Abstract: A method for inspecting an insulator for a spark plug to determine whether or not the insulator has a defect, the spark plug extending in an axial direction, includes a voltage applying step of applying a voltage between a first electrode disposed in an axial hole in the insulator and a second electrode disposed near an outer peripheral surface of the insulator. The voltage applying step is performed while a gap between a front end of the first electrode and an inner peripheral surface of the insulator that faces the front end of the first electrode in a radial direction is filled with a first insulating material without leaving a hollow space.

Abstract: A method for inspecting an insulator for a spark plug to determine whether or not the insulator has a defect, the spark plug extending in an axial direction, includes a voltage applying step of applying a voltage between a first electrode disposed in an axial hole in the insulator and a second electrode disposed near an outer peripheral surface of the insulator. The voltage applying step is performed while a gap between a front end of the first electrode and an inner peripheral surface of the insulator that faces the front end of the first electrode in a radial direction is filled with a first insulating material without leaving a hollow space.

Abstract: A spark plug manufacturing method providing a spark plug equipped with an insulator having dielectric strength by means of judging whether or not the insulator has a defect. A defect judgement step judges whether or not the insulator has a defect, by generating an electric potential difference between the center electrode and the metallic shell under conditions such that an assembly of the center electrode, metallic shell, and insulator is disposed within a pressure vessel; a high-pressure atmosphere higher in pressure than the atmospheric pressure is established within the pressure vessel; a space allowing the presence of insulating oil is a space surrounded by the packing, the metallic shell, the insulator, and an imaginary plane containing a forward end surface of the metallic shell; and the insulating oil is present at least in a region of the space where the distance between the ledge and insulator becomes shortest.

Abstract: A spark plug manufacturing method providing a spark plug equipped with an insulator having dielectric strength by means of judging whether or not the insulator has a defect. A defect judgement step judges whether or not the insulator has a defect, by generating an electric potential difference between the center electrode and the metallic shell under conditions such that an assembly of the center electrode, metallic shell, and insulator is disposed within a pressure vessel; a high-pressure atmosphere higher in pressure than the atmospheric pressure is established within the pressure vessel; a space allowing the presence of insulating oil is a space surrounded by the packing, the metallic shell, the insulator, and an imaginary plane containing a forward end surface of the metallic shell; and the insulating oil is present at least in a region of the space where the distance between the ledge and insulator becomes shortest.