Abstract: This invention has an object to provide a spark plug exhibiting high withstand voltage characteristics and high strength at high temperature, formed by an alumina-based sintered body prepared with excellent processability and high productivity, and a method for manufacturing the spark plug. This invention relates to a spark plug 1 including an insulator 3, wherein the insulator 3 includes a dense alumina-based sintered body having an average crystal particle diameter of 1.50 ?m or more, and the alumina-based sintered body contains Si, a Group 2 element component containing Ba and a Group 2 element other than those, and a rare earth element component, such that a ratio of a content S of the Si component to a total content of the content S and a content A of the Group 2 element component is 0.60 or more, and a method for manufacturing the spark plug 1 prepared through a grinding-shaping process in which the insulator 3 is ground before burning to shape the same.

Abstract: A device for inspecting a spark plug insulator for bending, the spark plug insulator including a cylindrical body having a radially enlarged flange at an axially intermediate portion thereof, and a through hole formed in the body along an axial direction of the body. The inspection device includes an insulator supporting means for supporting the spark plug insulator so as to be slidable in an axial direction of the spark plug insulator, a linear inspection pin that is to be inserted into the through hole of the spark plug insulator supported by the insulator supporting means, and a detecting means for detecting a sliding movement of the spark plug insulator when the spark plug insulator is caused to slide in the axial direction thereof by the inspection pin by the inspection pin prevented from completely penetrating the through hole of the spark plug insulator owing to bending of the spark plug insulator which exceeds a predetermined tolerance.

Abstract: An object of the invention is to provide a spark plug including an insulator which is difficult to cause breakdown and exhibits higher withstand voltage characteristics. This invention relates to a spark plug 1 including a center electrode 2, an insulator 3 and a grounding electrode 6, wherein the insulator is formed by an alumina-based sintered body which has a theoretical density ratio of 95.

Abstract: A method for manufacturing an insulator includes: preparing a press pin and a forming die having a cavity, the press pin including a pin-side spiral portion formed on a first position; filling a raw powder into the cavity; arranging the press pin within the cavity; pressing the raw powder within the cavity along with the press pin, and obtaining a green body formed with a green body-side spiral portion; releasing the green body along with the press pin from the cavity; retreating the press pin with respect to the green body while rotating the press pin relative to the green body around an axis, and extracting the press pin from the green body; and removing an unnecessary portion from the green body. The first position is positioned such that the green body-side spiral portion is located in the unnecessary portion of the green body.

Abstract: There are provided a ceramic heater in which a defect, such as generation of a gap at the interface between an insulating substrate and a heat-generating resistor, is unlikely to occur in the course of manufacture or use, and a glow plug using the ceramic heater. A ceramic heater 110 includes an insulating substrate 111 extending in the direction of an axis AX and a heat-generating resistor 115, which has a heat-generating portion 116, two lead portions 117, 117 and two lead lead-out portions 118a and 118b. The ceramic heater 110 satisfies an expression S1?0.34 Sa, where Sa is the area of a cross section of the ceramic heater 110 taken perpendicular to the direction of the axis AX, and S1 is the total cross-sectional area of the two lead portions 117, 117 as measured in the cross section.

Abstract: There is provided a test method for detecting the presence or absence of a defect in a spark plug insulator, including a reference voltage determination process, a test area determination process, a test voltage determination process and a current detection process. In the reference voltage determination process, a reference voltage VF is determined. In the test area and voltage determined processes, test area and voltage are determined so as not to incur a flashover on the basis of a reference insulator of the same material, shape and size as the spark plug insulator when the reference insulator is placed in position between first and second test electrodes. In the current detection step, the test voltage is applied between the first and second test electrodes to detect an electric current between the first and second test electrodes.