SPARK PLUG, AND PROCESS FOR PRODUCING SPARK PLUG

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PROBLEM TO BE SOLVED : To provide a downsized spark plug having both fouling-proof properties and voltage-withstanding characteristics over a long period of time, and to provide a process for producing the spark plug. SOLUTION : The spark plug 100 is equipped with an insulator 2 having a long-leg part 30 at its end side, a central electrode 3, and a body metal fitting 1 holding the insulator 2 together with an engaging projection part 56. In the spark plug 100, an inner diameter of the engaging projection part 56 denoted by DIN, a maximum outer diameter denoted by dOUT and an inner diameter denoted by dIN of the long-leg part 30 facing an inner periphery surface 59 of the engaging projection part 56, and a dielectric constant of the insulator 2 denoted by ε satisfy requirements given below. The manufacturing method of the spark plug 100 includes a process of manufacturing the insulator 2 by sintering, after pressure molding, raw material powder containing Al compound powder as well as Si compound powder, two or more kinds of group-2 element compound powder including Mg compound powder and Ba compound powder, and 0.5 to 4.0% by mass of rare earth element compound powder. Requirements : (DIN-dOUT)/2 ≤0.40 (mm), (dOUT-dIN)/2 ≤1.65 (mm) and ε≥9.4 (F/m). COPYRIGHT : (C)2011, JPO&INPIT