JP4824464B2 - Spark plug - Google Patents

Description

  The present invention relates to a spark plug used for an internal combustion engine such as an automobile engine.

  Conventionally, a spark plug has a structure in which a center electrode, an insulator holding the center electrode, and a metal shell provided with a ground electrode at the tip are supported and fixed in the metal shell. Are known. In such a spark plug, a large bowl-shaped portion of the insulator is inserted into a cylindrical metal shell, and one end of the metal shell is crimped to form a bowl-shaped portion of the insulator. In general, a structure in which an insulator is supported and fixed in the metal shell by pressing in the axial direction (see, for example, Patent Document 1).

  Further, in the spark plug as described above, thermal characteristics (also referred to as heat value) are important, and the range is defined particularly from the relationship between the ignition advance angle and the ignition temperature of the spark plug. The wider this range, the better the characteristics. As the adjustment of the heat value, a method of adjusting the range by adjusting the size of the area exposed to the combustion gas on the ignition part side of the insulator is generally used. However, only by changing the area, the heat value may not be adjusted sufficiently, and the spark plug may be easily smoldered or burnt easily. For this reason, wide range has been achieved by improving the heat dissipation from the ignition part side by using a center electrode with a copper core or the like.

Further, in the spark plug using the copper cored center electrode as described above, the length of the gas pocket formed between the metal shell and the insulator and into which the combustion gas enters, and the gas plug is disposed between the metal shell and the insulator. It is also known to adjust the heat value by making the sum of the lengths of the metal packing interposition parts constant and changing these lengths (see, for example, Patent Document 2). In this method, in order to improve heat dissipation, the length of the packing interposition part is increased and the length of the gas pocket is decreased. For this reason, there exists a problem that stain resistance, such as smoldering, worsens.
JP 2002-164147 A Japanese Utility Model Publication No. 61-39880

  As described above, in the prior art, there has been a problem that it is difficult to improve heat dissipation without causing deterioration in stain resistance such as smoldering. Further, the spark plug is required to have a smaller diameter.

  The present invention has been made to solve the above problems. SUMMARY OF THE INVENTION An object of the present invention is to provide a spark plug that can improve the fouling resistance and heat dissipation as compared with the prior art and can be reduced in diameter.

The spark plug of the present invention accommodates a center electrode extending in the axial direction, a cylindrical insulator for holding the center electrode, a part of the insulator, and a tip portion between the center electrode and the center electrode. A spark plug having a cylindrical metal shell provided with a ground electrode for generating a spark discharge, wherein the metal shell is a protruding portion that protrudes toward the inside of the metal shell. And a projecting base end and a projecting inner end having different axial positions from each other, the projecting inner end having a projecting portion that comes into contact with the insulator, and the projecting portion includes the metal shell and the insulator Among the abutting portions that are in contact with each other, the projection is positioned closest to the tip end in the axial direction , and the protruding portion and the insulator are in contact with each other by press-fitting .

  In the spark plug of the present invention having the above-described configuration, the projecting portion functions as a heat dissipation path between the insulator and the metal shell, which is positioned closest to the tip end in the axial direction. In this way, by forming the heat radiation path by the protruding portion that protrudes from the metal shell in the inner peripheral direction, for example, stepped portions that respectively face the metal shell and the insulator are formed, and between them, Compared to a conventional structure in which a heat transfer member such as a ring is arranged, it is not necessary to provide a structure such as a stepped portion, so that the leg length (gas pocket length) can be maximized. As a result, a spark plug with good antifouling properties can be obtained, and the diameter of the spark plug can be reduced without the need for an excessively large stepped portion. Furthermore, since the protruding portion is configured such that the protruding base end and the protruding inner end are displaced from each other in the axial direction, the tightening load due to the protruding portion on the insulator can be reduced. In other words, the protrusion, which is a portion that exerts a load on the insulator among the protrusions, can be deformed to bend and relieve stress, and an excessive increase in surface pressure due to the protrusion on the insulator can be suppressed. It becomes. This can prevent the insulator from being damaged by the tightening load. Moreover, the radial expansion | swelling of the metal shell by press-fitting can be prevented, and the fall of the screw precision of a thread part can be prevented.

  Further, the spark plug of the present invention is the above spark plug, wherein the insulator has a large diameter portion where a portion near the rear end thereof is exposed from the metal shell, and a small diameter diameter insulator than the large diameter portion. A body part, and an insulator leg long part that is exposed to combustion gas at least a part near its tip when attached to the engine with a smaller diameter than the middle body part, and the inner surface of the metal shell is the insulator A large hole portion opposed to the large diameter portion, a middle hole portion opposed to the insulator middle trunk portion, and a small hole portion opposed to the lever leg long portion, wherein the protruding portion is the most of the small hole portion. It is formed in the rear end side, The said hole part and the said small hole part are divided by the said protrusion part, It is characterized by the above-mentioned. In this way, by changing the position of the protruding portion, the leg length (the length of the gas pocket) can be adjusted, and a configuration that maximizes the leg length can be realized. Further, when manufacturing spark plugs of different product numbers having different heat values, the insulator can be made into a common part.

  The spark plug according to the present invention is characterized in that, in the spark plug, a portion of the insulator that contacts the projecting inner end is parallel to the axis. The insulator expands and contracts (expands and contracts) in the axial direction due to a thermal cycle. For this reason, the outer diameter of the insulator also changes, and the contact position in the radial direction of the protruding inner end of the protruding portion and the insulator changes. When the axial direction and the radial direction perpendicular to the axial direction are analyzed with respect to the change in the contact position, if the portion of the insulator that contacts the projecting inner end is not parallel, the radial direction is also affected by the expansion and contraction of the axial component of the insulator. It will affect the change of the contact position. However, the parallel contact as in this configuration does not change the radial contact position due to expansion and contraction in the axial direction, and a good contact state can be maintained. For this reason, breakage of the insulator is suppressed or reduced, and good heat dissipation can be realized. In addition, it is good also considering the site | part adjacent to the front end side of the site | part which contact | abuts this protrusion inner end as a shape which has the inclined surface which becomes a small diameter toward a front-end | tip. As a result, the insulator can be easily inserted into the metal shell.

  The spark plug according to the present invention is characterized in that, in the spark plug, the protruding portion has a shape whose thickness decreases from the protruding base end toward the protruding inner end. As a result, the tightening load due to the protrusions on the insulator can be further reduced.

  Further, the spark plug of the present invention is the above-described spark plug, wherein the metal shell holds the insulator by any one of press fitting, shrink fitting, and cold fitting at least on the rear end side of the protruding portion. It is characterized by providing.

  As described above, by holding the insulator by any one of press fitting, shrink fitting, and cold fitting at the fitting portion of the metal shell, a large size for engaging the crimping portion of the metal shell as in the conventional case. There is no need to provide a diameter portion in the insulator. Thereby, the maximum diameter of a spark plug can be made thinner than before. That is, the diameter of the spark plug can be reduced as compared with the conventional case. Moreover, the freedom degree of the structure of a thermal radiation part can be raised by setting it as the structure which maintains airtightness in a fitting part.

  As a means for holding the insulator in the fitting portion, press-fitting can be selected. In this case, a press-fitting introduction portion having a diameter smaller than that of the rear end side is provided on the front end side of the press-fitting portion of the insulator. It is preferable to provide it. This makes it possible to manufacture easily and to ensure a sufficient pull-out load.

  ADVANTAGE OF THE INVENTION According to this invention, while being able to aim at the improvement of stain resistance and heat dissipation compared with the past, the spark plug which can aim at diameter reduction can be provided.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an enlarged cross-sectional configuration of a main part of a spark plug 100 according to an embodiment of the present invention, FIG. 2 shows the overall appearance of the spark plug 100, and FIG. The main part structure enclosed with the dashed-dotted line is further expanded and shown. The spark plug 100 includes a substantially cylindrical metal shell 1 and a substantially cylindrical insulator 2 that is fitted into the metal shell 1 so that the tip portion protrudes. As shown by a dotted line in FIG. 1, a center electrode 3 with a copper core is disposed along the axial direction at the center portion on the tip side in the insulator 2, and the tip portion of the center electrode 3 is the insulator. 2 is in a state of protruding from the tip surface. And the ground electrode 10 is arrange | positioned so that the front-end | tip part of this center electrode 3 may be opposed. One end of the ground electrode 10 is coupled to the metal shell 1, and a spark discharge gap having a predetermined interval is formed between the ground electrode 10 and the center electrode 3.

  The insulator 2 is configured in a substantially cylindrical shape by a ceramic sintered body such as alumina. As shown by a dotted line in FIG. 1, a through hole for inserting the center electrode 3 along the axial direction of the insulator 2 is provided inside the insulator 2, and the terminal fitting 4 is inserted and fixed on the rear end side. The center electrode 3 is inserted and fixed on the tip side. These terminal fittings 4 and the center electrode 3 are electrically connected through a resistor 30 and a conductive glass seal layer 31 in the through hole of the insulator 2. The insulator 2 includes an insulator large diameter portion 21 including a portion near the rear end of the metal shell 1 and a portion exposed from the metal shell 1 on the rear end side of the metal shell 1, and an insulator on the tip side of the insulator large diameter portion 21. An insulator middle body portion 22 having a smaller diameter than the large diameter portion 21, and a portion closer to the front end of the insulator when it is attached to an internal combustion engine such as an engine that is smaller in diameter than the insulator intermediate body portion 22 on the tip side from the insulator center body portion 22. It has the insulator leg long part 23 exposed to gas. In the present embodiment, the insulator middle trunk portion 22 includes a large-diameter rear-end side insulator middle barrel portion 220 located on the rear end side, and a small-diameter distal-end-side insulator middle barrel portion 221 located on the distal end side. Yes.

  The metal shell 1 is formed in a cylindrical shape by a metal such as carbon steel or stainless steel, for example, S35C, S45C, SUS430, SUS630, etc., and constitutes a housing of the spark plug 100 and has a distal end side (lower in the figure). A threaded portion 7 for attaching the spark plug 100 to a plug attachment hole of an engine (not shown) is formed on the outer peripheral surface. A tool engaging portion 8 that engages a tool such as a spanner or a wrench when the spark plug 100 is attached to the plug attachment hole is provided on the outer peripheral portion on the rear end side from the screw portion 7. A fitting portion 9 is provided further on the rear end side than the tool engaging portion 8. 1 and 2, reference numeral 5 denotes a seating surface that forms an airtight sealing surface with the engine when the spark plug 100 is attached to the engine. For example, a ring-shaped seal member (gasket) for hermetic sealing may be disposed between the seat surface 5 and the contact surface of the engine.

  The inner peripheral surface of the metal shell 1 has a large hole portion 11 facing the large insulator diameter portion 21 of the insulator 2, a middle hole portion 12 facing the insulator middle body portion 22, and a small hole facing the insulator leg long portion 23. Part 13. Further, the middle hole portion 12 mainly includes a large-diameter rear-end-side middle hole portion 120 opposed to the rear-end-side insulator middle barrel portion 220 and a small-diameter front-end side portion mainly opposed to the distal-end-side insulator middle barrel portion 221. It is comprised from the hole part 121. FIG.

  An annular projecting portion 14 that projects toward the inner side of the metal shell 1 is formed on the most rear end side (upper side in FIG. 1) of the small hole portion 13, and the projecting portion 14 substantially substantially The hole 12 (the front end side middle hole 121) and the small hole 13 are divided. As shown in FIG. 3, the projecting portion 14 has a projecting base end 140 and a projecting inner end 141 having different axial positions, and the projecting inner end 141 is in contact with the insulator 2. That is, the projecting base end 140 and the projecting inner end 141 are disposed at positions shifted in the axial direction so that their axial positions (indicated by bidirectional arrows in FIG. 3) do not overlap. Yes. Note that the length indicated by the bidirectional arrow also corresponds to the “thickness” of the protrusion. That is, the “thickness” means the length of the protrusion that occupies the protrusion in the direction parallel to the axis in the cross section including the axis of the spark plug.

  The protruding portion 14 is maintained in a state in which the insulator 2 is press-fitted, similarly to the fitting portion 9 described in detail later. And this protrusion part 14 is located in the most axial direction front end side among the contact parts which the metal shell 1 and the insulator 2 contact. That is, the protrusion 14 acts as a heat dissipation path located closest to the tip end in the axial direction among the heat dissipation paths formed between the insulator 2 and the metal shell 1. In this way, by forming the heat dissipation path by the protruding portion 14 protruding from the metal shell 1 toward the inner circumferential direction, for example, stepped portions that respectively face the metal shell and the insulator are formed. Compared with the case where a ring-like heat transfer member is disposed between them, it is not necessary to provide a structure such as a stepped portion, so that the leg length (the length of the gas pocket) can be maximized. As a result, it is possible to obtain a spark plug with good antifouling property while maintaining good heat dissipation, and to reduce the overall diameter of the spark plug.

  Further, since the projecting portion 14 and the insulator 2 are in contact with each other by the radial tightening due to the press-fitting, it is more reliable in the case where the thermal cycle is applied than in the case where the projecting portion 14 and the insulator 2 are pressed and contacted in the axial direction. The contact state can be maintained. That is, when the thermal cycle is applied, the axial expansion and contraction becomes larger than the radial expansion and contraction of the insulator 2. For this reason, when the heat transfer member is pressed in the axial direction and heat is applied and the insulator 2 is extended, the heat transfer member is deformed, the temperature returns to room temperature, and the extension of the insulator 2 returns. Contact may not be maintained. On the other hand, according to the present embodiment, the expansion and contraction of the insulator 2 in the axial direction can be absorbed by sliding the protruding portion 14 and the insulator 2. In addition, since the expansion and contraction of the insulator 2 in the radial direction is slight as compared with the axial direction, there is no problem because the protrusion 14 itself is deformed to bend. For this reason, good heat dissipation can be maintained.

  Furthermore, as described above, the projecting portion 14 has a configuration in which the projecting base end 140 and the projecting inner end 141 are disposed at positions shifted in the axial direction so that the axial position does not overlap at all. ing. In other words, the projecting portion 14 is arranged so that a space is formed between the metal shell 1 and the insulator 2 in any radial cross section. As a result, the tightening load on the insulator 2 can be reduced. Since the projecting portion 14 is in contact with the small-diameter and thin-thick insulator leg long portion 23 of the insulator 2, suppressing the surface pressure in this way prevents the insulator 2 from being damaged. It is valid. Moreover, the radial expansion | swelling of the metal shell 1 by press injection can be prevented, and the fall of the screw precision of the screw part 7 can be prevented.

  Moreover, according to this embodiment, it is possible to adjust a heat value by changing the axial direction position of the protrusion part 14. Even in this case, the insulator 2 can be made into a common part.

  In the present embodiment, an annular heat transfer member 40 for forming a heat dissipation path is provided between the insulator 2 and the metal shell 1 on the rear end side from the protrusion 14 described above. ing. The heat transfer member 40 is made of a metal having good thermal conductivity, such as copper or aluminum. Thus, by forming a heat radiation path for radiating heat from the insulator 2 to the metal shell 1 at a plurality of locations (two locations) spaced apart in the axial direction of the insulator 2, heat radiation can be accurately controlled, A wide range can be achieved without degrading the fouling property.

  That is, in this case, the heat radiation path on the distal end side by the protrusion 14 in FIG. 1 mainly radiates heat from the distal end portion of the insulator 2 to the metal shell 1. Further, the heat radiation path on the rear end side by the heat transfer member 40 in FIG. 1 is provided adjacent to a portion where the center electrode 3 and the resistor 30 are joined, and the center with a copper core having good thermal conductivity is provided. Heat from the electrode 3 is radiated to the metal shell 1. Thereby, the temperature of these parts can be controlled to a desired temperature so as to match a desired heat value, and the wide range can be realized by preventing the occurrence of pre-ignition. Further, since it is not necessary to reduce the length of the gas pocket, the antifouling property such as smoldering does not deteriorate.

  The fitting portion 9 is for fitting and holding the insulator 2. In this embodiment, the fitting portion 9 is fitted and held in the radial direction by press-fitting the insulator 2. ing. Thus, by providing the fitting portion 9 on the rear end side with respect to the tool engaging portion 8, when the tool is engaged with the tool engaging portion 8 and the spark plug 100 is tightened in the plug mounting hole, etc. It is possible to prevent or reduce torsional torque and axial force from being applied to the fitting part 9, and to improve the reliability of the coupling part (fitting holding) in the fitting part 9. That is, even if the spark plug 100 is repeatedly attached to and detached from the plug attachment hole, the influence on the fitting portion 9 is small and the connection state with the insulator 2 is not loosened. . Moreover, by supporting the insulator 2 on the rear end side of the metal shell 1, the vibration frequency when the insulator 2 vibrates can be increased, and the vibration resistance can be improved.

  Further, if the fitting portion 9 as described above is provided, for example, in the portion of the screw portion 7, the screw portion 7 may swell due to the press-fitting of the insulator 2, and the screw accuracy may be reduced. Thus, by providing the fitting portion 9 on the rear end side with respect to the tool engaging portion 8, it is possible to prevent such a problem from occurring. Furthermore, it can be fitted on the side of the large-diameter portion 21 of the insulator 2 by being provided on the rear end side. The insulator large diameter portion 21 can be formed thicker than other parts, and the breaking load of the insulator 2 is higher than that of the small / medium diameter portion. It is possible to prevent the insulator 2 from being destroyed or damaged. Moreover, since it becomes comparatively low temperature when used with an engine, problems such as airtight leakage hardly occur.

  On the other hand, the insulator 2 is made into the insulator leg long part 23, the insulator middle trunk | drum 22, and the insulator diameter large part 21 in order from the front end side as mentioned above. The end portion 22 side end portion of the insulator large diameter portion 21 is formed with a taper of a predetermined angle and serves as a press-in introduction portion 24 for press-fitting into the fitting portion 9 of the metal shell 1. The taper angle of the press-fit introduction portion 24 is preferably about 1 to 5 degrees, and more preferably about 2 to 4 degrees. The reason why the taper angle is 1 degree or more, preferably 2 degrees or more is that the taper length (press-fit introduction part length) varies depending on the taper angle, and when the taper angle is less than 1 degree, the taper length increases rapidly. It is. The reason why the taper angle is set to 5 degrees or less, preferably 4 degrees or less is to ensure a sufficient fitting allowance after removal. The fitting allowance after removal indicates the difference in diameter (D2-D1) between the outer shape (D1) of the insulator 2 and the inner diameter (D2) of the press-fitted portion 9 when it is once pressed and then withdrawn. In order to obtain a sufficient fitting strength (a predetermined load or more), it is necessary to have a certain level of strength.

  As described above, in the present embodiment, the insulator 2 is press-fitted into the fitting portion 9 to be fitted and held, so that the caulking portion of the metal shell 1 is engaged as in the conventional case. It is not necessary to provide the large-diameter portion in the insulator 2, and the maximum diameter of the spark plug 100 can be reduced. Thereby, the diameter of the hole for attaching the spark plug 100 provided in the engine can be reduced, and the degree of freedom in engine design can be increased. In addition to press-fitting, the insulator 2 may be fitted to the fitting portion 9 by shrink fitting, cold fitting, or a combination thereof.

  Moreover, in the spark plug of this embodiment, it is necessary to increase the reliability of the fitting portion, that is, to increase the unloading load. However, the higher the unloading load, the higher the press-fit load. In such a case, the press-fitting load can be reduced while keeping the reliability of the fitting portion high by using a lubricant during press-fitting. In this case, the removal load increases by performing heat treatment after press-fitting. This is because the lubricant is decomposed by heat treatment and the lubrication effect is lost, and the contact state of the fitting portion 9 is in a point contact state before the heat treatment, but the point contact portion is locally subjected to high surface pressure. In addition, by applying heat to this state, the metal shell material is softened, and plastic deformation causes the contact state to change from a point to a surface contact, thereby increasing the true contact area of the fitting portion 9. Depending on the effect. As such a lubricant, for example, Paskin M30 (trade name), cellosol (trade name) or the like can be used.

  The heat treatment is preferably performed at a temperature of 300 ° C. for about 15 minutes, for example. When such heat treatment after press-fitting is not performed, the press-fitting load and the removal load are substantially the same. However, by performing the heat treatment as described above, for example, in the case of a spark plug having a fitting portion diameter (outer diameter of the insulator) of 10 mm, an example of data actually measured is as follows. The unloading load at room temperature was 610 kg, and the unloading load at 200 ° C. was 520 kg. For example, in the case of a spark plug having a fitting portion diameter (outer diameter of the insulator) of 8 mm, an example of data actually measured is 157 kg of press-fit load and 357 kg of unloading load at room temperature. The unloading load at ℃ was 276 kg. In this press-fitting, the insulator 2 is press-fitted while supporting the seat surface 5 of the metal shell 1. Since the ground electrode 10 is joined to the front end of the metal shell 1 by a known method (see FIG. 1), the seat surface 5 is supported in order to perform press-fit without deforming the ground electrode 10. It is preferable to press fit.

  Further, the fitting portion 9 can ensure the required airtightness with the outside of the insulator 2. When airtightness was measured when a pressure of 1.55 MPa was applied from the inside with the spark plug 100 attached, the leak rate was about 0 ml / min at room temperature and about 1 ml / min at 200 ° C. It was found that airtightness equivalent to or better than spark plugs by caulking was secured. As described above, in the spark plug 100 according to the present embodiment, since the airtightness is ensured in the fitting portion 9, filling with talc powder or the like that serves as a seal for ensuring airtightness as in the past. Therefore, the structure can be simplified.

  In the above, the present invention has been described with reference to the embodiment. However, the present invention is not limited to the above-described embodiment and the like, and it is needless to say that the present invention can be appropriately modified and applied without departing from the gist thereof. For example, in addition to the L-shaped ground electrode 10 described in the present embodiment, a combination of a plurality of ground electrodes, and further, the tip of a metal shell, which is one of the so-called creeping discharge types, also serves as a spark discharge electrode. It may be a type.

  Moreover, it is preferable that the length of the part which contacts an insulator in a fitting part shall be 1 mm or more. However, if the length is too long, an excessive press-fitting load is required. From the viewpoint of production, it is preferable to set the inner diameter of the fitting portion as the upper limit. Further, the protruding portion that is the main point of the present invention is not limited to the above embodiment. In the above-described embodiment, the projecting portion has an annular shape. However, for example, a radial slit may be provided in the projecting portion to facilitate press-fitting. In addition, when comprised in this way, it cannot be overemphasized that it needs to have a part which hold | maintains airtight in the rear end side rather than a protrusion part.

The figure which expands and shows the principal part cross-section structure of the spark plug which concerns on embodiment of this invention. The figure which shows the external appearance structure of the whole spark plug of FIG. The figure which expands and shows the principal part cross-section structure of the spark plug of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Main metal fittings, 2 ... Insulator, 3 ... Center electrode, 4 ... Terminal metal fittings, 7 ... Screw part, 8 ... Tool engagement part, 9 ... Fitting part, 10 ... Ground electrode , 14 ... Projection, 100 ... Spark plug.

Claims (7)

A central electrode extending in the axial direction;
A cylindrical insulator holding the center electrode;
A cylindrical metal shell provided with a ground electrode for accommodating a part of the insulator and generating a spark discharge with the center electrode at the tip,
A spark plug comprising:
The metal shell is a protruding portion that protrudes toward the inside of the metal shell, and has a protruding base end and a protruding inner end that are different from each other in the axial direction, and the protruding inner end is in contact with the insulator. Having a protruding portion,
The projecting portion is located on the most distal end side in the axial direction among the abutting portions with which the metal shell and the insulator are abutted ,
The spark plug, wherein the protrusion and the insulator are in contact with each other by press-fitting . The insulator is attached to the engine with a large-diameter portion in which a portion near the rear end of the insulator is exposed from the metal shell, a small-diameter intermediate body portion smaller than the large-diameter portion, and a smaller diameter than the central body portion. And at least a part near its tip when exposed to combustion gas,
The metal shell has a large hole portion with an inner peripheral surface facing the large insulator diameter portion, a medium hole portion facing the insulator medium trunk portion, and a small hole portion facing the insulator leg long portion. ,
2. The spark plug according to claim 1, wherein the protruding portion is formed on the most rear end side of the small hole portion, and the middle hole portion and the small hole portion are divided by the protruding portion.   The spark plug according to claim 1, wherein a portion of the insulator that contacts the projecting inner end is parallel to an axis.   The spark plug according to claim 3, wherein a portion of the insulator that is adjacent to the distal end side of the portion that contacts the projecting inner end has an inclined surface that decreases in diameter toward the distal end.   The spark plug according to any one of claims 1 to 4, wherein the protruding portion has a shape whose thickness decreases from the protruding base end toward the protruding inner end.   The said metal shell is provided with the fitting part which hold | maintains the said insulator by either press fit, shrink fitting, or cold fitting at least in the rear end side rather than the said protrusion part. The spark plug according to item 1.   The insulator is held by press-fitting at the fitting portion, and a press-fit introduction portion is provided in which at least the tip side of the portion that contacts the fitting portion of the insulator is smaller in diameter than the rear end side. The spark plug according to claim 6, wherein:

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