Patent Publication Number: US-7914353-B2

Title: Spark plug and method for manufacturing the same

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for manufacturing a spark plug mounted in an internal-combustion engine so as to ignite an air-fuel mixture, and to a spark plug manufactured by the method. 
     BACKGROUND OF THE INVENTION 
     A conventional spark plug for an internal-combustion engine is comprised of: a center electrode in which a front end thereof serves as an electrode for spark discharge; an insulator having an axial bore and accommodating the center electrode in a front end of the axial bore; and a metal shell surrounding and holding the insulator in a radial direction thereof. A male thread portion is formed on a front end side outer surface of the metal shell so as to engage with a threaded hole of an internal-combustion engine. Then, a spark is discharged in the internal-combustion engine to thereby ignite an air-fuel mixture. 
     The metal shell of such a spark plug typically includes an outward projection portion disposed toward a rear end side with respect to the fitting thread portion. An annular-shaped hollow gasket is disposed on a locating portion formed between the seal portion and the fitting thread portion. The hollow gasket is sandwiched between the surface of the engine block that surrounds the threaded hole therein and the seal portion of the metal shell when the spark plug is mounted on an internal-combustion engine. The hollow gasket is deformed to thereby improve its sealing properties and prevent the air leakage of the combustion chamber through the fitting threaded hole. Such a hollow gasket is produced by, for example, radially bending a ring-like plate member into an “S” shaped or a “C” shaped cross-section. As a result, the gasket is easily deformed when mounting the spark plug, and its sealing properties may be maintained after being deformed. 
     In the manufacturing process of the spark plug, the hollow gasket is inserted from the front end side of the metal shell having a thread ridge in the fitting thread portion, which is subjected to a cutting process, and is disposed on the locating portion. At this time, plural parts of an inner edge of the hollow gasket is compressed in an axial direction so as to form a nail-like portion, which radially inwardly projects with respect to a portion serving as the maximum outer diameter of the fitting thread portion. As a result, the gasket is retained on the metal shell and is prevented from falling from the metal shell over the fitting thread portion (e.g., refer to Patent Document 1). 
     In recent years, the improvement in an output of an automobile engine and fuel efficiency are highly in demand, and further improvement in spark plug components is also required. Regarding a gasket, a flat solid gasket assuming a thick disc shape and comprised of an alloy, which is mainly made of copper or the like, is considered. One of the advantages of using such a flat solid gasket is that after being once mounted on the engine, the spark plug is unlikely to loosen. Further, because such a gasket is a flat solid member, it is unlikely to be crushed. Furthermore, the position of the front end of the center electrode within the combustion chamber, with respect to an axis of the fitting threaded hole, is unlikely to vary, thereby stabilizing an igniting position. 
     In the manufacturing process of the spark plug using the flat solid gasket, in order to prevent the flat solid gasket from falling out from the metal shell, for example, a pipe shaped pressing member having an inner diameter slightly lager than the maximum outer diameter of the thread ridge is provided from the front end side of the metal shell in a state that the flat solid gasket is disposed on the locating portion of the metal shell in which the fitting thread portion has already been formed. Further, a front end opening of the pressing member being in contact with the flat solid gasket is pressed against the seal portion. As a result, the inner edge of the flat solid gasket radially inwardly projects from a portion serving as the maximum outer diameter of the fitting thread portion. 
     However, in the manufacturing process of a spark plug, since there is no large difference in diameters between an inner circumference of a pressing member and a thread ridge of a fitting thread portion of the metal shell when disposing a gasket on an locating portion of a metal shell after a thread rolling and processing for preventing a falling out of a gasket, and there is a tendency of producing a chip of the thread ridge. In order to prevent a loosening of a spark plug, the hardness of the gasket is necessary to be raised. However, when such a gasket is used, a pressing force against the gasket using a pressing member during a process of preventing the falling out of the gasket needs to be increased. As a result, a durability of a pressing member decrease, thereby causing a rise of a production cost. 
     An advantage of the present invention is a method for manufacturing a spark plug, and a spark plug manufactured by the method, wherein a gasket used for securing the air-tightness between the spark plug and an internal-combustion engine is prevented from falling from the metal shell of the spark plug with a simple step. 
     SUMMARY OF THE INVENTION 
     In order to solve the above problems, there is provided a method for manufacturing a spark plug according to a first aspect of the invention comprising: a center electrode; an insulator having an axial bore which extends in an axial direction and holding the center electrode in a front end side of the axial bore; and a metal shell surrounding and holding a radial circumference of the insulator and having a male-screw-shaped fitting thread portion formed on a front end side outer circumference face of the metal shell, a seal portion formed so as to radially outwardly project at a rear end side with respect to the fitting thread portion, and a locating portion formed between the seal portion and the fitting thread portion, where an annular gasket is disposed so as to seal between an opening peripheral portion of a fitting threaded hole of an internal-combustion engine and the seal portion when screwing the fitting thread portion into the fitting threaded hole, wherein the method for manufacturing a spark plug comprising the steps of: a cylindrical member formation step for forming a cylindrical member which serves as an original form of the metal shell and where the seal portion and the locating portion are formed but no fitting thread portion is formed; a gasket locating step for disposing the gasket on an outer circumference face of the cylindrical member after the cylindrical member formation step; and a fitting thread portion formation step for forming a fitting thread portion with a thread rolling on a thread forming portion of the cylindrical member after the gasket locating step. 
     In accordance with a second aspect of the invention, there is provided a method for manufacturing a spark plug as described above, wherein the gasket disposed on the outer circumferential face of the cylindrical member is pressed towards the seal portion with a die that is used for thread rolling, the gasket being disposed on the locating portion prior to a thread format step. 
     In accordance with a third aspect of the invention, there is provided a method for manufacturing a spark plug as described above, wherein an inner diameter of the gasket is larger than an outer diameter of the thread forming portion, and wherein a maximum outer diameter of the thread ridge after the fitting thread portion formation step is larger than the inner diameter of the gasket. 
     In accordance with a fourth aspect of the present invention, there is provided a method for manufacturing a spark plug, wherein the gasket is an annular plate. 
     In accordance with a fifth aspect of the invention, there is provided a spark plug manufactured by a method according to any one of the above aspects. 
     In the method for manufacturing a spark plug according to the first aspect, the thread ridge is formed on the thread-forming portion of the metal shell after the gasket is positioned on the outer circumferential face of the cylindrical member of the original form of the metal shell. Thus, an inner edge of the gasket is caught by the thread ridge after the thread rolling process. As a result, the gasket is retained on and does not fall from the fitting thread portion, thereby preventing the gasket from falling from the metal shell. Since a separate process to secure the gasket is not necessary after positioning the gasket on the locating portion of the metal shell, the production costs are reduced and the manufacturing process is simplified. As a result, the spark plug can be offered with a reasonable price. 
     According to the second aspect of the present invention, the die used for threading the thread ridge is also used to press the gasket on the locating portion of the metal shell. The gasket locating step and the fitting thread portion formation step can be performed in series. Thus, reduction in the production cost along with simplifying the manufacturing process of the spark plug can be achieved. As a result, the spark plug can be offered with a reasonable price. Further, since the gasket is disposed on the locating portion using the die, it can save any additional labor to dispose the gasket on the locating portion in the gasket locating step. 
     With respect to the third aspect of the present invention, the gasket is slid over the thread forming portion from a front end side of a cut body and is located on the locating portion of the cut body before the thread ridge is formed. After threading the thread ridge, the edge portion of the inner circumference of the gasket is captured by the formed screw thread whereby the gasket cannot fall from the locating portion. 
     With respect to the fourth aspect of the present invention, since the gasket does not require a separate manufacturing process to prevent it from falling from the shell, it is easy to secure a gasket even if a flat solid gasket made of an intractable plate material is used for the spark plug. Furthermore, when mounting a spark plug that has such a flat solid gasket on an internal-combustion engine, loosening of the spark plug as a result of vibrations of the internal-combustion engine can be prevented. Moreover, since the gasket deforms only slightly, the position of the front end of the center electrode within a combustion chamber, with respect to an axis of the fitting threaded hole, is unlikely to vary, thereby stabilizing an ignition position. 
     With respect to the fifth aspect of the invention, a spark plug manufactured according to the method of any one of above aspects, the gasket is unlikely to separate, i.e., fall from the metal shell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross sectional view of a spark plug  100 . 
         FIG. 2  is a perspective view of a gasket  5 . 
         FIG. 3  is an enlarged sectional view of a vicinity of a locating portion  59  of a metal shell  50 . 
         FIG. 4  shows a forging step of a manufacturing process of the spark plug  100 . 
         FIG. 5  shows a cutting step of the manufacturing process of the spark plug  100 . 
         FIG. 6  shows a gasket locating step of the manufacturing step of the spark plug  100 . 
         FIG. 7  is a partial cross sectional view of a cut body  220  for explaining the gasket locating step. 
         FIG. 8  shows a first thread rolling step of the manufacturing process of the spark plug  100 . 
         FIG. 9  shows a second thread rolling step of the manufacturing process of the spark plug  100 . 
         FIG. 10  is a partial cross sectional view of a cut body  220  for explaining the second thread rolling step. 
         FIG. 11  is a partial cross sectional view of the cut body  220  according to a modification, which uses a hollow gasket  105 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereafter, an embodiment of a method for manufacturing a spark plug and a spark plug manufactured by the method which carries out the present invention will be described with reference to the drawings. First, referring to  FIG. 1 , a composition of a spark plug  100  will be explained as an example of the spark plug produced by the method according to the present invention.  FIG. 1  is a partial cross-sectional view of a spark plug  100 . In  FIG. 1 , the direction of axis “O” of spark plug  100  is regarded as the top-to-bottom direction in the drawing. A lower side of the drawing is regarded as a front end side of spark plug  100  and an upper side of the drawing is regarded as a rear end side of spark plug  100 . 
     As shown in  FIG. 1 , the spark plug  100  is generally comprised of: an insulator  10 ; a metal shell  50  holding the insulator  10  therein; a center electrode  20  being held in the insulator  10  in an axis “O” direction; a ground electrode  30  having a base end portion  32  welded to a front end face  57  of the metal shell  50  and a front end portion  31  where a side face thereof faces a front end portion  22  of the center electrode  20 ; and a metal terminal fitting  40  provided at a rear end portion of the insulator  10 . 
     First, the insulator  10  constituting an insulator of the spark plug  100  will be explained. The cylindrical insulator  10  includes an axial bore  12  extending along an axis “O.” Insulator  10  is made of sintered alumina or the like as is commonly known. A flange portion  19  having the largest outer diameter is formed generally at a central location along the axis “O.” A rear end side body portion  18  is formed at the rear end side (upper side in  FIG. 1 ) with respect to the flange portion  19 . A front end side body portion  17  having a smaller outer diameter than that of the rear end side body portion  18  is formed at the front end side (lower side in  FIG. 1 ) with respect to the flange portion  19 . An elongated leg portion  13  having a smaller outer diameter than that of the front end side body portion  17  is formed at the front end side with respect to the front end side body portion  17 . The diameter of the long leg portion  13  is gradually tapered towards the front end side. The elongated leg portion  13  is exposed to a combustion chamber  208  when the spark plug  100  is mounted on an engine head  200 . A step portion  15  is formed between the elongated leg portion  13  and body portion  17 . 
     Next, the center electrode  20  is made of a nickel-system alloy or the like, such as, by way of example and not limitation, INCONEL (trade name)  600  or  601  in which a metal core  23  comprised of copper or a like metal with excellent thermal conductivity is provided. The front end portion  22  of the center electrode  20  projects from the front end face of the insulator  10  and is tapered towards the front end side. A noble metal tip  91  is welded to a front end face of the front end portion  22  so as to improve resistance to spark erosion. Further, the center electrode  20  is electrically connected to the metal terminal fitting  40  at the rear end side through a conductive seal material  4  and a ceramic resistance  3  both provided inside the axial bore  12 . A high-tension cable (not shown) is connected to the metal terminal fitting  40  through a plug cap (not shown), to which high voltage is applied. 
     Next, the ground electrode  30  will be described. The ground electrode  30  is comprised of a metal having an excellent corrosion resistance. As one of the examples, a nickel-system alloy such as INCONEL (trade name)  600  or  601  is used. The ground electrode  30  has a generally rectangular shape as seen from the cross-section in the longitudinal direction. The base end portion  32  of ground electrode  30  is welded to the front end face  57  of the metal shell  50 . The free end portion  31  of the ground electrode  30  is bent so that a side face thereof faces the front end portion  22  of the center electrode  20 . 
     Next, the metal shell  50  will be described. The metal shell  50  is a tubular metal fitting for fixing the spark plug  100  to the engine head  200  of an internal-combustion engine. The metal shell  50  holds therein the insulator  10  so as to surround an area from a part of the rear end side body portion  18  to the long leg portion  13 . The metal shell  50  is comprised of a low carbon steel material and includes a tool engagement portion  51 , dimensioned to engage with a spark plug wrench (not shown), and a fitting thread portion  52  having a thread ridge  521  dimensioned to engage with a threaded hole  201  in the engine head  200  provided in an upper part of the internal-combustion engine. 
     A flange-like seal portion  54  is formed between the tool engagement portion  51  and the thread portion  52  of the metal shell  50 . A locating portion  59  where a gasket  5 , which will be mentioned later, is to be disposed between a formation starting position  155 , which is formed in a rear end of the fitting thread portion  52  (i.e., the rear end of the thread ridge  521  formed in the fitting thread portion  52 ), and a seat face  55  (a face facing the front end side) of the seal portion  54 . A thin caulking portion  53  is formed at the rear end side with respect to the tool engagement portion  51  of the metal shell  50 . Similar to the caulking portion  53 , a thin buckling portion  58  is formed between the seal portion  54  and the tool engagement portion  51 . Annular ring members  6 ,  7  lie between an inner circumference face of the metal shell  50  where the tool engagement portion  51  and the caulking portion  53  are formed and an outer circumference face of the rear end side body portions  18  of the insulator  10 . Further, talc powder  9  is filled between both ring members  6 ,  7 . The insulator  10  extends through the ring members  6 ,  7  and the talc  9  and is pressed towards the front end side of the metal shell  50  by inwardly caulking an end portion  60  of the caulking portion  53 . Thus, in the screw portion  52  of the metal shell  50 , a step portion  56  projects inwardly and supports the step portion  15  of the insulator  10  through an annular packing  8 , thereby integrating the metal shell  50  and the insulator  10 . At this time, the air-tightness between the metal shell  50  and the insulator  10  is maintained by the packing  8 , thereby preventing combustion gas from flowing out. The buckling portion  58  is formed so as to outwardly deform with an application of compression force at the time of a caulking process. 
     Next, the assembly of the gasket  5  will be described with reference to  FIGS. 1 to 3 .  FIG. 2  is a perspective view of the gasket  5 .  FIG. 3  is an enlarged sectional view of a vicinity of the locating portion  59  of the metal shell  50 . 
     The gasket  5  shown in  FIG. 2  is an annular flat solid packing and is formed by a punching process from a plate-like material made of copper, or an alloy comprised mainly of copper. As shown in  FIG. 3 , the gasket  5  is disposed on the locating portion  59  of the metal shell  50 . As shown in  FIG. 1 , when the spark plug  100  is mounted on the engine head  200 , the gasket  5  is interposed between the seat face  55  of the seal portion  54  and an opening peripheral portion  205  of the fitting threaded hole  201  of the engine head  200  where the fitting thread portion  52  is engaged. With the tightening of the fitting thread portion  52  at the time of a mounting process, a surface (contact face) of the gasket  5 , which is in contact with the seat face  55  of the seal portion  54  and the opening peripheral portion  205  of the fitting threaded hole  201 , is plastically deformed. As a result, the gasket  5  adheres to the seat face  55  and the opening peripheral portion  205 , and forms a seal therebetween thereby preventing an air leak from the engine through the fitting threaded hole  201 . 
     As shown in  FIG. 3 , the annular-shaped gasket  5  has an inner diameter A that is smaller than an outer diameter of a portion B (hereinafter referred to as a “crest diameter”) serving as the maximum outer diameter of the fitting thread portion  52  (i.e., the crest of the thread ridge  521 ) and that is larger than an outer diameter C (hereinafter referred to as a “core diameter” or “root diameter”) of a portion serving as the minimum outer diameter of the fitting thread portion  52  (i.e., a bottom portion between the thread ridge  521 ). In the manufacturing process of the metal shell  50  of the spark plug  100 , which will be mentioned later, the thread ridge  521  of the fitting thread portion  52  is formed by a thread rolling process. Before the thread ridge  521  is formed, metal shell  50  begins as a cut body  220 , as shown in  FIG. 5 . The thread forming portion  152  of cut body  220  has an outer diameter D (hereinafter referred to as a “blank diameter”) (the outline is shown with a dotted line in  FIG. 3 ) that becomes the fitting thread portion  52  after the thread rolling process. Blank diameter D is generally equal to an effective diameter of the thread ridge  521 . Once the thread ridge  521  is formed, the crest diameter B of thread ridge  521  is larger than the blank diameter D, and the core diameter C of thread ridge  521  is smaller than the blank diameter D. The effective diameter of the thread ridge  521  and the crest diameter B and the core diameter C may vary depending on a material of the metal shell  50 , a specification of a rolling die (thread rolling cylindrical dies  300 ,  310  will be mentioned later) and/or pressing conditions at the time of the thread rolling or the like. In this embodiment, each diameter is defined according to the above conditions so as to meet a requirement of: the crest diameter B&gt;the inner diameter A&gt;the blank diameter D&gt;the core diameter C. By forming cut body  220  and gasket  5  as described above, the gasket  5  may be slid over the thread forming portion  152  of the cut body  220  (referred to in  FIG. 5 ) from the front end side, and be located on the locating portion  59  before forming the thread ridge  521 . After the thread ridge  521  is formed, the edge portion of the inner circumference of the gasket  5  is captured by the thread ridge  521 , whereby the gasket  5  is retained on the metal shell  50  and cannot fall from the locating portion  59 . The seal portion  54  that is formed in the rear end of the locating portion  59 , prevents the gasket  5  from moving towards the rear end side. Thus, the gasket  5  is unlikely to move towards a rear end side of the metal shell  50 , thereby preventing the gasket  5  from falling from the metal shell  50 . In other words, gasket  5  is captured on metal shell  50  between the formed thread ridge  521  and seal portion  54 . 
     In the manufacturing process of the spark plug  100  having such a composition, in this embodiment, the thread ridge  521  is formed along the thread forming portion  152  of metal shell  50  after disposing the gasket  5  on the locating portion  59  of the metal shell  50 . In this respect, the metal shell  50  is formed so as to satisfy the aforementioned size requirements (i.e., crest diameter B of the fitting thread portion  52 &gt;inner diameter A of the gasket  5 ). Thus, the gasket  5  is prevented from falling off of the metal shell  50 . Although the thread ridge  521  is formed in a second thread rolling step of the manufacturing process of the spark plug  100 , which will be mentioned later, the composition of the thread rolling cylindrical dies  300 ,  310  used for threading the thread ridge  521  in the thread forming portion  152  of the metal shell  50  will be briefly described with reference to  FIG. 9 .  FIG. 9  shows the second thread rolling step of the manufacturing process of the spark plug  100 . 
     As shown in  FIG. 9 , the thread rolling cylindrical dies  300 ,  310  are provided so that an axis line P of a rotation shaft  302  and an axis line Q of a rotation shaft  312  are parallel with the axis O of the cut body  220 . The rotation shafts  302 ,  312  are movable toward each other in a direction where the axis P and the axis Q can tie together (i.e., a horizontal direction in  FIG. 9 ) and also are movable in a direction along each axis P and Q (up-and-down or vertical direction in  FIG. 9 ). The thread rolling cylindrical dies  300 ,  310  have processing faces  301 ,  311 , respectively. A thread-shaped processing tooth is formed on the whole outer circumference of each face  301 ,  311 . Dies  300 ,  310  are rotated in the same direction by a rotation means (not illustrated) at a predetermined speed. One end of the cylindrical, thread rolling dies  300 ,  310  includes end faces  305 ,  315 . End faces  305 ,  315  face toward the rear end side of the cut body  220 , which is disposed between the thread rolling cylindrical dies  300 ,  310 . End faces  305 ,  315  are planar in shape and are perpendicular to the each axis P and Q. 
     In the manufacturing process for forming the spark plug  100 , when producing the metal shell  50 , the thread ridge  521  is formed in the thread forming portion  152  using the thread rolling cylindrical dies  300 ,  310  as described above. The method for manufacturing the spark plug  100  shall hereafter be described with reference to  FIGS. 4 to 11 .  FIG. 4  shows a forging step in the manufacturing process of the spark plug  100 .  FIG. 5  shows a cutting step in the manufacturing process of the spark plug  100 .  FIG. 6  shows a gasket locating step in the manufacturing process of the spark plug  100 .  FIG. 7  is a partial cross sectional view of a cut body  220  for explaining the gasket locating step.  FIG. 8  shows a first thread rolling step in the manufacturing process of the spark plug  100 .  FIG. 10  is a partial cross sectional view of the cut body  220  for explaining the second thread rolling step. 
     In manufacturing the metal shell  50 , as shown in  FIG. 4 , a rod-like steel material made of low-carbon-steel material (e.g., low-carbon-steel material of 6C to 35C, such as S10C or S15C) is set to a cold forging machine (not illustrated). A forging operation, such as an extrusion molding, is conducted to form a forged body  210  serving as the manufactured metal shell  50 . The cylindrical forged body  210  has a through hole  215  used for accommodating the insulator  10 . The outer periphery of the forged body  210  is formed to include a rear end side cylindrical portion  211  to be used in forming the caulking portion  53 , the tool engagement portion  51  and the buckling portion  58 , an intermediate cylindrical portion  212  to be used in forming the seal portion  54  and a front end side cylindrical portion  213  to be used in forming the locating portion  59  and the thread forming portion  152  (fitting thread portion  52  after forming the thread ridge  521 ). The intermediate cylindrical portion  212  and the front end side cylindrical portion  213  are formed in a cylindrical shape, and the rear end side cylindrical portion  211  is formed in a hexagonal shape so as to engage with an outer shape of the manufactured tool engagement portion  51  (refer to  FIG. 1 ). It is noted that forged body  210  may be formed of a low-carbon-steel material in a rod shape, or may be made from a pipe-like steel material. 
     Next, the forged body  210  is set to a cutting machine (not illustrated) so that the outer circumference face thereof and the inside of the through hole  215  are cut, i.e., machined, into the respective shape of the metal shell  50 . That is, in the through hole  215 , the front end side is cut to form the step portion  56 , and through hole  215  is cut, i.e., machined, to establish a clearance (refer to  FIG. 1 ) between an inner wall of the through hole  215  and the long leg portion  13  when receiving the insulator  10  in the through hole  215  in the assembly of the spark plug  100 . Further, the caulking portion  53  having an annular outer circumference face and the buckling portion  58  is formed in the rear end side cylindrical portion  211 , and the tool engagement portion  51  is formed in the remainder. The tool engagement portion  51  is not necessarily a hexagonal shape, but may be other shape, such as a BI-HEX shape. 
     The seal portion  54  is formed in the intermediate cylindrical portion  212 , and the thread forming portion  152  which has not yet had the thread ridge  521  is formed in the front end side cylindrical portion  213 . As mentioned above, the blank diameter D of the thread forming portion  152  is cut so as to be smaller than the inner diameter A (refer to  FIG. 3 ) of the gasket  5  produced in a separate step. Further, the groove-like locating portion  59  is formed between the seal portion  54  and the thread forming portion  152 . 
     Subsequently, as shown in  FIG. 6 , the base end portion  32  of the ground electrode  30  (produced in a separate step) is joined for example, by resistance welding to the front end face  57  of the cut body  220 . The cut body  220  is oriented so that the front end face  57  faces upwards or sideways, and the gasket  5  produced in a separate step is inserted from the front end side of the cut body  220  so as to slide over the thread forming portion  152 . As shown in  FIG. 7 , since the blank diameter D of the thread forming portion  152  of the cut body  220  is smaller than the inner diameter A of the gasket  5 , the gasket  5  slides or slips past the thread forming portion  152  and reaches the locating portion  59 . Thus, the gasket  5  can be in contact with the seat face  55  of the seal portion  54 . 
     Next, the thread ridge  521  is formed in the thread forming portion  152  of the cut body  220  with the thread rolling. As shown in  FIG. 8 , in this embodiment, the cut body  220  is supported pivotally with a holding jig (not illustrated) so as to rotate about its central axis, i.e., around the axis O, see  FIGS. 1 and 3 . Cut body  220  is disposed, i.e., sandwiched between the thread rolling cylindrical dies  300 ,  310  that are dimensioned to form the thread ridge  521 . 
     First, the rotation axes  302 ,  312  of the thread rolling cylindrical dies  300 ,  310  are moved by a driving means (not illustrated) to a position where each processing face  301 ,  311  does not touch the cut body  220 , and an edge portion of each end face  305 ,  315  contacts the gasket  5  disposed on the locating portion  59  of the cut body  220 . The gasket  5  abuts the seal portion  54 , which prevents further movement towards the rear end side in the axis O direction. In this respect, the gasket  5  is pressed by each end face  305 ,  315  of the thread rolling cylindrical dies  300 ,  310  so as to be located on the locating portion  59 . The end faces  305 ,  315  prevent gasket  5  from moving towards the front end side in the axis O direction. 
     With the gasket  5  maintained on the locating portion  59 , as shown in  FIG. 9 , each rotation axis  302 ,  312  slides inward toward each other in the direction where the axis P and the axis Q can join together so that the cut body  220  is sandwiched between the thread rolling cylindrical dies  300 ,  310 . Then, portion  152  of the cut body  220  is pressed by the processing face  301 ,  311  of the thread rolling cylindrical dies  300 ,  310  to thereby create, i.e., thread, the thread ridge  521 . As mentioned above, the thread rolling cylindrical dies  300 ,  310  rotate in the same direction, and the cut body  220  that is sandwiched therebetween, follows and rotates the opposite direction to that of the thread rolling cylindrical dies  300 ,  310 . 
     As shown in  FIG. 10 , an outer circumference face of the thread forming portion  152  of the metal shell  50  is plastically deformed due to the pressure from the processing tooth of the processing face  301 ,  311  of the thread rolling cylindrical dies  300 ,  310 . The formed thread ridge  521  has a crest diameter B and a core diameter C, which differ from each other. As mentioned above, in this embodiment, the material of the metal shell  50 , the spec of the thread rolling cylindrical dies  300 ,  310 , the pressing conditions at the time of the thread rolling or the like are established so that the crest diameter B of the thus-formed thread ridge  521  is larger than the inner diameter A of the gasket  5 . After the thread rolling, since an edge portion of the inner circumference of the gasket  5  is caught by the thread ridge  521 , the gasket  5  is confined to the locating portion  59 . Gasket  5  is thereby prevented from falling from the metal shell  50 . Thereafter, each part such as the insulator  10  integrated with the center electrode  20  is assembled by a known technique in the metal shell  50  where the thread ridge  521  has been formed. As a result, the spark plug  100  shown in  FIG. 1  is completed. 
     As will be appreciated by those skilled in the art, various kinds of modifications are possible in the present invention. For example, although the thread rolling dies  300 ,  310  for threading the thread ridge  521  are cylindrical, rolling die with a flat type or a rotary type rolling die may be used, as long as the die has a face for pressing the gasket  5  so that the gasket  5  is maintained in the locating portion  59  during the thread rolling step. Further, the cut body  220  is disposed between the thread rolling dices and allowed to slide towards an axis of the rolling dice to form the thread ridge  521  of the thread forming portion  152 . In this case, the gasket  5  is controlled not to move towards the front end side of the thread forming portion  152  by the end face of the thread rolling dice. Further, along with the movement of the cut body  220 , the gasket  5  may be disposed in the locating portion  59  after the thread rolling. Furthermore, when pivotally supporting the cut body  220  with a holding jig during the thread rolling, the axis O of the cut body  220  may be supported perpendicularly or horizontally. 
     In the embodiment heretofore described, the gasket  5  is an annular flat solid packing. As shown in  FIG. 11 , a spark plug may use a conventional gasket  105  assuming an S-shape or C-shape in the cross section formed by radially bending a ring-like plate material. Similar to the above embodiment, if the thread ridge  521  of the thread forming portion  152  of the cut body  220  is formed so that the initial blank diameter D before the thread rolling is smaller than the inner diameter E of the gasket  105  and the crest diameter B of the thread ridge  521  after the thread rolling is larger than the inner diameter E of the gasket  105 , the gasket  105  does not fall out from the locating portion  59 . Furthermore, any process is not necessary to an inner edge of the gasket  105  for preventing it from falling out whereby the manufacturing process of the spark plug may be simplified. 
     Although the gasket  5  is disposed on the locating portion  59  in the gasket locating step, the gasket  5  may be positioned on a front end side of the forming portion  152  away from the locating portion  59 . In this case, the gasket  5  may be pressed, i.e., moved, by the end face  305 ,  315  of the thread rolling cylindrical dies  300 ,  310  in the first thread rolling step to allow the gasket to be positioned in the locating portion  59 . 
     The present invention may be applicable to one, such as a spark plug, a temperature sensor or a gas sensor, having a gasket for preventing a gas leaking through a mounting bore where a metal shell is fitted.