JP5096546B2 - Spark plug - Google Patents

Description

  The present invention relates to a spark plug for an internal combustion engine.

  Conventionally, spark plugs for ignition are used in internal combustion engines. A general spark plug has a metal shell for holding an insulator with a center electrode inserted therein, and a ground electrode welded to the front end surface of the metal shell, and the other end of the ground electrode, A spark discharge gap is formed facing the tip of the center electrode. Then, a spark discharge is performed between the center electrode and the ground electrode. In order to assemble such a spark plug in an internal combustion engine, normally, a male screw provided on the outer peripheral surface of the metal shell is screwed into a female screw provided on the engine head.

  In such a spark plug, there is a risk that screw loosening may occur due to heat or vibration transmitted from the engine. Therefore, in the conventional spark plug, the initial diameter is increased by increasing the effective diameter of the screw or by providing a washer (gasket) with high spring performance between the seat surface of the engine head and the seat surface of the metal shell. By improving the axial force, screw loosening was prevented. The “effective diameter” refers to a virtual cylindrical or conical diameter such that the width of the thread of the male screw portion is equal to the width of the thread groove.

  By the way, the temperature in the combustion chamber tends to rise as the engine output of automobiles in recent years increases. The spark plug attached to such an engine is subjected to a larger heat load than before, so that it is necessary to sufficiently dissipate the electrode portion in order to ensure the life of the spark plug. For this purpose, it is effective to enlarge the water jacket of the engine head to increase the heat dissipation efficiency from the spark plug. In accordance with this, the screw reach (length of the male thread) of the metal shell that engages the engine head is increased. A spark plug is used (see, for example, Patent Document 1). When such a spark plug is attached to the engine head, the following forces are acting on the joint surface (mutual thread portion) between the metal shell and the engine head.

  In the metal shell, when the thread of the male thread part is screwed with the female thread part of the engine head, a force in the tightening direction acts on the inside with the seat surface or gasket as a base point. This force is a state in which stress is exerted as a surface pressure between the screw threads in the respective thread portions of the metal shell and the engine head. Before starting the engine with the spark plug attached to the engine head, that is, when the engine is cold, the surface pressure is higher than the front side of the threaded portion (outer surface side opposite to the combustion chamber side in the engine head) from the combustion chamber. On the side, it works a lot.

Japanese Patent Laid-Open No. 11-273727

  However, when the engine is driven, the combustion chamber on the tip side of the threaded portion becomes high temperature, so that the metal shell and the engine head have a heat distribution from high temperature to low temperature from the combustion chamber side to the base portion side (this is It is called hot time.) As the temperature of the combustion chamber rises, the metal shell and the engine head thermally expand based on the coefficient of thermal expansion of each material. At this time, an excessive surface pressure is generated on the tip end side of the threaded portion due to the difference in thermal expansion coefficient between the two. Since this excessive surface pressure places a load on the threaded portion, plastic deformation accompanied by creep may occur on the combustion chamber side of the threaded portion. This deformation may occur repeatedly every time the engine is driven, that is, due to the cold heat of the engine. As a result, when the threaded portion is plastically deformed, the surface pressure at the deformed portion is lowered, and there is a possibility that the screw is liable to be loosened. In addition, by repeatedly causing deformation, there is a possibility that the deformation that has occurred only on the combustion chamber side of the screw portion may gradually progress to the base portion, and consequently the screw pitch between the metal shell and the engine head will be different. There is also a concern that the spark plug for repair cannot be used. The “thread portion” is a name obtained by integrating both a male screw formed on the metal shell and a female screw formed on the engine head.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a spark plug capable of preventing loosening of a metal shell screwed into an engine head.

In order to achieve the above object, a spark plug according to a first aspect of the present invention includes a center electrode having a tip portion as a firing portion, and an axial hole extending in an axial direction of the central electrode. An insulator that holds the center electrode inside, and a cylindrical body that surrounds and holds the periphery of the insulator in the radial direction, and is formed on the outer periphery on the tip side of the seating surface for mounting on the engine head of an internal combustion engine A spark plug comprising a metal shell having a male threaded portion that is screwed into a mounting hole of the engine head, wherein the male threaded portion is a portion on the distal end side in the mounting direction of the metal shell to the engine head. It is composed of a front part, a base part which is a part on the rear end side, and a body part which is a part between the front part and the base part, and the body part has a part where the effective diameter is maximum. and, the effective diameter of the outermost The effective diameter of the portion ranging the tip portion from the site where the, characterized in that it is formed smaller than the effective diameter of the portion to be the maximum.

The spark plug according to claim 2, in addition to the configuration of the invention according to claim 1, the pre-Symbol externally threaded portion, and further, effective from the site where the effective diameter is the largest portion ranging the root portions The diameter is smaller than the effective diameter of the maximum portion.
According to a third aspect of the present invention, in addition to the configuration of the first or second aspect of the invention, the spark plug of the invention has a configuration in which the effective diameter of the portion where the effective diameter of the male screw portion is maximized is the attachment of the engine head. It is substantially the same as the effective diameter of the hole, and when the metal shell is attached to the engine head, the slope of the screw thread of the male screw portion is the portion of the mounting hole at the portion where the effective diameter of the male screw portion is maximum. In the part where the effective diameter of the male threaded portion is minimized, the threaded slope of the male threaded portion is in contact with the threaded slope of the mounting hole. It is characterized in that a part is in contact.

  In the spark plug of the invention according to claim 1, it has a portion where the effective diameter of the male screw portion is maximum in the body portion or the base portion, and has a portion where the effective diameter is minimum in at least the front portion, When the metal shell is screwed into the mounting hole of the engine head, the contact area between the screw threads can be increased in the body portion or the base portion rather than the tip portion, thereby increasing the surface pressure. Since the effective diameter has been increased on the base side, even when the engine is running hot, the heat from the combustion chamber also escapes to the engine head before reaching the base. Similarly, it is possible to obtain a surface pressure by tightening. In addition, since the effective diameter is reduced at the tip (combustion chamber side), the surface pressure applied to the internal thread of the engine head can be reduced, and plastic deformation accompanying creep of the internal thread can be reduced. It becomes. Since the surface pressure at the tip of the threaded portion is reduced, the surface pressure applied during hot operation can be reduced compared to the conventional one, and in addition to being able to reduce deformation at the tip, deformation that proceeds to the base portion Can also be suppressed. Thus, since the deformation of the screw portion is suppressed while maintaining the surface pressure at the base portion, it is possible to prevent the occurrence of screw loosening.

1 is a partial cross-sectional view of a spark plug 100. FIG. It is the side view of the spark plug 100 which expanded the external thread part 52 vicinity. FIG. 6 is a cross-sectional view showing a state in which the base portion 522 of the male screw portion 52 is screwed with the female screw portion 202 of the engine head 200. FIG. 6 is a cross-sectional view showing a state of screwing with a female screw portion 202 of an engine head 200 at a tip portion 521 of a male screw portion 52. It is a schematic diagram which shows the external shape of a drum-type metal shell. It is a schematic diagram which shows the external shape of a straight type, a taper type, and a thick metal shell. It is a graph which shows the relationship between the distance (maximum diameter distance) from the rear end of an external thread part to the position where an effective diameter becomes the maximum, and a return torque.

  Hereinafter, an embodiment of a spark plug embodying the present invention will be described with reference to the drawings. First, the structure of a spark plug 100 will be described with reference to FIG. 1 as an example of the spark plug according to the present invention. FIG. 1 is a partial cross-sectional view of a spark plug 100. In the direction of the axis O shown in FIG. 1, the side on which the center electrode 20 is provided is described as the front end side (front side) of the spark plug 100, and the side on which the connection terminal 40 is provided is described as the rear end side (rear side). To do.

  As shown in FIG. 1, the spark plug 100 generally includes an insulator 10, a metal shell 50 that holds the insulator 10, a center electrode 20 that is held in the shaft hole 12 of the insulator 10, and a metal shell. The ground electrode 30 is joined to the front end portion 31 of the center electrode 20 so as to be opposed to the front end portion 22 of the center electrode 20, and the connection terminal 40 is provided on the rear end side of the insulator 10.

  First, the insulator 10 of the spark plug 100 will be described. The insulator 10 is a cylindrical insulating member that is formed by firing alumina or the like and has an axial hole 12 in the direction of the axis O as is well known. A flange portion 19 having the largest outer diameter is formed substantially at the center in the direction of the axis O, and a rear end side body portion 18 is formed on the rear end side. Further, a corrugation portion 16 for increasing a creepage distance is formed further on the rear end side than the rear end side body portion 18. A front end side body portion 17 having an outer diameter smaller than that of the rear end side body portion 18 is formed on the front end side from the flange portion 19, and the outer diameter of the front end side body portion 17 is further closer to the front end side than the front end side body portion 17. A small leg length portion 13 is formed. The long leg portion 13 is reduced in diameter toward the distal end side, and is exposed to the combustion chamber when the spark plug 100 is assembled to an internal combustion engine (not shown).

  Next, the center electrode 20 will be described. In the center electrode 20, a core material 23 made of copper or copper alloy for radiating heat is embedded in the center of an electrode base material made of nickel-based alloy such as Inconel (trade name) 600 or 601. It is a rod-shaped electrode. The distal end portion 22 of the center electrode 20 protrudes from the distal end surface of the insulator 10 and is formed so as to become smaller in diameter toward the distal end side. A noble metal tip 90 made of a noble metal for improving spark wear resistance is joined to the distal end surface of the distal end portion 22.

  The center electrode 20 is electrically connected to a connection terminal 40 held on the rear end side of the shaft hole 12 via a sealing material 14 provided in the shaft hole 12 and the ceramic resistor 3. The rear end portion 42 of the connection terminal 40 is exposed from the rear end of the insulator 10, and a high voltage cable (not shown) is connected to the rear end portion 42 via a plug cap (not shown), and a high voltage is applied. It has become so.

  Next, the ground electrode 30 will be described. The ground electrode 30 is made of a metal having high corrosion resistance. As an example, a nickel alloy such as Inconel (trade name) 600 or 601 is used. The ground electrode 30 has a substantially rectangular cross section perpendicular to the longitudinal direction of the ground electrode 30, and has a bent rectangular bar-like outer shape. The base 32 on the base end side in the shape of a square bar is joined to the front end surface 57 on the front end side of the metal shell 50 by resistance welding. On the other hand, the tip 31 of the ground electrode 30 opposite to the base 32 is bent so as to face the tip 22 of the center electrode 20. The inner surface 33 of the ground electrode 30, which is the surface facing the center electrode 20, is substantially orthogonal to the axis O, and the noble metal tip similar to the noble metal tip 90 is located at a position where the noble metal tip 90 of the center electrode 20 faces. 91 is resistance welded. A spark discharge gap is formed between the noble metal tip 91 and the noble metal tip 90.

  Next, the metal shell 50 will be described. The metal shell 50 is for holding the insulator 10 and fixing the spark plug 100 to an internal combustion engine (not shown). The metal shell 50 holds the insulator 10 so as to surround the flange portion 19, the distal end side trunk portion 17, and the leg length portion 13 from the rear end side barrel portion 18 in the vicinity of the flange portion 19 of the insulator 10. The metal shell 50 is formed of a low carbon steel material, and a tool engaging portion 51 into which a spark plug wrench (not shown) is fitted, and a female screw portion 202 (FIGS. 3 and 4) formed on the engine head 200 of the internal combustion engine (not shown). And a male threaded portion 52 that is screwed onto (see).

  Further, the metal shell 50 has a caulking portion 53 on the rear end side of the tool engaging portion 51. By caulking the caulking portion 53, a step 56 formed on the inner periphery of the metal shell 50 is provided. The step portion 15 between the front end side body portion 17 and the leg long portion 13 of the insulator 10 is supported via the plate packing 8, and the metal shell 50 and the insulator 10 are integrated. In order to complete sealing by caulking, an annular shape is formed between the inner peripheral surface in the vicinity of the caulking portion 53 of the metal shell 50 and the outer peripheral surface of the rear end side body portion 18 in the vicinity of the flange portion 19 of the insulator 10. Ring members 6 and 7 are interposed, and talc (talc) 9 powder is filled between the ring members 6 and 7. A flange 54 is formed at the center of the metal shell 50, and the gasket 5 is fitted into the vicinity of the rear end side (upper part in FIG. 1) of the male screw 52, that is, the seat surface 55 of the flange 54. Yes.

  Here, as shown in FIG. 2, in the male thread portion 52 of the metal shell 50, a portion located on the front end side (the front end surface 57 side of the main metal shell 50) of the spark plug 100 in the direction of the axis O is the front portion 521 and the rear end side. A portion located on the seat surface 55 side of the metal shell 50 is referred to as a base portion 522, and a portion located between the tip portion 521 and the base portion 522 is referred to as a trunk portion 523. More specifically, the tip portion 521 indicates a portion from the starting point of the thread on the tip side in the direction of the axis O (thread formation start position) to the fifth thread, and the base portion 522 is the rear end. The part from the starting point of the side thread to the fifth thread. In the spark plug 100 according to the present embodiment, the shape of the virtual cylindrical body constituted by the effective diameter of the male screw portion 52 becomes a tapered shape that gradually becomes thinner from the base portion 522 to the tip portion 521 through the body portion 523. It is configured as follows. That is, the effective diameter B at the base portion 522 is configured to be larger than the effective diameter A at the tip portion 521 of the male screw portion 52.

  In FIG. 2, for the sake of explanation, the difference between the effective diameter A of the front portion 521 and the effective diameter B of the base portion 522 is illustrated, but in this embodiment, the effective diameter A and the effective diameter B are illustrated. When the difference from the diameter B is d and the screw reach (the length between both starting points of the threads of the male screw portion 52 in the direction of the axis O) is L, (d / L) × 100 is 0.8 (%) or more. It is desirable to be 1.97 (%) or less. When (d / L) × 100 is larger than 1.97 (%), a gap is generated between the male threaded portion 52 of the metal shell 50 and the female threaded portion 202 (see FIG. 3) of the engine head 200, and the center electrode 20. In addition, it becomes difficult to efficiently release the heat of the ground electrode 30 to the engine head 200 through the metal shell 50, and the durability of the spark plug 100 is lowered. Also, if (d / L) × 100 is less than 0.8 (%), the effective diameter of the base part 522 cannot be made sufficiently larger than the effective diameter of the front part 521, and the engine head 200 and the spark plug It is difficult to reliably maintain the screw engagement with 100 on the base portion 522 side, and based on the result of an evaluation test described later, it is not possible to effectively prevent the occurrence of screw loosening.

  Such a spark plug 100 is assembled to an engine head 200 in which a female screw portion 202 (see FIG. 3) having an effective diameter D substantially the same as the effective diameter B of the base portion 522 of the male screw portion 52 of the metal shell 50 is formed. In this case, as shown in FIG. 3, in the base portion 522 of the male screw portion 52, the threaded slope 525 of the base portion 522 abuts on the substantially entire surface of the threaded slope 205 of the female screw portion 202. On the other hand, as shown in FIG. 4, since the effective diameter A is smaller than the effective diameter D at the tip 521 of the male screw portion 52, the thread slope 526 of the tip 521 is in contrast to the thread slope 206 of the female thread 202. A part of them will come into contact.

  By the way, the spark plug 100 is heated by the heat generated when the internal combustion engine is driven, and this heat is released to the engine head 200 through the metal shell 50. Since heat from the combustion gas directly flows into the metal shell 50 itself, in the male screw part 52, the temperature difference due to the cooling cycle is larger in the tip part 521 than in the base part 522. Therefore, due to the difference in thermal expansion between the metal shell 50 and the engine head 200, the screw thread of the female screw portion 202 of the engine head 200 and the screw thread of the male screw portion 52 of the metal shell 50 in contact with the female screw portion 202 are accompanied by creep. Plastic deformation (creep deformation) is likely to occur. However, in the thread portion of the male screw portion 52 and the thread portion of the female screw portion 202 of the engine head 200 that abuts on the base portion 522, the creep cycle is less affected by the cooling cycle farther from the combustion chamber than the front portion 521 side. Hard to occur. In the spark plug 100 according to the present embodiment, the thread on the base part 522 side rather than the front part 521 side makes contact with the thread on the female thread part 202 of the engine head 200 with a smaller contact area. On the side of the base portion 522 having a large area, the surface pressure is maintained, and the occurrence of deformation of the screw portions 52 and 202 is suppressed, thereby preventing the occurrence of screw loosening.

  However, in order to obtain the effect of preventing such screw loosening, the shape of the virtual cylindrical body constituted by the effective diameter of the male threaded portion 52 is not necessarily tapered from the base portion 522 toward the tip portion 521. There is no need. In other words, if the position where the effective diameter is maximized is not the tip portion 521 of the male screw portion 52 but the trunk portion 523 or the base portion 522, the effect of the cooling cycle is small, and the same effect as described above can be obtained. For this purpose, the shape of the virtual cylindrical body composed of the effective diameter of the male screw portion 52 may be a tapered shape that tapers from the base portion 522 toward the tip portion 521 as in the present embodiment, At least the tip portion 521 may have a shape having the smallest effective diameter, and the base portion 522 and the body portion 523 may have a shape having the largest effective diameter. In the present embodiment, based on the result of an evaluation test described later, the effective diameter is at a position within 20 mm in the direction of the axis O from the rear end of the male screw portion 52, that is, the starting point of the screw thread on the rear end side of the male screw portion 52. The largest part is formed.

  Thus, in order to confirm the effect by having prescribed | regulated the site | part in which the effective diameter of the external thread part 52 of the metal shell 50 becomes the maximum, the following evaluation test was done.

[Example 1]
In this evaluation test, as shown in FIG. 5, the screw reach of the metal shell (the length of the male screw portion in the direction of the axis O) is L, and the distance from the seat surface of the collar portion to the position where the effective diameter is maximized (maximum diameter). A sample of a plurality of metal shells (drum type) having a distance) a and a different combination of the screw reach L and the maximum diameter distance a was produced. Each of these samples was assembled into an aluminum bush made of an aluminum material, and after applying a thermal load, the return torque (torque required for removing the metal shell) was measured and evaluated.

  The aluminum bush was manufactured by forming an internal thread portion having an effective diameter of M12 internal thread of the spark plug described in JIS B8031 (median value: 11.278 mm ± 0.02 mm) by NC processing on an aluminum bar material. . The metal shells with screw reach L of 12.7 mm, 19.0 mm, 26.5 mm, 34.0 mm are prepared, and the maximum diameter distance a of each male screw part is 2.5 mm, 5 mm, 10 mm, 15 mm, Threads were formed with possible combinations of 20 mm, 25 mm, and 30 mm. The effective diameter of the male thread portion of each sample is set to 11.125 mm with the maximum diameter (effective diameter at the distance a) so that it is within the range of the M12 male thread effective diameter of the spark plug described in JIS B8031. It was set as 10.993 mm so that the front part of the part might become the minimum in an external thread part. In addition, each sample produced in this way measured the effective diameter using the well-known three-needle method, and confirmed whether each said value was satisfy | filled.

  As a comparative example, the screw reach L shown in FIG. 6 is 26.5 mm, and the effective diameter from the base part to the tip part is tapered (tapered type), thicker (taped type), A non-straight (straight type) metal shell was produced. The tapered metal shell is threaded so that the effective diameter of the first thread of the first thread of the male thread is 11.05 mm and the effective diameter of the first thread of the leading thread is 11.99 mm. Formed. The tip-shaped metal shell was formed with a thread so that the effective diameter of the first thread of the first part was 11.99 mm, and the effective diameter of the first thread of the first part was 11.05 mm. . The straight metal shell was threaded so that the effective diameter of the first thread of each of the base part and the tip part was 11.05 mm (see Table 1).

  A copper washer was provided on the seating surface of each sample, and each sample was assembled and fixed to the aluminum bush with a predetermined tightening torque. Next, these samples were put in a heater at 200 ° C., heated from room temperature to 200 ° C. over 1 hour, then heated at 200 ° C. for 10 hours, and cooled down to room temperature over 5 hours. And about each sample, the return torque at the time of removing a metal shell from an aluminum bush was measured. FIG. 7 shows a graph of the results of this evaluation test.

  As shown in FIG. 7, in any metal shell having a screw reach L of 12.7, 19.0, 26.5, 34.0 (mm), if the maximum diameter distance a is less than 15 mm, the return torque is A value greater than 12.5 N · m was exhibited. Further, if the maximum diameter distance a is less than 20 mm, 5 N · m as the limit value of the return torque that may cause problems such as blowout of combustion gas (combustion due to loosening of the screw when the spark plug is assembled and driven to the internal combustion engine) It is a return torque that does not cause pressure blow-off, and has been confirmed to be about 5 N · m by a preliminary test conducted separately from this evaluation test. On the other hand, in each of the tapered, straight, and thick metal shells as comparative examples, the return torques were 16.0, 13.0, and 3.0 (N · m), respectively (see Table 1). In addition, the metal shell as this comparative example used that whose screw reach L is 26.5 mm, and measured the return torque.

この評価試験の結果、主体金具の雄ねじ部には、その元部で鍔部の座面から２０ｍｍ以内の部位に有効径が最大となる部位が形成されるように、ねじ山を形成すれば、少なくとも上記限界値より大きい戻しトルクを得られ、冷熱サイクルによるねじ緩みを防止することができることがわかった。また、鍔部の座面から１５ｍｍ以内の部位に有効径が最大となる部位が形成されるようにねじ山を形成すれば、ストレート型よりも大きな戻しトルクを得ることができ望ましい。さらには、有効径が最大となる部位が鍔部に近づくほど、より大きな戻しトルクを得られることがわかった。すなわち、スパークプラグの雄ねじ部の有効径からなる仮想円筒の形状は、図６に示す先細型、図５に示す太鼓型、図６に示すストレート型、そして先太型の順に、戻しトルクが小さくなるが、いずれも上記限界値よりは大きく、ねじ緩みを防止できることが分かった。そして先太型のスパークプラグでは、上記限界値よりも戻しトルクが小さくなり、ねじ緩みが発生してしまう虞があることが分かった。

As a result of this evaluation test, if the screw thread is formed in the male screw part of the metal shell so that the part having the maximum effective diameter is formed in the part within 20 mm from the seating surface of the collar part at the base part, It has been found that a return torque greater than at least the above limit value can be obtained, and screw loosening due to the thermal cycle can be prevented. Further, it is desirable that a return torque larger than that of the straight type can be obtained if a thread is formed so that a portion having the maximum effective diameter is formed within a portion within 15 mm from the seating surface of the buttocks. Furthermore, it has been found that a larger return torque can be obtained as the portion having the maximum effective diameter approaches the buttocks. That is, the shape of the virtual cylinder comprising the effective diameter of the male thread portion of the spark plug is such that the return torque decreases in the order of taper type shown in FIG. 6, drum type shown in FIG. 5, straight type shown in FIG. However, both were larger than the above limit values, and it was found that screw loosening can be prevented. In addition, it has been found that with a thick-type spark plug, the return torque is smaller than the above limit value, and screw loosening may occur.

  The present invention is not limited to the above embodiment, and various modifications can be made. For example, although the shape of the virtual cylindrical body configured from the effective diameter of the male screw portion 52 is a tapered shape that gradually decreases from the base portion 522 toward the tip portion 521, at least the base portion 522 or the body portion 523 has an effective diameter. May be formed in a step shape so that the effective diameter other than that portion is constant.

  In addition, the seating surface 55 of the metal shell 50 is configured as a surface orthogonal to the axial direction of the spark plug 100 in the present embodiment, but may be a tapered sheet. Further, a flat washer may be used as the gasket 5, and the gasket 5 is not limited to the one formed so as to have an S-shaped cross section by processing a flat plate as in the present embodiment.

  The spark plug may include a plurality of ground electrodes. The ground electrode may not project in a rod shape, and a spark discharge gap may be formed between the tip of the metal shell and the center electrode. Of course, it doesn't ask for the presence of precious metal tips.

  The present invention can be applied to a spark plug or a glow plug in which a male thread portion formed on the outer periphery of a metal shell is screwed into an engine head of an internal combustion engine.

DESCRIPTION OF SYMBOLS 10 Insulator 12 Shaft hole 20 Center electrode 50 Main metal fitting 52 Male screw part 100 Spark plug 521 Tip part 522 Base part 523 Trunk part

Claims (3)

A central electrode having its own tip as an ignition part, an axial hole extending in the axial direction of the central electrode, an insulator holding the central electrode inside the axial hole, and a radial periphery of the insulator And a metal shell having a male threaded portion that is formed on the outer periphery of the front end side of the seat surface for mounting on the engine head of the internal combustion engine and has a male threaded portion that is screwed into the mounting hole of the engine head. A spark plug provided,
The male screw portion is a tip portion that is a tip side portion in a direction in which the metallic shell is attached to the engine head, a base portion that is a rear end side portion, and a portion between the tip portion and the base portion. is composed of a certain body part has its effective diameter is maximum portion on the barrel, and the effective diameter of the portion ranging the tip portion from the site where the effective diameter is maximum, and the maximum A spark plug characterized by being formed smaller than the effective diameter of the part . Claims before Symbol externally threaded portion, and further, the effective diameter of the portion ranging the source unit from the site where the effective diameter is maximized, characterized in that it is formed smaller than the effective diameter of the portion to be the maximum The spark plug according to 1 . The effective diameter of the portion where the effective diameter of the male screw portion is maximized is substantially the same as the effective diameter of the mounting hole of the engine head,
When the metal shell is attached to the engine head,
In the portion where the effective diameter of the male screw portion is the maximum, the slope of the thread of the male screw portion abuts on the substantially entire surface of the slope of the thread of the mounting hole,
The part of the slope of the thread of the male thread part abuts against the slope of the thread of the mounting hole at a portion where the effective diameter of the male thread part is minimum. 2. The spark plug according to 2.

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