Patent ID: 8143773

Claim:
A spark plug comprising: an insulator holding a center electrode in an interior of an axial hole thereof which is formed in an axial direction, the insulator comprising: a front end side body portion having a stepped portion on a front end side of an outer circumferential surface thereof; an intermediate body portion formed on a rear end side of the front end side body portion and having a larger diameter than that of the front end side body portion; and a rear end side body portion formed on a rear end side of the intermediate body portion with interposing a shoulder portion and having a smaller diameter than that of the intermediate body portion; a metal shell comprising: a tool engagement portion for mounting the spark plug on an internal combustion engine; a crimping portion formed on a rear end side with respect to the tool engagement portion; and a ledge portion formed on a front end side with respect to the tool engagement portion within an inner hole of the metal shell and projecting radially inwards, wherein the metal shell holds a portion of the insulator which lies from the shoulder portion to the stepped portion of the insulator between the clamping portion and the ledge portion; and an annular packing interposed between the ledge portion and the stepped portion, wherein in τ A=LAX max/ ZX max, and τ B =( LAC·LBY max)/( LBC·ZY max), at least either τA or τB is 0.47 or larger, and 0.71≦τA/τB≦1.27 is satisfied, where, in the axial direction, A represents a position where a rear end of the insulator is situated; B represents a position where the insulator and the packing are first brought into contact with each other from a front end side of the insulator; C represents a position where the insulator and the clamping portion are first brought into contact with each other from a rear end side of the insulator; LAC represents a length between the position A and the position C; LBC represents a length between the position B and the position C; X represents an arbitrary position between the position A and the position C; Y represents an arbitrary position between the position B and the position C; LAX represents a length between the position A and the position X; LBY represents a length between the position B and the position Y; ZX represents a modulus of section of the insulator at the position X; ZY represents a modulus of section of the insulator at the position Y; Xmax represents a position of the position X where LAX/ZX takes a maximum value; Ymax represents a position of the position Y where LBY/ZY takes a maximum value; LAXmax represents a length between the position A and the position X max; LBYmax represents a length between the position B and the position Ymax; ZXmax represents a modulus of section of the insulator at a portion of the position Xmax; and ZYmax represents a modulus of section of the insulator at a portion of the position Ymax.