Patent Publication Number: US-9843166-B2

Title: Spark plug and method for manufacturing spark plug

Description:
This application claims the benefit of Japanese Patent Applications No. 2014-187258, filed Sep. 16, 2014, which is incorporated by reference in its entity herein. 
     FIELD OF THE INVENTION 
     The present disclosure relates to a spark plug. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a spark plug is used for ignition of an air-fuel mixture or the like within a combustion chamber of an internal combustion engine. As a spark plug, for example, a spark plug including a housing and a cap fixed to the housing by means of welding has been proposed. The cap includes a plurality of orifices. Flame jets out from the orifices, whereby a flame jet for ignition is generated around the cap. 
     PRIOR ART DOCUMENT 
     Patent Document 
     [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2012-199236 
     [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2011-214492 
     Problems to be Solved by the Invention 
     In the case of welding components of a spark plug (e.g., the case of welding a cap to a housing), a problem may occur due to the welding. For example, by gas such as air being trapped in a portion melted by the welding, a small void may be formed in the portion that has been cooled and solidified. Such a void can cause a problem such as cracking. 
     The present disclosure provides a technique to reduce a possibility of occurrence of a problem due to welding. 
     SUMMARY OF THE INVENTION 
     Means for Solving the Problems 
     According to modes of the present disclosure, for example, the following application examples are provided. 
     Application Example 1 
     A spark plug including: 
     a metallic shell having a through hole extending in a direction of an axis; 
     a cap covering that is located at a front side of the spark plug and covers an opening in the metallic shell; 
     a melt portion provided between the cap and the metallic shell, said melt portion joining the cap and the metallic shell to each other; and 
     a gap extending from a specific space which is surrounded by the metallic shell and the cap to the melt portion and interposed between the metallic shell and the cap. 
     According to this configuration, at the time of formation of the melt portion, degassing can be performed through the gap, and thus a possibility of occurrence of a problem due to welding can be reduced. 
     Application Example 2 
     The spark plug described in the application example 1, wherein the gap includes: 
     a first gap which communicates with the specific space; and 
     a second gap which communicates with the first gap and the specific space and is larger than the first gap. 
     According to this configuration, at the time of formation of the melt portion, degassing can be appropriately performed through the second gap larger than the first gap, and thus the possibility of occurrence of a problem due to welding can be reduced. 
     Application Example 3 
     The spark plug described in the application example 2, wherein 
     the metallic shell has a first surface which is not perpendicular to the axis, and 
     the cap has a second surface which faces the first surface of the metallic shell to provide the first gap and the second gap. 
     According to this configuration, the possibility of occurrence of a problem due to welding can be reduced while misalignment of the cap relative to the metallic shell (in particular, misalignment in a direction perpendicular to the axis) is suppressed. 
     Application Example 4 
     The spark plug described in the application example 3, wherein 
     the first surface of the metallic shell is located at the front side and is a portion of an inner peripheral surface of the metallic shell, 
     the cap includes a projection portion which projects toward a rear side of the spark plug and is located at an inner peripheral side of the first surface of the metallic shell, and 
     the second surface of the cap is located at an outer peripheral side of the metallic shell and is a surface of the projection portion. 
     According to this configuration, the possibility of occurrence of a problem due to welding can be reduced while misalignment of the cap relative to the metallic shell (in particular, misalignment in the direction perpendicular to the axis) is suppressed. 
     Application Example 5 
     The spark plug described in any one of the application examples 1 to 4, wherein the gap is annular. 
     According to this configuration, degassing is possible from any position in the circumferential direction at the time of formation of the melt portion, and thus the possibility of occurrence of a problem due to welding can be reduced. 
     Application Example 6 
     The spark plug described in any one of the application examples 1 to 4, wherein the gap is provided at a plurality of positions in a circumferential direction when being seen from the direction of the axis. 
     According to this configuration, at the time of formation of the melt portion, degassing can be appropriately performed through the gap provided at the plurality of positions in the circumferential direction, and thus the possibility of occurrence of a problem due to welding can be reduced. 
     Application Example 7 
     The spark plug described in any one of the application examples 2 to 4, wherein 
     the metallic shell and the cap form N second gaps whose positions in the circumferential direction are different from each other, N being an integer which is equal to or higher than 3, and 
     when the N second gaps are projected parallel to the axis, onto a projection surface perpendicular to the axis, and the projection surface is divided into three regions each having a center angle of 120 degrees centered at the axis, each of the three regions includes one or more of the second gaps. 
     According to this configuration, uneven arrangement of the N second gaps in the circumferential direction is suppressed, and thus the possibility of occurrence of a problem due to welding can be appropriately reduced. 
     Application Example 8 
     A method for manufacturing a spark plug, the method comprising the steps of: 
     placing a cap at a specific position which covers an opening in a metallic shell located at a front side of the spark plug, said metallic shell having a through hole extending in a direction of an axis; and 
     welding the cap placed at the specific position to the metallic shell, 
     wherein the metallic shell and the cap placed at the specific position form an annular gap which communicates with a specific space surrounded by the metallic shell and the cap and is interposed between the metallic shell and the cap, 
     the annular gap includes:
         a first gap which communicates with the specific space; and   a second gap which communicates with the specific space and is larger than the first gap, and       

     the step of welding the cap to the metallic shell further comprises the sub-steps of:
         welding a specific portion whose position in a circumferential direction is different from that of a specific second gap, of a boundary between the cap and the metallic shell which communicates with the annular gap; and   after welding the specific portion, welding a portion whose position in the circumferential direction is the same as that of the specific second gap, of the boundary.       

     According to this configuration, degassing can be performed through the specific second gap portion when the specific portion is welded, and thus the possibility of occurrence of a problem in welding can be reduced. 
     The present invention can be embodied in various forms. For example, the present invention can be embodied in forms such as a spark plug, a method for manufacturing a spark plug, and a spark plug manufactured by the manufacturing method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein like designations denote like elements in the various views, and wherein: 
         FIG. 1  is a cross-sectional view of an embodiment of a spark plug. 
         FIG. 2  is a cross-sectional view, of a front end portion of a spark plug  100 , including a central axis CL. 
         FIG. 3  is a cross-sectional view, of the front end portion of the spark plug  100 , perpendicular to the central axis CL. 
         FIG. 4  is a cross-sectional view, of the front end portion of the spark plug  100 , perpendicular to the central axis CL. 
         FIG. 5  is a flowchart showing an example of a method for manufacturing the spark plug  100 . 
         FIGS. 6(A) to 6(C)  are cross-sectional views showing an arrangement of a cap  300  with respect to a metallic shell  50 . 
         FIG. 7  is a schematic cross-sectional view in welding. 
         FIGS. 8(A) to 8(C)  are schematic cross-sectional views showing a situation in which the welding proceeds. 
         FIGS. 9(A) to 9(C)  are schematic cross-sectional views showing the situation in which the welding proceeds. 
         FIG. 10  is a schematic diagram showing an order of the welding. 
         FIGS. 11(A) to 11(C)  are schematic cross-sectional views showing a situation in which welding in a reference example proceeds. 
         FIG. 12  is a cross-sectional view of a spark plug  100   b  of a second embodiment. 
         FIG. 13  is a cross-sectional view of a spark plug  100   c  of a third embodiment. 
         FIG. 14  is a cross-sectional view of a spark plug  100   d  of a modified embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. First Embodiment 
     A1. Device Configuration 
       FIG. 1  is a cross-sectional view of an embodiment of a spark plug. In the drawing, the central axis CL (also referred to as “axis CL”) of a spark plug  100  is shown. The cross section shown is a cross section including the central axis CL. Hereinafter, a direction parallel to the central axis CL is referred to as “direction of the axis CL” or merely as “axial direction”. The radial direction of a circle centered on the central axis CL is referred to merely as “radial direction”, and the circumferential direction of the circle centered on the central axis CL is referred to as “circumferential direction”. In the direction parallel to the central axis CL, the downward direction in  FIG. 1  is referred to as a front end direction Df, and the upward direction in  FIG. 1  is referred to as a rear end direction Dfr. The front end direction Df is a direction from a metal terminal  40  described later toward electrodes  20  and  30  described later. In addition, the front end direction Df side in  FIG. 1  is referred to as a front side of the spark plug  100 , and the rear end direction Dfr side in  FIG. 1  is referred to as a rear side of the spark plug  100 . 
     The spark plug  100  includes an insulator  10  (also referred to as “ceramic insulator  10 ”, the center electrode  20 , the ground electrodes  30 , the metal terminal  40 , a metallic shell  50 , a conductive first seal portion  60 , a resistor  70 , a conductive second seal portion  80 , a front packing  8 , a talc  9 , a first rear packing  6 , a second rear packing  7 , and a cap  300 . 
     The insulator  10  is a substantially cylindrical member having a through hole  12  (hereinafter, also referred to as “axial bore  12 ”) which extends along the central axis CL to penetrate the insulator  10 . The insulator  10  is formed by baking a material containing alumina (another insulating material may be used). The insulator  10  includes a leg portion  13 , a first reduced outer diameter portion  15 , a front trunk portion  17 , a flange portion  19 , a second reduced outer diameter portion  11 , and a rear trunk portion  18  which are arranged in order from the front side toward the rear end direction Dfr. The flange portion  19  is a portion having a largest outer diameter in the insulator  10 . The outer diameter of the first reduced outer diameter portion  15  gradually decreases from the rear side toward the front side. Near the first reduced outer diameter portion  15  of the insulator  10  (in the front trunk portion  17  of the embodiment in  FIG. 1 ), a reduced inner diameter portion  16  is formed which has an inner diameter gradually decreasing from the rear side toward the front side. The outer diameter of the second reduced outer diameter portion  11  gradually decreases from the front side toward the rear side. 
     The center electrode  20  is inserted in the front side of the axial bore  12  of the insulator  10 . The center electrode  20  includes a bar-shaped axial portion  27  extending along the central axis CL, and a tip  200  joined to the front end of the axial portion  27 . The axial portion  27  includes a leg portion  25 , a flange portion  24 , and a head portion  23  which are arranged in order from the front side toward the rear end direction Dfr. The tip  200  is joined to the front end of the leg portion  25  (i.e., the front end of the axial portion  27 ) (e.g., by means of laser welding). The tip  200  is exposed outside from the axial bore  12  at the front side of the insulator  10 . A surface, at the front end direction Df side, of the flange portion  24  is supported by the reduced inner diameter portion  16  of the insulator  10 . In addition, the axial portion  27  also includes an outer layer  21  and a core portion  22 . The outer layer  21  is formed of a material having more excellent oxidation resistance than that of the core portion  22 , that is, a material which less wears when being exposed to a combustion gas within a combustion chamber of an internal combustion engine (e.g., pure nickel, an alloy containing nickel and chromium, etc.). The core portion  22  is formed of a material having a higher coefficient of thermal conductivity than that of the outer layer  21  (e.g., pure copper, a copper alloy, etc.). A rear end portion of the core portion  22  is exposed from the outer layer  21  to form a rear end portion of the center electrode  20 . The other portion of the core portion  22  is covered with the outer layer  21 . However, the entirety of the core portion  22  may be covered with the outer layer  21 . In addition, the tip  200  is formed by using a material having more excellent durability against discharge than the axial portion  27  (e.g., noble metals such as iridium (Ir) and platinum (Pt), tungsten (W), and an alloy containing at least one metal selected from these metals). 
     A portion of the metal terminal  40  is inserted in the rear side of the axial bore  12  of the insulator  10 . The metal terminal  40  is formed by using a conductive material (e.g., a metal such as low-carbon steel). 
     Within the axial bore  12  of the insulator  10 , the resistor  70  which has a substantially columnar shape and serves to suppress electrical noise is disposed between the metal terminal  40  and the center electrode  20 . The resistor  70  is formed by using, for example, a material containing a conductive material (e.g., carbon particles), ceramic particles (e.g., ZrO 2 ), and glass particles (e.g., SiO 2 —B 2 O 3 —Li 2 O—BaO-based glass particles). The conductive first seal portion  60  is disposed between the resistor  70  and the center electrode  20 , and the conductive second seal portion  80  is disposed between the resistor  70  and the metal terminal  40 . Each of the seal portions  60  and  80  is formed by using, for example, a material containing metal particles (e.g., Cu) and the same glass particles as those included in the material of the resistor  70 . The center electrode  20  and the metal terminal  40  are electrically connected to each other via the resistor  70  and the seal portions  60  and  80 . 
     The metallic shell  50  is a substantially cylindrical member having a through hole  59  which extends along the central axis CL to penetrate the metallic shell  50 . The metallic shell  50  is formed by using a low-carbon steel material (another conductive material (e.g., a metal material) may be used). The insulator  10  is inserted in the through hole  59  of the metallic shell  50 . The metallic shell  50  is fixed to the outer periphery of the insulator  10 . At the rear side of the metallic shell  50 , the rear end of the insulator  10  (in the present embodiment, a portion, at the rear side, of the rear trunk portion  18 ) is exposed outside from the through hole  59 . 
     At the front side of the metallic shell  50 , the front end of the center electrode  20  (here, the front end of the tip  200 ) is disposed within the through hole  59 . Each bar-shaped ground electrode  30  is fixed to the inner peripheral surface of the metallic shell  50 . Each ground electrode  30  extends from the inner peripheral surface of the metallic shell  50  to a position facing a side surface of the tip  200 . Each ground electrode  30  and the side surface of the tip  200  form a gap sg. In the gap sg (hereinafter, referred to as “discharge gap sg”), spark discharge occurs. Each ground electrode  30  is formed by using a material having excellent oxidation resistance (e.g., an alloy containing nickel and chromium) (another material may be used). The cap  300  is fixed to the front side of the metallic shell  50  so as to cover an opening, at the front side, of the metallic shell  50 . The cap  300  is formed by using a material having excellent oxidation resistance (e.g., an alloy containing nickel and chromium) (another material may be used). The center electrode  20 , each ground electrode  30 , and the cap  300  will be described in detail later. 
     The metallic shell  50  includes a trunk portion  55 , a seat portion  54 , a deformable portion  58 , a tool engagement portion  51 , and a crimp portion  53  which are arranged in order from the front side toward the rear side. The seat portion  54  is a flange-like portion. The trunk portion  55  is a substantially cylindrical portion extending from the seat portion  54  toward the front end direction Df along the central axis CL. The outer peripheral surface of the trunk portion  55  has a thread  52  formed thereon for screwing into a mount hole of an internal combustion engine. An annular gasket  5  which is formed by bending a metal plate is fitted between the seat portion  54  and the thread  52 . 
     The metallic shell  50  includes a reduced inner diameter portion  56  disposed at the front end direction Df side of the deformable portion  58 . The inner diameter of the reduced inner diameter portion  56  gradually decreases from the rear side toward the front side. The front packing  8  is interposed between the reduced inner diameter portion  56  of the metallic shell  50  and the first reduced outer diameter portion  15  of the insulator  10 . The front packing  8  is an O-shaped ring made of iron (another material (e.g., a metal material such as copper) may be used). The front packing  8  seals between the metallic shell  50  and the insulator  10 . 
     The tool engagement portion  51  is a portion for engaging with a tool for tightening the spark plug  100  (e.g., a spark plug wrench). In the present embodiment, the external shape of the tool engagement portion  51  is substantially a hexagonal column extending along the central axis CL. In addition, the crimp portion  53  is disposed at the rear side of the second reduced outer diameter portion  11  of the insulator  10  and forms the rear end of the metallic shell  50  (i.e., an end at the rear end direction Dfr side). The crimp portion  53  is bent inward in the radial direction. At the front end direction Df side of the crimp portion  53 , the first rear packing  6 , the talc  9 , and the second rear packing  7  are arranged between the inner peripheral surface of the metallic shell  50  and the outer peripheral surface of the insulator  10  in this order toward the front end direction Df. In the present embodiment, the rear packings  6  and  7  are C-shaped rings made of iron (another material may be used). 
     In manufacturing of the spark plug  100 , the crimp portion  53  is crimped so as to be bent inward. Then, the crimp portion  53  is pressed to the front end direction Df side. Accordingly, the deformable portion  58  deforms, and the insulator  10  is pressed toward the front side within the metallic shell  50  via the packings  6  and  7  and the talc  9 . The front packing  8  is pressed between the first reduced outer diameter portion  15  and the reduced inner diameter portion  56  to seal between the metallic shell  50  and the insulator  10 . In this manner, the insulator  10  is fixed to the metallic shell  50 . 
     Next, a portion, at the front end direction Df side, of the spark plug  100  will be described.  FIG. 2  is a cross-sectional view, of a front end portion of the spark plug  100 , including the central axis CL. In  FIG. 2 , the upward direction corresponds to the front end direction Df, and the downward direction corresponds to the rear end direction Dfr. In the drawing, a front end portion of the metallic shell  50  (here, a front end portion of the trunk portion  55 ), a front end portion of the tip  200 , the ground electrodes  30 , and the cap  300  are shown. An inward direction Di shown in the lower portion in the drawing is a direction toward the inner side in the radial direction, and an outward direction Do shown in the lower portion in the drawing is a direction toward the outer side in the radial direction. 
       FIG. 3  is a cross-sectional view, of the front end portion of the spark plug  100 , perpendicular to the central axis CL. The cross-sectional view shows a cross section taken along a line A-A in  FIG. 2 . In the drawing, the tip  200  of the center electrode  20 , the ground electrodes  30 , and the trunk portion  55  of the metallic shell  50  are shown.  FIG. 4  is a cross-sectional view, of the front end portion of the spark plug  100 , perpendicular to the central axis CL. This cross-sectional view shows a cross section taken along a line B-B in  FIG. 2 , which is a cross section at the front end direction Df side of the cross section (A-A cross section) in  FIG. 3 . In the drawing, the trunk portion  55  of the metallic shell  50  and a later-described projection portion  310  of the cap  300  are shown. The cross-sectional view in  FIG. 2  shows a cross section taken along a line C-C in  FIGS. 3 and 4 . 
     As shown in  FIGS. 2 and 3 , in the present embodiment, each ground electrode  30  is a bar-shaped electrode having a rectangular cross section. One end portion of each ground electrode  30  is joined to an inner peripheral surface  55   i  of the trunk portion  55  (e.g., by means of laser welding). The other end portion of each ground electrode  30  faces the side surface  200   s  of the tip  200  across the discharge gap sg. In the present embodiment, four ground electrodes  30  are disposed at substantially equal intervals in the circumferential direction. The side surface  200   s  of the tip  200  is surrounded by the four ground electrodes  30 . 
     As shown in  FIG. 2 , the cap  300  is a member which covers an opening OPf, at the front end direction Df side, of the metallic shell  50 . In the present embodiment, the cross-sectional shape of the cap  300  is a substantially U shape which projects to the front end direction Df side. The shape of the cap  300  is a cup shape obtained by rotating this cross-sectional shape about the central axis CL. 
     The cap  300  is welded to the front end portion of the metallic shell  50  (here, the trunk portion  55 ). A melt portion  350  is formed between the cap  300  and the metallic shell  50 . The melt portion  350  is formed by a portion of the cap  300  and a portion of the trunk portion  55  which are melted at the time of welding. The melt portion  350  joins the trunk portion  55  and the cap  300  to each other. Specifically, the melt portion  350  joins an annular end portion, at the rear end direction Dfr side, of the cap  300  and an annular end portion, at the front end direction Df side, of the metallic shell  50  to each other. 
     The cap  300  and the metallic shell  50  form a space Sa (referred to as “specific space Sa”) surrounded by a surface  300   i  at the inner side (referred to as “inner surface  300   i ”) of the cap  300  and the inner peripheral surface  55   i  of the metallic shell  50  (here, the trunk portion  55 ). The discharge gap sg (that is, the tip  200  of the center electrode  20  and the ground electrodes  30 ) is located within the specific space Sa. The cap  300  has a plurality of holes  390  formed so as to provide communication between the inner side (i.e., the specific space Sa side) and the outer side of the cap  300 . Although not shown in detail, in the present embodiment, the cap  300  has one hole  390  provided on the central axis CL and four holes  390  located around the central axis CL. 
     As shown in  FIG. 2 , the cap  300  includes the projection portion  310  which projects at the inner peripheral side near a joined portion of the cap  300  and the trunk portion  55  toward the rear end direction Dfr side. The projection portion  310  is disposed at the inner peripheral side of the front end portion of the trunk portion  55 . A gap g 2  is formed between the inner peripheral surface  55   i  of the trunk portion  55  and an outer peripheral surface  310   o  of the projection portion  310 . The gap g 2  extends from the specific space Sa to the melt portion  350 . 
       FIG. 4  shows a cross section of the projection portion  310 . In the cross section in  FIG. 4 , the shape of the inner peripheral surface  55   i  of the trunk portion  55  is a substantially circular shape. The projection portion  310  is an annular portion disposed at the inner peripheral side of the inner peripheral surface  55   i  of the trunk portion  55 . An annular gap g is formed between the outer peripheral surface  310   o  of the projection portion  310  and the inner peripheral surface  55   i  of the trunk portion  55 . The projection portion  310  has a plurality of (here, four) recesses  310   r  formed as portions obtained by recessing the outer peripheral surface  310   o  toward the inner peripheral side. The plurality of recesses  310   r  are located at substantially equal intervals along the circumferential direction. The gap g includes second gaps g 2  formed by the recesses  310   r  and first gaps g 1  formed by the other portion of the outer peripheral surface  310   o.    
     The left portion of  FIG. 4  shows a partial cross section including the first gap g 1  and a partial cross section including the second gap g 2 . These partial cross sections are cross sections including the central axis CL, similarly to the cross section in  FIG. 2 . Of the inner peripheral surface  55   i  of the metallic shell  50 , the portion forming the gaps g 1  and g 2  is the inner peripheral surface of a portion, at the front end direction Df side, of the inner peripheral surface  55   i , and is parallel to the central axis CL. The outer peripheral surface  310   o  of the projection portion  310  faces the inner peripheral surface  55   i  to form the gaps g 1  and g 2 . 
     Each first gap g 1  communicates with the specific space Sa and extends from the specific space Sa to the melt portion  350 . Each second gap g 2  communicates with both the first gaps g 1  and the specific space Sa and extends from the specific space Sa to the melt portion  350 . In addition, each second gap g 2  is larger than each first gap g 1 . In particular, a size d 2 , in the radial direction, of each second gap g 2  is larger than a size d 1 , in the radial direction, of each first gap g 1 . The reason why the gaps g 1  and g 2  having different sizes are formed as described above will be described later. 
     An operation of the spark plug  100  described above will be described. The spark plug  100  is mounted to an internal combustion engine such as a gas engine when being used. A voltage is applied between the ground electrodes  30  and the center electrode  20  of the spark plug  100  by an igniter (e.g., a full-transistor igniter). As a result, spark discharge occurs in the discharge gap sg formed by each ground electrode  30  and the center electrode  20 . The spark discharge occurs within the specific space Sa. Meanwhile, an air-fuel mixture within a combustion chamber of the internal combustion engine is introduced through the holes  390  of the cap  300  into the specific space Sa. The air-fuel mixture within the specific space Sa is ignited by spark caused within the specific space Sa. Flame generated by combustion of the ignited air-fuel mixture jets out through the holes  390  of the cap  300  to the outside (i.e., the combustion chamber). The air-fuel mixture within the combustion chamber is ignited by the flame having jetted out. As a result, in particular, even in the case of an internal combustion engine including a combustion chamber having a relatively large volume, the entire air-fuel mixture within the combustion chamber can be rapidly combusted. As the configuration of the holes  390  of the cap  300  (e.g., the total number, the arrangement, and the inner diameters thereof), various configurations different from the configuration described with reference to  FIG. 2  can be used. In general, the configuration of the holes  390  may be experimentally determined such that an appropriate jet of flame is achieved. 
     A2. Manufacturing Method 
       FIG. 5  is a flowchart showing an example of a method for manufacturing the spark plug  100 . It is assumed that components of the spark plug  100  such as the metal terminal  40 , the metallic shell  50 , and the cap  300  have been already produced. The cap  300  can be produced, for example, by means of pressing. 
     In step S 100 , an assembly including the insulator  10 , the center electrode  20 , and the metal terminal  40  is produced. As a method for producing the assembly, a publicly known method can be used. For example, the center electrode  20 , the material of the first seal portion  60 , the material of the resistor  70 , and the material of the second seal portion  80  are inserted into the through hole  12  of the insulator  10  in this order. Then, in a state where the insulator  10  is heated, the metal terminal  40  is inserted into the through hole  12 , to produce the assembly. 
     In step S 110 , the ground electrodes  30  is joined to the inner peripheral surface  55   i  of the metallic shell  50 . Steps S 100  and S 110  can proceed independently of each other. 
     In step S 120 , the assembly is fixed to the metallic shell  50 . Specifically, the front packing  8 , the assembly in step S 100 , the second rear packing  7 , the talc  9 , and the first rear packing  6  are placed into the through hole  59  of the metallic shell  50 , and the crimp portion  53  of the metallic shell  50  is crimped so as to be bent inward, whereby the insulator  10  is fixed to the metallic shell  50 . 
     In step S 130 , the distance of the discharge gap sg is adjusted. For example, a gauge having a predetermined thickness is inserted into the discharge gap sg, and each ground electrode  30  is bent such that the distance of the discharge gap sg is equal to the thickness of the gauge. 
     In step S 140 , the cap  300  is fixed to the front end portion of the metallic shell  50 . First, the cap  300  is placed at a specific position covering the opening, at the front end direction Df side, of the metallic shell  50  (S 142 ). Then, the cap  300  placed at the specific position is welded to the metallic shell  50  (S 144 ). Through the above, the spark plug  100  is completed. 
       FIGS. 6(A) to 6(C)  are cross-sectional views showing an arrangement of the cap  300  with respect to the metallic shell  50  in step S 142  in  FIG. 5 .  FIG. 6(A)  shows a cross section of the entirety of the cap  300  and a portion, at the front end direction Df side, of the metallic shell  50 ,  FIG. 6(B)  shows a cross section around the second gap g 2 , and  FIG. 6(C)  shows a cross section around the first gap g 1 . These cross sections are cross sections including the central axis CL similarly to the cross section in  FIG. 2 . 
     The metallic shell  50  before welding has an end surface  55   f  at the front end direction Df side. The end surface  55   f  is an annular surface which extends around the central axis CL. In the present embodiment, the end surface  55   f  is a flat surface substantially perpendicular to the central axis CL. At a corner connecting the end surface  55   f  and the inner peripheral surface  55   i  (i.e., a corner, at the inner peripheral side, of an end portion, at the front end direction Df side, of the metallic shell  50 ), a chamfered portion  55   x  is formed such that the inner diameter thereof gradually decreases toward the rear end direction Dfr. The chamfered portion  55   x  is formed over the entire circumference of the corner, at the inner peripheral side, of the metallic shell  50 . 
     The cap  300  before welding has an end surface  300   r  at the rear end direction Dfr side. The projection portion  310  is connected to the inner peripheral side of the end surface  300   r , and projects in the rear end direction Dfr from the end surface  300   r . The end surface  300   r  is an annular surface which extends around the central axis CL. In the present embodiment, the end surface  300   r  is a flat surface substantially perpendicular to the central axis CL. The outer diameter of the end surface  300   r  is substantially equal to the outer diameter of the end surface  55   f  of the metallic shell  50 . 
     As shown in  FIG. 6(A) , in step S 142 , the projection portion  310  of the cap  300  is inserted into the through hole  59  through the opening OPf, at the front end direction Df side, of the metallic shell  50 . Accordingly, the projection portion  310  is located within the through hole  59  of the metallic shell  50 . The end surface  300   r  of the cap  300  is in contact with the end surface  55   f  of the metallic shell  50 . The cap  300  covers the opening OPf, at the front end direction Df side, of the metallic shell  50 . At this position, the cap  300  is welded to the metallic shell  50 . Hereinafter, the position at which the cap  300  is welded to the metallic shell  50  is referred to as “specific position”. 
     As shown in  FIGS. 6(B) and 6(C) , a gap gr is formed between the chamfered portion  55   x  of the metallic shell  50  and the cap  300 . The gap gr communicates with the gap g (the first gaps g 1  and the second gaps g 2 ). 
     The metallic shell  50  and the cap  300  disposed at the specific position form an annular gap gx which communicates with the specific space Sa surrounded by the metallic shell  50  and the cap  300  and is interposed between the metallic shell  50  and the cap  300 . The gap gx includes the gap g (i.e., the gaps g 1  and g 2 ) described with reference to  FIG. 4  and the gap gr described with reference to  FIGS. 6(B) and 6(C) . 
     Since the chamfered portion  55   x  is formed at an end portion, at the front end direction Df side, of the inner peripheral surface of the metallic shell  50 , insertion of the projection portion  310  is easy. Therefore, the size d 1  of each first gap g 1  can be reduced. By reducing the size d 1 , misalignment of the cap  300  relative to the metallic shell  50  (in particular, misalignment in a direction perpendicular to the central axis CL) can be decreased. 
       FIG. 7  is a schematic cross-sectional view in the welding in step S 144  in  FIG. 5 . In the drawing, the same cross-sectional view as in  FIG. 6(A)  is shown. In the drawing, arrows Lz denote a laser beam. In the present embodiment, the laser beam Lz is applied to the outer peripheral side of a boundary portion between the metallic shell  50  and the cap  300 , in a direction which is perpendicular to the central axis CL and toward the inner side in the radial direction. Accordingly, a portion, forming the end surface  55   f , of the metallic shell  50  and a portion, forming the end surface  300   r , of the cap  300  are melted, thereby forming the melt portion  350  which joins the metallic shell  50  and the cap  300  to each other. The melt portion  350  is formed so as to extend from the outer peripheral side to the inner peripheral side. 
       FIGS. 8(A) to 8(C) and 9(A) to 9(C)  are schematic cross-sectional views showing a situation in which the welding proceeds.  FIGS. 8(A) to 8(C)  show a region around the second gap g 2 , and  FIGS. 9(A) to 9(C)  show a region around the first gap g 1 . Each cross-sectional view is a cross section including the central axis CL, similarly to the cross sections in  FIGS. 6(A) to 6(C) . 
     Around the second gap g 2 , the welding proceeds in order of  FIG. 8(A) ,  FIG. 8(B) , and  FIG. 8(C) . By the application of the laser beam Lz, the cap  300  and the metallic shell  50  are melted. As the welding proceeds, a melted portion  350   m  extends in the inward direction Di from the outer peripheral surfaces of the cap  300  and the metallic shell  50  to the gap gr. Then, as a result of end of the application of the laser beam Lz, the melted portion  350   m  is cooled and solidified to form the melt portion  350 . At the time of formation of the melt portion  350  (i.e., at the time of welding), gas GS (e.g., air) present between the end surfaces  55   f  and  300   r  and within the gap gr is exhausted out through the second gap g 2 . 
     Around the first gap g 1 , the welding proceeds in order of  FIG. 9(A) ,  FIG. 9(B) , and  FIG. 9(C) . Similarly to  FIGS. 8(A) to 8(C) , as the welding proceeds, a melted portion  350   m  extends in the inward direction Di from the outer peripheral surfaces of the cap  300  and the metallic shell  50  to the gap gr. Then, as a result of end of the application of the laser beam Lz, the melted portion  350   m  is cooled and solidified to form the melt portion  350 . At the time of formation of the melt portion  350 , gas (e.g., air) present between the end surfaces  55   f  and  300   r  and within the gap gr can be exhausted out through the first gap g 1 . In addition, the gas can move through the gap gr to the second gap g 2  at an un-welded position in the circumferential direction and can be exhausted out through the second gap g 2 . 
     As shown in  FIGS. 8(A) to 8(C) and 9(A) to 9(C) , in the present embodiment, the entirety of the end surface  55   f  of the metallic shell  50  is welded. That is, the entirety of a portion between the outer peripheral surface and the inner peripheral surface  55   i  of the front end portion of the metallic shell  50  is welded. Therefore, the strength of joining can be enhanced as compared to the case where only a part of the portion is welded. 
       FIG. 10  is a schematic diagram showing an order of the welding. In the drawing, the same cross section as in  FIG. 4  is shown. The positions of arrows which denote the laser beam Lz in the drawing indicate positions of the laser beam Lz in the circumferential direction. In the method in  FIG. 10 , the laser beam Lz moves from a first position Lz 1  as a specific position counterclockwise around the cap  300  and the metallic shell  50 . Accordingly, the boundary portion between the cap  300  and the metallic shell  50  is welded over the entire circumference thereof. 
     The first position Lz 1  is a position slightly shifted from the position, in the circumferential direction, of one second gap g 2  (referred to as “specific second gap g 2 S”) in the moving direction of the laser beam Lz (here, in the counterclockwise direction). At the first position Lz 1 , the laser beam Lz does not form the melt portion  350  in the cross section ( FIGS. 8(A) to 8(C) ) including the specific second gap g 2 S, and forms the melt portion  350  in the cross section ( FIGS. 9(A) to 9(C) ) including the first gap g 1  adjacent to the specific second gap g 2 S. 
     In the drawing, a second position Lz 2  indicates the position, in the circumferential direction, of a portion to be welded lastly. The second position Lz 2  coincides with the position of the specific second gap g 2 S in the circumferential direction. The laser beam Lz which moves around in the circumferential direction as described above lastly forms the melt portion  350  in the cross section ( FIGS. 8(A) to 8(C) ) including the specific second gap g 2 S. 
     During welding at the same position in the circumferential direction as that of the first gap g 1 , gas is exhausted out via the gap gr through the second gap g 2  at an un-welded position in the circumferential direction (e.g., the specific second gap g 2 S). Then, welding at the same position in the circumferential direction as that of the specific second gap g 2 S is performed lastly. Therefore, regardless of a position in the circumferential direction where welding is performed, degassing can be appropriately performed at least through the specific second gap g 2 S. 
       FIGS. 11(A) to 11(C)  are schematic cross-sectional views showing a situation in which welding of a cap  300   z  and a metallic shell  50  in a reference example proceeds. The difference from the cap  300  of the embodiment in  FIGS. 8(A) to 8(C) and 9(A) to 9(C)  is only that the recesses  310   r  are omitted in a projection portion  310   z . In the case of using the cap  300   z  of the reference example, the second gaps g 2  are omitted, and an annular first gap g 1  is formed (not shown). The configuration of the other portion of the cap  300   z  of the reference example is the same as that of the portion corresponding to the cap  300  of the embodiment. Of the elements of the cap  300   z , the same elements as those of the cap  300  are designated by the same reference numerals, and the description thereof is omitted. The metallic shell  50  is the same as the metallic shell  50  of the embodiment. 
     The welding proceeds in order of  FIG. 11(A) ,  FIG. 11(B) , and  FIG. 11(C) . By application of a laser beam Lz, the cap  300   z  and the metallic shell  50  are melted. As the welding proceeds, a melted portion  354   m  extends in the inward direction Di from the outer peripheral surfaces of the cap  300   z  and the metallic shell  50  to a gap gr. Then, as a result of end of the application of the laser beam Lz, the melted portion  354   m  is cooled and solidified to form a melt portion  354 . 
     In the reference example, the large second gaps g 2  are not formed, and thus gas present within the gap gr may not be able to be sufficiently exhausted out. The gas that has not been exhausted out and has remained can be trapped in the melted portion  354   m  to form voids  352  within the melt portion  354 . Such voids  352  can cause cracking. Then, due to the voids  352 , the strength of joining can decrease. 
     In the present embodiment, since degassing can be appropriately performed through the specific second gap g 2 S at the time of welding as described above, a possibility can be reduced that the voids  352  are formed in the melt portion  350 . Therefore, a decrease in the strength of joining can be suppressed. 
     B. Second Embodiment 
       FIG. 12  is a cross-sectional view of a spark plug  100   b  of a second embodiment. In the drawing, the configuration in the same cross section as in  FIG. 4  is shown. The difference from the spark plug  100  in  FIG. 4  is only that the total number of recesses  310   r  provided in a projection portion  310   b  of a cap  300   b  is eight. The eight recesses  310   r  are located at substantially equal intervals along the circumferential direction. An annular gap gb is formed between an outer peripheral surface  310   bo  of the projection portion  310   b  and the inner peripheral surface  55   i  of the metallic shell  50 . The gap gb includes first gaps g 1  and second gaps g 2 . A method for welding the cap  300   b  and the metallic shell  50  is the same as the method described with reference to  FIGS. 6(A)  to  10 . In the second embodiment, since the total number of the recesses  310   r , that is, the total number of the second gaps g 2 , is large, degassing can be appropriately performed at the time of welding. Therefore, a decrease in the strength of joining can be suppressed. The configuration of the other portion of the spark plug  100   b  is the same as that of the portion corresponding to the spark plug  100  of the first embodiment (the same elements as the corresponding elements are designated by the same reference numerals, and the description thereof is omitted). In addition, the spark plug  100   b  can be manufactured by the manufacturing method in  FIG. 5 . 
     C. Third Embodiment 
       FIG. 13  is a cross-sectional view of a spark plug  100   c  of a third embodiment. In the drawing, the configuration in the same cross section as in  FIG. 4  is shown. The difference from the spark plug  100  in  FIG. 4  is only that the recesses  310   r  are omitted in a projection portion  310   c  of a cap  300   c , and instead, a plurality of (here, four) recesses  55   r  are formed in a portion, at the front side, of a trunk portion  55   c  of a metallic shell  50   c  as portions obtained by recessing an inner peripheral surface  55   ci  toward the outward direction Do. Such recesses  55   r  can be formed, for example, by means of cutting. The shape of an outer peripheral surface  310   co  of the projection portion  310   c  is a cylindrical shape about the central axis CL. The configuration of the other portion of the spark plug  100   c  is the same as that of the portion corresponding to the spark plug  100  (the same elements as the corresponding elements are designated by the same reference numerals, and the description thereof is omitted). In addition, the spark plug  100   c  can be manufactured by the manufacturing method in  FIG. 5 . 
     As shown, an annular gap gc is formed between the outer peripheral surface  310   co  of the projection portion  310   c  of the cap  300   c  and the inner peripheral surface  55   ci  of the metallic shell  50   c  (here, the trunk portion  55   c ). The gap gc includes second gaps g 2   c  formed by the recesses  55   r  and first gaps g 1  formed by the other portion of the inner peripheral surface  55   ci.    
     The cap  300   c  and the metallic shell  50   c  form a space Sc (referred to as “specific space Sc”) surrounded by the inner peripheral surface  55   ci  of the metallic shell  50   c  and a surface at the inner side of the cap  300   c  which surface is not shown. Although not shown, a discharge gap sg is located within the specific space Sc. 
     The left portion of  FIG. 13  shows a partial cross section including the first gap g 1  and a partial cross section including the second gap g 2   c . These partial cross sections are cross sections including the central axis CL, similarly to the cross section in  FIG. 2 . The configuration in the cross section including the first gap g 1  is the same as that in the cross section including the first gap g 1  as shown in  FIG. 4 . 
     In the partial cross section including the second gap g 2   c , the recess  55   r  is shown. As shown, the recess  55   r  extends from a position at the rear end direction Dfr side of the projection portion  310   c  toward the front end direction Df side. The second gap g 2   c  communicates with the first gaps g 1  and the specific space Sc. The second gap g 2   c  extends from the specific space Sc to a melt portion  350   c . A method for welding the cap  300   c  and the metallic shell  50   c  is the same as the method described with reference to  FIGS. 6(A)  to  10 . At the time of welding, degassing can be easily performed through the second gaps g 2   c.    
     D. Modified Embodiments 
     (1) As the method for welding the cap to the metallic shell, other various methods can be used instead of the method described with reference to  FIG. 10 . For example, after welding in the entire range of the positions of the first gaps g 1  in the circumferential direction is completed, welding may be performed at the positions of the second gaps g 2  or g 2   c  in the circumferential direction. In addition, welding at a plurality of positions in the circumferential direction may proceed in parallel. In either case, after welding at the positions, in the circumferential direction, of the first gaps g 1  which communicate with the second gaps g 2  or g 2   c , welding is preferably performed at the positions of second gaps g 2  or g 2   c  in the circumferential direction. By so doing, degassing can be appropriately performed through the second gaps g 2  or g 2   c  at the time of welding. 
     In general, as a method for fixing the cap and the metallic shell, the following method is preferably used. The cap is placed at a specific position with respect to the metallic shell ( FIG. 5 : S 142 ). The specific position is a position at which the cap covers the opening, at the front side, of the metallic shell, and is a position at which the cap is welded to the metallic shell. The metallic shell and the cap placed at the specific position form a specific space which is a space surrounded by the metallic shell and the cap (e.g., the specific space Sa ( FIG. 4 ) or the specific space Sc ( FIG. 13 )). In addition, the metallic shell and the cap placed at the specific position form an annular gap which communicates with the specific space and is interposed between the metallic shell and the cap (e.g., the gap gx ( FIGS. 6(A) to 6(C) ,  FIG. 10 )). The annular gap includes a first gap which communicates with the specific space, and a second gap which communicates with the specific space and is larger than the first gap (e.g., the first gaps g 1  and the second gaps g 2  ( FIG. 10 )). 
     Then, the cap is welded to the metallic shell ( FIG. 5 : S 144 ). In this welding, the boundary between the cap and the metallic shell which communicates with the annular gap is welded (e.g., a portion including the end surfaces  55   f  and  300   r  ( FIGS. 8(A) to 8(C) and 9(A) to 9(C) )). Here, of the boundary between the metallic shell and the cap, a specific portion which is a portion whose position in the circumferential direction is different from the position, in the circumferential direction, of the specific second gap is welded. For example, in the example in  FIG. 10 , the position, in the circumferential direction, of the specific portion is a remaining portion obtained by excluding the position, in the circumferential direction, of the specific second gap g 2 S from the entire range in the circumferential direction. Then, after the welding at the specific portion, a portion whose position in the circumferential direction is the same as the position, in the circumferential direction, of the specific second gap, of the boundary between the metallic shell and the cap, is welded. Because of the above, when the welding at the specific portion is performed, degassing can be appropriately performed through the specific second gap. In addition, by performing welding over the entire circumference of the boundary between the metallic shell and the cap, the strength of joining can be enhanced. 
     The method for welding the cap to the metallic shell can be changed in accordance with the configurations of the cap and the metallic shell. For example, a gap may be provided in only a part of the range in the circumferential direction. In this case, welding may be performed at only the position in the circumferential direction at which the gap is provided, of the entire range in the circumferential direction. In either case, if the gap includes a first gap which communicates with the specific space and a second gap which communicates with the specific space and is larger than the first gap, after welding is performed at the same position in the circumferential direction as that of the first gap, welding is preferably performed at the same position in the circumferential direction as that of the second gap. By so doing, degassing can be appropriately performed through the second gap. Here, in order to appropriately perform degassing, the first gap further preferably communicates with the second gap. In addition, the method for manufacturing the spark plug is not limited to the method shown in  FIG. 5 , and other various methods can be used. 
     (2) The outer diameter of the projection portion  310 ,  310   b , or  310   c  may be equal to or larger than the inner diameter of the portion, at the front end direction Df side, of the metallic shell  50  or  50   c  (here, the trunk portion  55  or  55   c ). In this case, the size d 1  of each first gap g 1  is zero (i.e., each first gap g 1  is omitted).  FIG. 14  is a cross-sectional view of a spark plug  100   d  of a modified embodiment. In the drawing, the configuration in the same cross section as in  FIG. 4  is shown. The difference from the spark plug  100  in  FIG. 4  is only that the size d 1  of each first gap g 1  is zero, that is, each first gap g 1  is omitted. Of an outer peripheral surface  310   do  of a projection portion  310   d  of a cap  300   d , a portion other than recesses  310   r  is in close contact with the inner peripheral surface  55   i  of the metallic shell  50 . The configuration of the other portion of the spark plug  100   d  is the same as that of the portion corresponding to the spark plug  100  (the same elements as the corresponding elements are designated by the same reference numerals, and the description thereof is omitted). In addition, the spark plug  100   d  can be manufactured by the manufacturing method in  FIG. 5 . 
     In step S 142  in  FIG. 5 , the cap  300   d  can be placed at a specific position by press-fitting the projection portion  310   d  of the cap  300   d  into the through hole  59  through the opening OPf of the metallic shell  50 . As shown in  FIG. 14 , a plurality of gaps g 2  are provided at a plurality of positions in the circumferential direction when being seen from the direction of the axis CL (i.e., when being seen from a direction parallel to the axis CL). Before welding, each of the plurality of gaps g 2  communicate with an annular gap gr formed by the chamfered portion  55   x  ( FIG. 6(B) ). In step S 144  in  FIG. 5 , welding is performed by the same method as described with reference to FIG.  10 . By so doing, gas within the gap gr can be exhausted out from the gap gr through the second gap g 2  at an un-welded position in the circumferential direction. Therefore, occurrence of the voids  352  in the melt portion can be suppressed. 
     In addition, the gap which extends from the specific space to the melt portion may be provided in only a part of the range in the circumferential direction. For example, in each of the above-described embodiments, each first gap g 1  may be provided only near the second gap g 2  or g 2   c . In this case, a plurality of gaps which each include a first gap g 1  and a second gap g 2  or g 2   c  and are separated from each other are located at a plurality of positions in the circumferential direction. Here, the second gap g 2  or g 2   c  may be omitted. In this case, a plurality of first gaps g 1  which are separated from each other are located at a plurality of positions in the circumferential direction. As described above, regardless of the size of each gap, a plurality of gaps which are separated from each other may be located at a plurality of positions in the circumferential direction. Of the entire range in the circumferential direction, in a range where the size of the gap is zero (i.e., in a range where the gap is omitted), the cap and the metallic shell may not be welded, and in the range where the gaps are provided, the cap and the metallic shell may be welded. From the standpoint that desired joining strength between the cap and the metallic shell is ensured, the cap and the metallic shell are preferably welded over the entire range in the circumferential direction. 
     As the size of each gap, the maximum outer diameter of a sphere which can be disposed within the gap can be used. In addition, in order to achieve gas exhaust out through the first gap, the size of the first gap is preferably equal to or greater than 0.1 mm. In order to suppress misalignment of the cap relative to the metallic shell, the first gap is preferably equal to or less than 0.2 mm. However, the size of the first gap may be less than 0.1 mm or exceed 0.2 mm. In addition, the size of the second gap may be, for example, equal to or greater than 0.3 mm. In order to suppress an excessive increase in the size of the projection portion, the size of the second gap is preferably equal to or less than 1.0 mm. However, the size of the second gap may be less than 0.3 mm or exceed 1.0 mm. 
     (3) In each of the above-described embodiments, a part of the cap (here, the projection portion  310 ,  310   d ,  310   c , or  310   d ) is located at the inner peripheral side of the melt portion  350  or  350   c  which joins the cap  300 ,  330   b ,  300   c , or  300   d  and the metallic shell  50  or  50   c . Therefore, scattering of a melted material into the specific space Sa or Sc at the time of welding can be suppressed. If the melted material scatters into the specific space Sa or Sc at the time of welding, the material that has scattered can be attached to the electrodes  20  and  30 . As a result, discharge can occur along an unintended discharge path. In order to suppress such a problem, preferably, the cap includes, at the inner peripheral side of the inner peripheral surface of the metallic shell, a projection portion which projects to the rear end direction Dfr side, and the projection portion is located at the inner peripheral side of the melt portion. Instead of this, the metallic shell may include, at the inner peripheral side of the inner peripheral surface of the cap, a projection portion which projects to the front end direction Df side, and the projection portion may be located at the inner peripheral side of the melt portion. However, such a projection portion may be omitted. 
     (4) In each of the above-described embodiments, of the surface of the metallic shell  50  or  50   c , the surface  55   i  or  55   ci  (referred to as “first surface”) which forms the first gaps g 1  and the second gaps g 2  or g 2   c  is parallel to the central axis CL. In addition, of the surface of the cap  300 ,  300   b ,  300   c , or  300   d , the surface  310   o ,  310   bo ,  310   co , or  310   do  (referred to as “second surface”) which forms the first gaps g 1  and the second gaps g 2  or g 2   c  is parallel to the central axis CL. Therefore, by causing the second surface of the cap to face the first surface of the metallic shell, misalignment of the cap relative to the metallic shell (in particular, misalignment in the direction perpendicular to the central axis CL) can be suppressed. 
     In each of the above-described embodiments, the first surface is a portion, at the front side, of the inner peripheral surface  55   i  or  55   ci  of the metallic shell  50  or  50   c . The projection portion  310 ,  310   b ,  310   c , or  310   d  of the cap  300 ,  300   b ,  300   c , or  300   d  projects toward the rear end direction Dfr and is located at the inner peripheral side of the first surface. The second surface is the outer peripheral surface  310   o ,  310   bo ,  310   co , or  310   do  of the projection portion  310 ,  310   b ,  310   c , or  310   d . Therefore, By inserting the projection portion  310 ,  310   b ,  310   c , or  310   d  into the through hole  59  of the metallic shell  50  or  50   c , misalignment of the cap with respect to the metallic shell (in particular, misalignment in the direction perpendicular to the central axis CL) can be easily suppressed. 
     In general, the first surface of the metallic shell is preferably not perpendicular to the central axis CL. Here, among the angles formed between the normal of the first surface of the metallic shell and the central axis CL, the acute angle is preferably equal to or greater than 45 degrees, particularly preferably equal to or greater than 70 degrees, and most preferably 90 degrees (i.e., the first surface is parallel to the central axis CL). 
     Both the first surface and the second surface are preferably annular surfaces which extend around the central axis CL. According to this configuration, by disposing the cap with respect to the metallic shell such that the second surface faces the first surface, misalignment of the cap with respect to the metallic shell (in particular, misalignment in the direction perpendicular to the central axis CL) can be suppressed. However, at least one of the first surface and the second surface may be formed in only a part of the range in the circumferential direction. 
     (5) Of the surface of the metallic shell, the surface that forms the first gaps which communicate with the specific space surrounded by the metallic shell and the cap and the second gaps which communicate with the first gaps and the specific space and are larger than the first gaps, may be a portion of the metallic shell that is different from the inner peripheral surface thereof. Similarly, of the surface of the cap, the surface that forms the first gaps and the second gaps may be a portion of the cap that is different from the surface, at the outer peripheral side, of the projection portion. For example, the front end surface of the metallic shell and the rear end surface of the cap may form the first gaps and the second gaps. 
     (6) The gap which extends from the specific space surrounded by the metallic shell and the cap to the melt portion (the gap interposed between the metallic shell and the cap) is preferably an annular gap which extends around the central axis CL, like the above-described gaps g, gb, and gc. According to this configuration, degassing is possible from any position in the circumferential direction, and thus a possibility of occurrence of a problem due to welding can be reduced. 
     (7) The gap gr (i.e., the chamfered portion  55   x  ( FIG. 6(A) ) may be omitted. In this case, at the time of welding, gas present at the boundary between the cap and the metallic shell (e.g., between the end surfaces  55   f  and  300   r ) can be exhausted out via an un-welded boundary through the gap (e.g., the gaps g 1  and g 2 ) which communicates with the specific space (e.g., the specific space Sa). Even in the case where the gap gr (i.e., the chamfered portion  55   x ) is omitted as described above, at the time of welding, gas can be appropriately exhausted out through the gap which communicates with the specific space. 
     In order to make it easy to insert the projection portion (e.g., the projection portion  310  in  FIG. 6(A) ) of the cap into the through hole of the metallic shell, a chamfered portion is preferably formed in at least one of the metallic shell and the projection portion. For example, at a corner, at the outer peripheral side, of the end portion, at the rear end direction Dfr side, of the projection portion of the cap, a chamfered portion may be formed such that the outer diameter thereof gradually decreases toward the rear end direction Dfr. 
     (8) As the arrangement of the plurality of second gaps, other various arrangements can be used instead of the arrangements shown in  FIGS. 4, 12, 13, and 14 . For example, the plurality of second gaps may be arranged unequally along the circumferential direction. In general, an arrangement described below is preferably used. The metallic shell and the cap form N second gaps whose positions in the circumferential direction are different from each other, and N is an integer which is equal to or higher than 3. The N second gaps are projected parallel to the axis CL, onto a projection surface perpendicular to the axis CL. For example, the cross-sectional views in  FIGS. 4, 12, 13 , and  14  correspond to the above projection surface. Then, the projection surface is divided into three regions each having a center angle of 120 degrees centered at the axis CL. In each of the drawings, three regions A 1 , A 2 , and A 3  each having a center angle of 120 degrees centered at the axis CL are shown. The plurality of second gaps are preferably arranged such that each of the three regions includes one or more of the second gaps. In each of the embodiments in  FIGS. 4, 12, 13, and 14 , each of the regions A 1 , A 2 , and A 3  includes one or more of the second gaps g 2  or g 2   c . When the N second gaps g 2  are arranged so as to be dispersed in the three regions A 1 , A 2 , and A 3  each having a center angle of 120 degrees as described above, uneven arrangement of the N second gaps g 2  in the circumferential direction can be suppressed. Therefore, for example, a problem that appropriate degassing cannot be performed in the case of welding at a specific position in the circumferential direction (e.g., a position distant from any second gap g 2 ) can be suppressed. 
     The number of the second gaps included in each of the three regions A 1 , A 2 , and A 3  can be changed by changing the positions, in the circumferential direction, of the three regions A 1 , A 2 , and A 3 . For example, in the embodiments in  FIGS. 4, 12, 13, and 14 , when boundary lines between the three regions A 1 , A 2 , and A 3  are rotated clockwise, the number of the second gaps g 2  or g 2   c  in each of the regions A 1 , A 2 , and A 3  changes. In general, the N second gaps may be arranged so as to allow the projection surface to be divided into three regions each having a center angle of 120 degrees centered at the axis CL such that each of the three regions include one or more of the second gaps. 
     Although the arrangement of the second gaps has been described above, in the case where a plurality of gaps separated from each other regardless of the sizes of the gaps are arranged at a plurality of positions in the circumferential direction, the plurality of gaps are preferably arranged so as to be dispersed in the three regions A 1 , A 2 , and A 3 , similarly to the above arrangement of the second gaps. 
     (9) The configuration of the spark plug including the cap and the metallic shell is not limited to the configuration of each of the embodiments and modified embodiments described above, and various configurations which allow degassing to be performed at the time of welding can be used. In general, a configuration described below is preferably used. The spark plug includes: a metallic shell having a through hole extending in the direction of the axis; a cap covering an opening, at the front side, of the metallic shell; and a melt portion formed between the cap and the metallic shell to join the cap and the metallic shell to each other. The spark plug includes a gap extending from a specific space which is a space surrounded by the metallic shell and the cap to the melt portion and interposed between the metallic shell and the cap. When such a configuration is used, degassing is possible through the gap at the time of formation of the melt portion (at the time of welding), and thus occurrence of a problem due to welding can be suppressed. 
     In the spark plug, as the configuration of the portion other than the joined portion of the cap and the metallic shell, any configuration can be used. For example, the tip  200  of the center electrode  20  may be omitted. In this case, the leg portion  25  preferably includes a portion corresponding to the tip  200 . In addition, a tip formed by using a material having excellent durability against discharge may be provided at the portion, forming the discharge gap sg, of each ground electrode  30 . Moreover, the configurations of the center electrode and each ground electrode (e.g., the configuration of the portion forming the discharge gap sg) is not limited to the configuration in  FIGS. 2 and 3 , and any other configuration can be used. 
     Although the present invention has been described above based on the embodiments and the modified embodiments, the above-described embodiments of the invention are intended to facilitate understanding of the present invention, not as the present invention. The present invention can be changed and modified without departing from the gist thereof and the scope of the claims and equivalents thereof are encompassed in present invention. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               5 : gasket 
               6 : first rear packing 
               7 : second rear packing 
               8 : front packing 
               9 : talc 
               10 : insulator (ceramic insulator) 
               11 : second reduced outer diameter portion 
               12 : through hole (axial bore) 
               13 : leg portion 
               15 : first reduced outer diameter portion 
               16 : reduced inner diameter portion 
               17 : front trunk portion 
               18 : rear trunk portion 
               19 : flange portion 
               20 : center electrode 
               21 : outer layer 
               22 : core portion 
               23 : head portion 
               24 : flange portion 
               25 : leg portion 
               27 : axial portion 
               30 : ground electrode 
               40 : metal terminal 
               50 ,  50   c : metallic shell 
               51 : tool engagement portion 
               52 : thread 
               53 : crimp portion 
               54 : seat portion 
               55 ,  55   c : trunk portion 
               55   f : end surface 
               55   i ,  55   ci : inner peripheral surface 
               55   r : recess 
               55   x : chamfered portion 
               56 : reduced inner diameter portion 
               58 : deformable portion 
               59 : through hole 
               60 : first seal portion 
               70 : resistor 
               80 : second seal portion 
               100 ,  100   b ,  100   c : spark plug 
               200 : tip 
               200   s : side surface 
               310 ,  310   b ,  310   c ,  310   z : projection portion 
               300 ,  300   b ,  300   c ,  300   z : cap 
               300   i : inner surface 
               300   r : end surface 
               310   o ,  310   bo ,  310   co : outer peripheral surface 
               310   r : recess 
               350 ,  354 : melt portion 
               350   m ,  354   m : melted portion 
               352 : void 
               390 : hole 
             sg: discharge gap 
             g, gb, gc, gr, gx: gap 
             g 1 : first gap 
             g 2 , g 2   c : second gap 
             g 2 S: specific second gap 
             A 1 , A 2 , A 3 : region 
             CL: central axis (axis) 
             GS: gas 
             Sa, Sc: specific space 
             Df: front end direction 
             Dfr: rear end direction 
             Di: inward direction 
             Do: outward direction 
             Lz: laser beam 
             Lz 1 : first position 
             Lz 2 : second position 
             OPf: opening