Spark plug that has an insulator less likely to be damaged
An insulator for a spark plug including a first portion; a second portion located on the rear side of the first portion, an outer diameter F at a front end of the second portion being smaller than an outer diameter of the first portion; a third portion located on the front side of the first portion, an outer diameter D at a rear end of the third portion being smaller than the outer diameter of the first portion and the outer diameter F at the front end of the second portion; a first slope portion connecting the first portion and the second portion; and a second slope portion connecting the first portion and the third portion. A length A of the second slope portion and a length B of the first slope portion satisfy 2.0≤A/B≤3.9, and 0.50≤D/F≤0.88 is satisfied.
1. A spark plug comprising an insulator in which an axial hole extending along an axial line from a front side to a rear side is formed, wherein
the insulator includes
a cylindrical first portion,
a cylindrical second portion located on the rear side of the first portion, an outer diameter F at a front end of the second portion being smaller than an outer diameter of the first portion,
a cylindrical third portion located on the front side of the first portion, an outer diameter D at a rear end of the third portion being smaller than the outer diameter of the first portion and the outer diameter F at the front end of the second portion,
a first slope portion connecting the first portion and the second portion, and having an outer diameter that reduces toward the rear side, and
a second slope portion connecting the first portion and the third portion, and having an outer diameter that reduces toward the front side, and
a length A in an axial-line direction of the second slope portion and a length B in the axial-line direction of the first slope portion satisfy 2.0≤A/B≤3.9, and 0.50≤D/F≤0.88 is satisfied.
2. The spark plug according to claim 1 , wherein
0.5≤D/F≤0.58 is satisfied.
3. The spark plug according to claim 1 , further comprising a metal shell provided around an outer circumference of the insulator, wherein
the insulator includes a diameter reducing portion adjacent to a front end of the third portion,
the metal shell includes a ledge portion engaging with the diameter reducing portion from the front side, and
2.2D+7.8≤Z≤34 and 4.5≤D≤8 are satisfied, where Z is a total length of a length in the axial-line direction of the third portion and the length A in the axial-line direction of the second slope portion.
4. The spark plug according to claim 3 , wherein
24≤Z≤34 is satisfied.
5. The spark plug according to claim 1 , further comprising a metal shell provided around an outer circumference of the insulator, wherein
the metal shell has an external thread formed on an outer circumference thereof, and
a nominal diameter of the external thread is not greater than 12 mm.