Patent ID: 6909226

Claim:
A method for manufacturing a spark plug comprising a tubular metallic shell and a rodlike insulator axially inserted into said metallic shell, said spark plug having a spark discharge gap at one end of said insulator with respect to a direction of an axis of said insulator, a side toward said spark discharge gap with respect to the direction of said axis of said insulator being defined as a front side, said spark plug further comprising the features that: said metallic shell is configured such that a crimped portion is formed at a rear end portion thereof so as to be curved toward an outer circumferential surface of said insulator, and such that a tool engagement portion having an opposite side-to-side dimension of not greater than 14 mm is formed in an outer surface region adjacent to a front side of said crimped portion; said insulator is located into said metallic shell in the direction of said axis and a compressed sealing-material-powder layer is provided in a circumferential gap formed between an inner circumferential surface of a rear end portion of said metallic shell and an outer circumferential surface of said insulator; and said compressed sealing-material-powder layer is formed from a sealing material powder which contains talc in an amount of from 75% to 99.7% by mass and at least one of magnesite and dolomite in a total amount of from 0.3% to 25% by mass, and satisfies 0.5≦M≦1.3 and 0.5 ≦L≦ 2×( M× 4.5), wherein L in mm represents height as measured in the direction of said axis, and M in mm represents thickness as measured radially with respect to said axis, and has a density in the range of from 2 to 2.9 g/cm 3 , said method comprising: a sealing-material-powder charging step which comprises inserting said insulator into said metallic shell in the direction of said axis, and charging a sealing material powder into a circumferential gap formed between an inner circumferential surface of a rear end portion of said metallic shell and an outer circumferential surface of said insulator, such that said sealing material powder is divided into a plurality of charge units and such that a step of charging one charge unit into said gap and preliminarily compressing said charge unit in said gap is repeated to thereby form preliminarily compressed sealing-material-powder layers in said gap; and a crimping step which comprises curving said portion-to-be-crimped toward the outer circumferential surface of said insulator to thereby be crimped, whereby said preliminarily compressed sealing-material-powder layers are formed into a compressed sealing-material-powder layer which satisfies 0.5≦M≦1.3 and 0.5 ≦L≦ 2×( M× 4.5), wherein L in mm represents height as measured in the direction of said axis, and M in mm represents thickness as measured radially with respect to said axis.