JP2007059079A - Spark plug - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spark plug enabled to be further downsized without giving rise to increase or the like of possibility of generating screw-thread breakage due to over-clamping. <P>SOLUTION: A fitting center body part 6, a tool-engagement part 8, and an insertion-coupling part 9 are provided at a side further toward rear of a screw part 7 formed at a tip side of a main body fitting 1, and an insulator 2 with a center electrode 3 fitted inside is pressed into the insertion-coupling part 9 for retention. A slanted-face bearing surface 5 with its outer periphery side located further toward a tip than an inner periphery side is formed at the fitting center body part 6, and airtightness is maintained by having the bearing surface 5 directly in contact with the engine. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

  Conventionally, as a spark plug, a center electrode, an insulator holding the center electrode, and a metal shell having a ground electrode at the tip and a tool engaging portion for engine mounting are insulated in the metal shell. A structure in which an insulator is supported and fixed is known. Such a spark plug has a structure in which the insulator is supported and fixed in the metal shell by inserting the insulator into the cylindrical metal shell and crimping one end of the metal shell. It is general (see, for example, Patent Document 1).

  In the spark plug having the above-described structure, it is necessary to form a flange-shaped large diameter portion on the insulator in order to crimp and engage the crimped portion of the metal shell. For this reason, the maximum diameter of the spark plug cannot be reduced. Therefore, a spark plug has been proposed in which an insulator is supported and fixed to a metal shell by welding, adhesive bonding, shrink fitting, or the like (see, for example, Patent Document 2).

In general, an annular gasket (seal ring in Patent Document 2) or the like is attached to a portion where the spark plug is attached to an engine or the like to ensure airtightness. However, there has also been proposed a spark plug in which a seat surface that is a contact portion with the engine is a tapered surface that tapers and is brought into direct contact with the seat surface of the engine for airtightness (for example, Patent Document 3). reference.).
JP 2002-164147 A JP 2002-158078 A JP 2001-118659 A

  In the above-described conventional technology, the above-described conventional spark plug in which the metal shell and the insulator are fixed by caulking can ensure sufficient fixing strength and high reliability, but it is difficult to reduce the size. is there.

  In addition, the above-described conventional spark plug in which the metal shell and the insulator are fixed by welding, adhesive bonding, shrink fitting, etc., can be downsized to some extent, but vibration resistance and sufficient reliability of the joint are sufficient. It has not yet been put to practical use because it is difficult to ensure the properties. In addition, since a gasket or the like is attached to secure the airtightness of the attachment portion with the engine or the like, the diameter of the portion constituting the seating surface on which the gasket abuts is increased, so that the size can be reduced. There is a limit.

  Further, in the above-described conventional spark plug that does not use a gasket or the like, the tapered surface of the spark plug and the tapered surface formed at the spark plug attachment port portion of the engine or the like are brought into contact with each other. Therefore, it is difficult to obtain a tightening torque, and this tendency becomes remarkable particularly when a lubricant such as oil adheres. For this reason, there exists a problem that a screw break may generate | occur | produce by overtightening by further tightening.

  The present invention has been made to solve the above problems. An object of the present invention is to provide a spark plug that can be reduced in size as compared with the conventional one without causing an increase in the possibility of thread breakage due to overtightening.

  A spark plug according to the present invention includes a center electrode extending in the axial direction, a substantially cylindrical insulator that holds the center electrode, a screw portion provided on the tip side for engine mounting, and a screw portion of the screw portion. A tool engaging portion provided on the rear end side, a fitting portion in which the insulator is fitted and held by any one of press fitting, shrink fitting, and cold fitting, and a ground electrode provided at the tip portion, A spark plug comprising a substantially cylindrical metal shell, wherein the metal shell is in direct contact with the engine at the time of installation of the engine at least on the tip side of the tool engaging portion to maintain airtightness. It is characterized by comprising a metal shell inner body portion that forms an inclined seat surface that is a surface and whose outer peripheral side is located on the tip side from the inner peripheral side.

  In the spark plug of the present invention, the insulator is held in the fitting portion of the metal shell by any one of press fitting, shrink fitting, and cold fitting. Thereby, it is not necessary to provide the insulator with a large-diameter portion for engaging the caulking portion of the metal shell as in the prior art, and the maximum diameter of the spark plug can be reduced. Further, an inclined surface (for example, a reverse taper shape) seating surface in which the outer peripheral side formed in the metal shell middle part is located on the tip side from the inner peripheral side is brought into direct contact with the engine to maintain airtightness. As a result, the outer diameter of the inner middle portion of the metal fitting can be reduced, and the size can be further reduced. Further, by bringing the seating surface having such a shape into direct contact with the engine, even when a lubricant such as oil adheres, a tightening torque can be obtained, which increases the possibility of thread breakage due to excessive tightening. There is nothing.

  As the shape of the seating surface, when the seating surface is seen in a cross section including the axis, the angle between the line segment connecting the inner peripheral base point and the outer peripheral base point of the seat surface with respect to a straight line perpendicular to the axis However, it is preferable to set it as the shape made into 10-15 degree | times. Thereby, the maximum surface pressure can be increased and the airtightness can be increased.

  The outer diameter of the spark plug is such that the outer diameter of the threaded portion is 8 mm or less, the outer diameter of the metal fitting middle body portion is larger than the screw portion, and the minimum outer diameter of the tool engaging portion is the metal fitting middle body portion. The outer diameter is larger than 11 mm. Thereby, the outer diameter of the tool engaging portion is substantially the maximum diameter of the metal shell, and the maximum diameter of the entire spark plug. As a result, the overall spark plug can be reduced in size.

  As a means for holding the insulator in the fitting portion, press-fitting can be selected. In this case, it is preferable to provide a press-fitting introduction portion having a taper formed on the tip side of the press-fitting portion of the insulator. As a result, it is possible to manufacture in a simpler process and to secure a sufficient removal load.

  In the case of downsizing, the screw neck strength and the strength of the connecting portion with the fitting portion are required. For this reason, it is preferable that the Vickers hardness (value measured at a load of 10 N by a method defined in JIS Z2244 (1988)) is 310 or more. In addition, when the Vickers hardness is 500 or more, it becomes brittle and cracks in the metal fitting may occur after fitting. For this reason, it is preferable that at least the fitting portion of the metal shell has a Vickers hardness of 310 to 500.

  When a material with high hardness is used, if the wall thickness (outer diameter) of the fitting part of the metal shell is too large, the holding force of the fitting part becomes extremely high, so there is a high possibility that the insulator will crack. Become. On the other hand, when the minimum thickness is less than 0.25 mm, the productivity is deteriorated. For this reason, it is preferable that the outer diameter of the fitting portion of the metal shell is not more than the minimum outer diameter of the tool engaging portion, and the minimum thickness of the fitting portion is not less than 0.25 mm.

  According to a seventh aspect of the present invention, there is provided a spark plug comprising: a center electrode; a substantially cylindrical insulator that holds the center electrode; a screw portion provided on the front end side for engine attachment; and a rear end side of the screw portion. A tool engaging portion provided in the base, a fitting portion in which the insulator is fitted and held by any one of press fitting, shrink fitting, and cold fitting, and a ground electrode provided at the tip portion, and is substantially cylindrical. A spark plug provided with a metal fitting, and a plane perpendicular to the axial direction that is in direct contact with the engine when the engine is mounted and is kept airtight at the tip side of the tool engagement portion of the metal fitting. A metal fitting middle barrel portion that forms a seating surface, the outer diameter of the screw portion is 8 mm or less, the outer diameter of the metal fitting middle barrel portion is larger than the screw portion, and the tool engaging portion The minimum outer diameter is larger than the outer diameter of the middle part of the metal fitting and is 11 mm or less. I am characterized in.

  According to the spark plug, the insulator is held at the fitting portion of the metal shell by any one of press fitting, shrink fitting, and cold fitting. Thereby, it is not necessary to provide the insulator with a large-diameter portion for engaging the caulking portion of the metal shell as in the prior art, and the maximum diameter of the spark plug can be reduced. In addition, since the flat seat surface perpendicular to the axial direction formed in the metal shell middle part is brought into direct contact with the engine to maintain airtightness, the outer diameter of the metal shell middle part is reduced. Therefore, the size can be further reduced. As a result, the outer diameter of the threaded portion is 8 mm or less, the outer diameter of the metal fitting middle barrel portion is larger than the screw portion, and the minimum outer diameter of the tool engaging portion is larger than the outer diameter of the fitting middle drum portion. It becomes possible to set it as 11 mm or less. Further, by bringing the seating surface having such a shape into direct contact with the engine, even when a lubricant such as oil adheres, a tightening torque can be obtained, which increases the possibility of thread breakage due to excessive tightening. There is nothing.

  According to the spark plug of the present invention, it is possible to reduce the size of the spark plug as compared with the related art without causing an increase in the possibility of thread breakage due to excessive tightening.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a state before the insulator is assembled to the metal shell, and FIG. 2 shows the spark plug according to the embodiment of the present invention after the assembly. 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. A central electrode 3 is disposed along the axial direction of the central portion in the insulator 2, and a tip portion of the central electrode 3 protrudes from the insulator 2. 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 formed in a substantially cylindrical shape by a ceramic sintered body such as alumina or aluminum nitride, and has a through-hole for fitting the center electrode 3 along its own axial direction. . The terminal fitting 4 is inserted and fixed on one end side of the through hole, and the center electrode 3 is inserted and fixed on the other end side. A resistor 11 is disposed between the terminal fitting 4 and the center electrode 3 in the through hole. Both ends of the resistor 11 are electrically connected to the center electrode 3 and the terminal fitting 4 through the conductive glass seal layer.

  The metal shell 1 is formed in a cylindrical shape from a metal such as SUS630 (Vickers hardness 455), for example. The metal shell 1 constitutes the housing of the spark plug 100 and the outer peripheral surface on the tip side (the lower side in the figure) A threaded portion 7 for attaching the spark plug 100 to an engine block (not shown) is formed. A tool engaging portion 8 for engaging a tool such as a spanner or a wrench when the metal shell 1 is attached to the engine block is provided on the outer peripheral portion on the rear end side of the screw portion 7.

  On the rear end side of the screw portion 7, the metal shell middle portion 6 is provided on the front end side of the tool engaging portion 8, that is, between the screw portion 7 and the tool engaging portion 8. A surface on the front end side (lower side in the drawing) of the metal shell middle body 6 is a seat surface 5 that is in direct contact with the engine (engine) and maintains airtightness. As shown in an enlarged view in FIG. 4, the seat surface 5 is an inclined surface (reverse tapered surface) whose outer peripheral side is positioned on the tip side from the inner peripheral side. In this seating surface 5, a reverse taper angle (when the seating surface 5 is viewed in a cross section including the axis line, a line segment connecting the inner peripheral side base point and the outer peripheral side base point of the seat surface 5 forms a straight line perpendicular to the axis line. The included angle (angle θ shown in FIG. 4) affects the surface pressure when the spark plug 100 is attached to the engine. These relationships are shown in FIG. 3 where the vertical axis represents the maximum surface pressure and the horizontal axis represents the reverse taper angle. As shown in FIG. 3, when the reverse taper angle is in the range of 10 to 15 degrees, the maximum surface pressure is higher than in other angles. For this reason, when the seat surface 5 is a reverse tapered surface, it is preferable that the reverse taper angle is in the range of 10 to 15 degrees from the viewpoint of increasing the surface pressure and improving the airtightness. The seat surface 5 is not limited to the reverse tapered surface, and may be an inclined surface whose outer peripheral side is positioned on the tip side from the inner peripheral side, and may be a curved R surface as shown in FIG. 5, for example. As shown in FIG. 6, the seat surface 5 is provided on the distal end side (lower side in the drawing) of the tool engaging portion 8, and the metal fitting middle body portion 6 provided separately from the tool engaging portion 8 is not provided. It may be the case. In this case, the tool engaging portion 8 can be substantially regarded as a metal fitting middle barrel portion, and as shown in FIG. 2, the metal fitting middle drum portion 6 is not provided in a form independent of the tool engaging portion 8. There is no problem. That is, the seat surface 5 only needs to have a form in which the outer peripheral side is positioned on the tip side from the inner peripheral side, and the metal fittings that form the seat surface 5 on the inner side of the minimum diameter portion of the tool engaging portion 8 as shown in FIG. It can be said that there should be a torso.

  On the other hand, a fitting portion 9 is provided on the rear end side from the tool engaging portion 8. The fitting portion 9 is for fitting and holding the insulator 2. In this embodiment, the fitting portion 9 is fitted and held by press-fitting the insulator 2. . In this manner, by providing the fitting portion 9 on the rear end side with respect to the tool engaging portion 8, the fitting is performed when the tool is engaged with the tool engaging portion 8 and the spark plug 100 is fastened to the engine block. It is possible to prevent torsional torque and axial force from being applied to the joint portion 9 and improve the reliability of the joint portion (fitting holding) in the fitting portion 9. That is, even if the spark plug 100 is repeatedly attached to and removed from the engine block, the torsion torque and the axial force are not applied to the fitting portion 9, so that the coupling state with the insulator 2 is loosened. It does not occur. 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. In this press-fitting, the insulator 2 is press-fitted while supporting the seat surface 5 of the metal shell 1. Since the metal shell 1 has a ground electrode 10 joined to the tip thereof by a known manufacturing method (see FIG. 1), in order to press fit without deforming the ground electrode 10, the seat surface 5 is supported. It is desirable to press fit.

  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, it can prevent that such a malfunction arises by providing in the back end side rather than the tool engaging part 8. FIG.

  On the other hand, as shown in FIG. 1, the insulator 2 has a small diameter portion 21, a medium diameter portion 22, and a large diameter portion 23 in order from the distal end side. A taper of a predetermined angle is formed at the end of the large diameter portion 23 on the middle diameter portion 22 side, and serves as a press-in introduction portion 24 for press-fitting into the fitting portion 9 of the metal shell 1.

  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 is engaged as in the conventional case. It is not necessary to provide a large-diameter portion in the insulator 2, and the maximum diameter of the spark plug 100 can be reduced. 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.

  In addition, as described above, in this embodiment, the inclined seat surface 5 with the outer peripheral side formed on the metal shell inner barrel portion 6 positioned on the tip side from the inner peripheral side is brought into direct contact with the engine to maintain airtightness. Therefore, it is not necessary to provide a large-diameter portion for sandwiching and pressing a gasket or the like, and the outer diameter of the metal shell inner barrel portion 6 can be reduced. For this reason, further miniaturization can be achieved. Further, by bringing the seating surface having such a shape into direct contact with the engine, even when a lubricant such as oil adheres, a tightening torque can be obtained, which increases the possibility of thread breakage due to excessive tightening. There is nothing. For comparison, FIG. 8 shows a configuration of a conventional spark plug 200. In the spark plug 200, the insulator 202 is supported by caulking by a caulking portion 209 at the rear end of the metal shell 201, and on the seat surface 205 side provided on the front end side of the metal shell inner body 206. A gasket 220 is provided. In order to hold the gasket 220 between the seating surface 205 and the abutting surface of the engine and press the gasket 220 to maintain airtightness, the metal shell middle body portion 206 has a large diameter. In FIG. 8, 204 is a terminal fitting, 207 is a threaded portion, 208 is a tool engaging portion, and 210 is a ground electrode.

  In the present embodiment, the outer diameter of the threaded portion 7 is 8 mm, the outer diameter of the metal fitting middle barrel portion 6 is larger than that of the screw portion 7, and the minimum outer diameter of the tool engaging portion 8 is larger than the outer diameter of the metal fitting middle barrel portion 7. The maximum is 11 mm. Accordingly, the outer diameter of the tool engaging portion 8 is substantially the maximum diameter of the metal shell 1 and the maximum diameter of the entire spark plug. Thereby, the maximum diameter of the spark plug 100 can be reduced and the size can be reduced. Thereby, the diameter of the hole for attaching the spark plug 100 provided in the engine block can be reduced, and the degree of freedom in engine design can be increased.

  When the spark plug 100 is downsized as described above, the screw neck strength and the strength of the joint portion with the fitting portion are required. For this reason, it is preferable that the metal shell 1, particularly the fitting portion 9, be configured so that the Vickers hardness (value measured at a load of 10 N by a method prescribed in JIS Z2244 (1988)) is 310 or more. Moreover, when the Vickers hardness becomes 500 or more, it becomes brittle, and the metal fitting crack may occur after the insulator 2 is press-fitted into the fitting portion 9. For this reason, it is preferable that at least the fitting portion 9 of the metal shell 1 is configured to have a Vickers hardness of 310 to 500.

  Further, when a material having high hardness such as Vickers hardness of 310 to 500 is used as described above, if the thickness (T shown in FIG. 1) of the fitting portion 9 of the metal shell 1 is too large, the fitting portion Since the holding power of 9 becomes extremely high, there is a high possibility that the insulator 2 will be cracked. On the other hand, when the minimum thickness is less than 0.25 mm, the productivity is deteriorated. For this reason, it is preferable that the outer diameter of the fitting portion 9 of the metal shell 1 is set to be equal to or smaller than the minimum outer diameter of the tool engaging portion 8 and the minimum thickness of the fitting portion 9 is set to 0.25 mm or more. Moreover, it is preferable that the length of the part which contacts the insulator 2 in the fitting part 9 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 9 as the upper limit.

  By the way, as above-mentioned, a thing with high hardness becomes difficult to process. Therefore, when processing, process with relatively good workability (low hardness), finish the dimensions roughly (may be finished dimensions), adjust the hardness by quenching, tempering or precipitation hardening, and then Finishing with regular dimensions improves efficiency. Moreover, when manufacturing the metal shell 1 by plastic working such as cold forging, the material before cold forging is in a low hardness state, and at the same time making the shape by plastic working, using the work hardening at that time, The method of adjusting the shape and hardness when cold forging is complete is also efficient.

  When the insulator 2 is press-fitted into the metal shell 1, if a lubricant is used, the press-fitting load can be reduced. Then, after completion of the press-fitting, if the lubricant is decomposed by heat treatment, the bondability can be improved. As such a lubricant, for example, Paskin M30 (trade name), cellosol (trade name) or the like can be used.

  Moreover, in the spark plug 100 of this embodiment, after the insulator 2 is press-fitted into the metal shell 1, heat treatment is performed. Thus, if the heat treatment is performed after press-fitting, it is possible to increase the removal load. This is considered to be because the press-fitted portion that was in the point contact state at the time of press-fitting becomes a surface contact state by heat treatment. The heat treatment is preferably performed, for example, in the atmosphere at a temperature of 300 ° C. for about 15 minutes. When such heat treatment after press-fitting is not performed, the press-fitting load and the removal load are substantially the same. However, when heat treatment as described above is performed, for example, in the case of a spark plug having a threaded portion with a diameter of 8 mm, an example of data measured actually is 157 kg of press-fit load and 357 kg of load at room temperature. The unloading load at 200 ° C. was 276 kg.

  Furthermore, in the spark plug 100 of this embodiment, the required airtightness can be ensured between the inner surface of the fitting portion 9 and the outer side 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, talc powder or the like serving as a seal for securing the airtightness is filled as in the prior art. Therefore, the structure can be simplified.

  FIG. 7 shows a configuration of a spark plug 110 according to another embodiment. In the spark plug 110, the seat surface 50 of the metal shell middle section 6 is formed in a plane shape perpendicular to the axial direction, and the engine (engine). At the time of installation, the engine and the planar seating surface 50 are brought into direct contact with each other so as to maintain airtightness. Further, in the spark plug 110 of this embodiment, the outer diameter of the screw portion 7 is 8 mm or less, the outer diameter of the metal fitting middle body portion 6 is larger than that of the screw portion 7, and the minimum outer diameter of the tool engaging portion 8 is the middle of the metal fitting. The outer diameter of the body 6 is larger than 11 mm.

  According to the spark plug 110 having the above-described configuration, the same effects as those of the above-described embodiment can be obtained, and since the seating surface 50 is planar, the processing can be performed relatively easily, and the manufacturing process can be simplified. Can be planned.

  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.

The figure which shows the state before the press injection of the spark plug which concerns on embodiment of this invention. The figure which shows the state after the press injection of the spark plug of FIG. The figure which shows the relationship between a reverse taper angle and the maximum surface pressure. Sectional drawing which expands and shows the principal part structure of the spark plug of FIG. Sectional drawing which expands and shows the principal part structure of the modification of the spark plug of FIG. The partially cutaway sectional view which expands and shows the principal part composition of the modification of the spark plug of an embodiment. The figure which shows the state before the press injection of the spark plug which concerns on other embodiment. The figure which shows the structure of the conventional spark plug.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Main metal fittings, 2 ... Insulator, 3 ... Center electrode, 5 ... Bearing surface, 6 ... Middle body part of metal fittings, 7 ... Screw part, 8 ... Tool engagement part, 9 ... Fit Joint part, 100 ... Spark plug.

Claims (7)

A central electrode extending in the axial direction;
A substantially cylindrical insulator holding the center electrode;
The screw part for engine attachment provided on the front end side, the tool engagement part provided on the rear end side of the screw part, and the insulator are fitted and held by press fitting, shrink fitting, or cold fitting A spark plug having a fitting portion and a ground electrode provided at the tip portion, and having a substantially cylindrical metal shell,
The metallic shell is a seating surface that is in direct contact with the engine and keeps hermeticity at the time of mounting the engine at least on the tip side of the tool engaging portion, and the outer peripheral side is located on the tip side from the inner peripheral side. A spark plug, comprising: a metal barrel portion forming a cylindrical seating surface. The spark plug according to claim 1, wherein
When the seating surface is viewed in a cross section including the axis, the included angle formed by a line segment connecting the inner peripheral base point and the outer peripheral base point of the seat surface with respect to a straight line perpendicular to the axis is 10 to 15 degrees. A spark plug characterized by The spark plug according to claim 1 or 2,
The outer diameter of the screw portion is 8 mm or less, the outer diameter of the metal shell middle portion is larger than the screw portion, and the minimum outer diameter of the tool engaging portion is larger than the outer diameter of the metal fitting middle drum portion. A spark plug characterized by being 11 mm or less. In the spark plug according to any one of claims 1 to 3,
A spark plug, wherein the insulator is held by press fitting at the fitting portion. In the spark plug according to any one of claims 1 to 4,
A spark plug characterized in that at least the fitting portion of the metal shell has a Vickers hardness of 310 to 500. In the spark plug according to any one of claims 1 to 5,
The spark plug according to claim 1, wherein an outer diameter of the fitting portion of the metal shell is less than or equal to a minimum outer diameter of the tool engaging portion, and a thickness of the fitting portion is 0.25 mm or more. A center electrode;
A substantially cylindrical insulator holding the center electrode;
The screw part for engine attachment provided on the front end side, the tool engagement part provided on the rear end side of the screw part, and the insulator are fitted and held by press fitting, shrink fitting, or cold fitting A spark plug having a fitting portion and a ground electrode provided at the tip portion, and having a substantially cylindrical metal shell,
At the tip side of the tool engaging portion of the metal shell, a metal fitting middle body portion is provided that forms a flat seat surface perpendicular to the axial direction that is in direct contact with the engine and maintains airtightness when the engine is mounted. ,
The outer diameter of the screw portion is 8 mm or less, the outer diameter of the metal shell middle portion is larger than the screw portion, and the minimum outer diameter of the tool engaging portion is larger than the outer diameter of the metal fitting middle drum portion. A spark plug characterized by being 11 mm or less.

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