Electrode-attached insulator and spark plug

1. An electrode-attached insulator comprising:

an insulator provided with an axial hole which extends in the direction of an axis, the insulator having a diametrically expanded portion where the axial hole is diametrically expanded towards the front-end side in the direction of the axis at the front end thereof; and

a center electrode is disposed in the axial hole on a front-end side of the axial hole wherein a front end portion is exposed from a front end of the insulator, said center electrode having a large diameter portion, said large diameter portion having an outside diameter that is larger than a minimum bore diameter of the axial hole and being disposed at a portion of the center electrode which is exposed from the front end of the insulator, and

a locking member having a collar portion adapted to be locked on a step portion formed on an inner circumferential surface of the axial hole,

wherein the center electrode and the locking member are joined together within the axial hole, and the large diameter portion is locked on the diametrically expanded portion, and

wherein a distance L, that extends from a front end of the step portion to the front end of the insulator along the axis CL 1 , is equal to or larger than 15 mm.

2. The electrode-attached insulator according to claim 1 , wherein

the axial hole includes:

a front-end side axial hole which is positioned further forward towards the front-end side in the direction of the axis than the step portion; and

a rear-end side axial hole that lies adjacent to a rear-end side of the front-end side axial hole in the direction of the axis and of which a bore diameter is larger than a bore diameter of the front-end side axial hole, and

at least part of a joining portion between the center electrode and the locking member is positioned in the rear-end side axial hole.

3. The electrode-attached insulator according to claim 1 , wherein

a sealing member which fixes the insulator and the locking member together is provided in the axial hole, and wherein

the center electrode is in contact with the sealing member.

4. The electrode-attached insulator according to claim 3 , wherein

the sealing member is a glass seal containing a glass component.

5. The electrode-attached insulator according to claim 1 , wherein

the large diameter portion is in contact with the insulator.

6. The electrode-attached insulator according to claim 1 , wherein

the center electrode and the locking member are screw joined together.

7. The electrode-attached insulator according to claim 6 , wherein

an electrode rotation restricting portion for restricting its relative rotation to the locking member is provided at a rear end portion of the center electrode.

8. The electrode-attached insulator according to claim 1 , wherein

a locking member rotation restricting portion for restricting its relative rotation to the insulator is provided on the locking member.

9. The electrode-attached insulator according to claim 1 , wherein

a sealing member for fixing the insulator and the locking member together is provided in the axial hole, and wherein

the rear end portion of the center electrode projects further rearward towards the rear-end side in the direction of the axis than a rear end of the locking member to thereby be in contact with the sealing member.

10. A spark plug comprising:

the electrode-attached insulator according to claim 1 ;

a cylindrical metal shell provided on an outer circumference of the electrode-attached insulator; and

a ground electrode configured to define a gap between the center electrode and itself.

11. The spark plug according to claim 10 , wherein

the gap is defined between a side circumferential surface of the large diameter portion and the ground electrode.