm ≦M≦1.3 mm);
• and
• a ground electrode electrically connected to the metallic shell.

A spark plug made by a method comprising:

• forming a metallic shell having a male-threaded portion, a tightening portion, a seat portion located between the male-threaded portion and the tightening portion and having a diameter greater than that of the male-threaded portion, a curved portion extending between the tightening portion and the seat portion, and a distance between two opposed parallel surfaces of the tightening portion is not greater than 14 mm;
• charging a cushion material into a cylindrical space defined by an outer surface of an insulator and an inner surface of the metallic shell to thereby form a cushion-material charged portion having an axial length L of from 0.5 mm to 10.0 mm inclusive (0.5 mm≦L≦10.0 mm) and a thickness M of from 0.5 mm to 1.3 mm inclusive (0.5 mm≦M≦1.3 mm);
• and
• pressing the tightening portion and the seat portion toward each other while applying current thereto so as to heat the curved portion to buckle the curved portion.