Spark plug for internal combustion engine, having an annular ground electrode facing an outer circumference of a center electrode

A spark plug for an internal combustion engine comprising:

• a cylindrical housing which is designed to be mounted in an internal combustion engine so as to be exposed to a combustion chamber of the internal combustion engine;
• a cylindrical porcelain insulator which is retained inside the housing;
• a center electrode which is retained inside the porcelain insulator and has a head extending outside a front end of the porcelain insulator;
• and
• an annular ground electrode which is secured to a front end of the housing,
• wherein the housing has a small-diameter portion formed on the front end thereof, the small-diameter portion having an inner diameter which is smaller than that of a rest of the housing,
• wherein the ground electrode protrudes from a front end surface of the small-diameter portion and has an inner peripheral surface facing an outer peripheral surface of the center electrode,
• wherein the ground electrode has an outer diameter which is smaller than an outer diameter of the front end surface of the small-diameter portion,
• wherein the porcelain insulator has a mounting shoulder formed on an outer periphery thereof, and the housing has a seat shoulder formed on an inner periphery thereof, the mounting shoulder riding on the seat shoulder from a base side of the spark plug in an axial direction of the spark plug to retain the porcelain insulator in the housing,
• wherein a pocket is formed between the inner periphery of the housing and a portion of the porcelain insulator which is located closer to a front end of the porcelain insulator than the mounting shoulder is, and
• wherein the housing has formed therein an air vent which extends from outside the ground electrode in a radial direction of the spark plug to the pocket to establish a communication between the pocket and the combustion chamber of the internal combustion engine.