| 主权项 |
1. An ignition plug comprising:
a tubular insulator having an axial hole extending in a direction of an axial line; a center electrode inserted into a forward end portion of the axial hole; and a metallic shell provided around the insulator and having a protrusion protruding radially inward, wherein the insulator has an engagement portion which is engaged directly or indirectly with a receiving surface of the protrusion which is a rear-end-side surface thereof, and an intermediate trunk portion extending rearward from a rear end of the engagement portion, the metallic shell has, on its outer circumference, an attachment screw portion located on a radially outer side of the protrusion, a seating portion located rearward of the screw portion and projecting radially outward, and a tube portion located between the screw portion and the seating portion, the tube portion being located on the radially outer side of the intermediate trunk portion and having a diameter smaller than that of the seating portion, the screw portion has a screw diameter of 10 mm or less; and a relation A≦1.70 and a relation B≧1.20 are satisfied, where A is a thickness (mm) of the metallic shell along a direction which passes through a center of the receiving surface and is orthogonal to the axial line on a cross section including the axial line, and B is a minimum thickness (mm) of the metallic shell at the tube portion along the direction orthogonal to the axial line. |
