| 主权项 |
1. A spark plug for internal-combustion engines, comprising:
a cylindrical insulator having an axial bore that penetrates in an axial direction; a center electrode inserted in the axial bore; a cylindrical metal shell surrounding an outer circumference of the insulator; a ground electrode provided on a front end face of the metal shell so that a front end portion of the ground electrode faces a front end face of the center electrode; and a noble metal tip joined to the ground electrode so as to form a spark discharge gap between a front end portion of the noble metal tip and a front end portion of the center electrode, and the noble metal tip made of a platinum alloy that contains platinum as a principal component,
wherein a projection length in an axial direction of the spark plug, from a main body of the ground electrode to a front end face of the noble metal tip, falls within the range from 0.4 mm or more to 1.6 mm or less,wherein the platinum alloy has a mean particle size of 70 micrometers or less after being heated at 1100 degrees C. under an air atmosphere for 50 hours,wherein the metal shell has a thread portion on an outer circumference thereof so as to engage with a mounting hole of an engine head of an internal-combustion engine, andwherein the spark plug satisfies the following expression of:
H>=0.5M, where “M” is an outer diameter of the thread portion, andwhere “H” is a distance from a front end face of the metal shell in the axial direction to a molten portion formed by which the noble metal tip and the main body of the ground electrode or a convex portion projecting from the main body of the ground electrode are melted together. |
