Patent Publication Number: US-2013241409-A1

Title: Non axis symmetric spark plug with offset bore

Description:
CROSS-REFERENCE 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/611,874, filed Mar. 16, 2012 and entitled “Small Diameter Non Axis Symmetric Spark Plug with Offset Bore,” the entire disclosure of which is incorporated herein. 
    
    
     BACKGROUND 
     This application relates generally to spark plugs for internal combustion engines, and more particularly, to the construction of a small diameter spark plug. More particularly, the present description relates to a small diameter spark plug having increased dielectric strength. 
     Conventional spark plugs for internal combustion engines generally include a threaded outer metal shell and an insulator disposed within the outer shell, the insulator formed to include a bore in which an electrode is arranged. The bore is traditionally aligned with the center axis of the spark plug. The bore may extend through the entire length of the insulator and be aligned symmetrically down the middle of the insulator and the outer shell. In this way, the insulator and outer shell are symmetrical about the electrode extending through the bore of the insulator, and the electrode extends through the insulator along the center axis of the spark plug. When installed in the internal combustion engine, conventional spark plugs are configured to have a standard outer diameter configured to fit inside a customary threaded hole provided in an engine head to protrude into a combustion chamber. 
     The insulator of the spark plug may be measured by its dielectric strength. The dielectric strength of a material refers to the maximum electrical potential that a material can withstand without failure. For example, in the field of electrical insulators, the dielectric strength of the insulator refers to the amount of electrical potential the insulator can withstand without experiencing failure of its insulation properties. Among the factors that may affect the dielectric strength of the material is the thickness of the material. For example, an increase in the thickness of the insulation material may lead to a higher dielectric strength. 
     A small diameter spark plug may include a threaded outer shell used for installation in a standard threaded hole in the engine head. Because of the mechanical strength necessary to support the mechanical threads on the outer shell, the outer shell wall must be thicker adjacent the threads. However, because such a spark plug must be useable in standard or conventional threaded holes of an engine head, the outer diameter of the spark plug must remain constant. Thus, use of thicker material for the outer shell wall translates into reduction in the amount and thickness of the insulator of the small diameter spark plug. The limited thickness of the insulator may reduce the potential dielectric strength of the insulator. A result of this design is that the spark plug includes a thin ceramic wall insulator that requires lower voltage ignition systems in order to reduce the likelihood of failure. In addition, the spark plug may require a smaller spark gap between the electrode and a ground electrode attached to the metal shell due to the lower voltage requirement. 
     Accordingly, while existing spark plug systems are suitable for their intended purpose, the need for improvement remains and it may be desirable to provide a small diameter spark plug in which the thickness of the insulator is maintained or increased in order to maintain or increase the dielectric strength of the insulator, while maintaining the size of the outer diameter of the spark plug in order to be compatible with standard ignition systems. 
     SUMMARY 
     According one exemplary embodiment, there is provided a spark plug including an outer shell defining a first longitudinal axis. The spark plug also includes an insulator having a first opening at a first end thereof and a second opening at a second end thereof, at least a portion of the insulator extending within the outer shell. An insulator bore extends between the first opening and second opening of the insulator, and includes a first bore section extending along first longitudinal axis, a second bore section extending along a second longitudinal axis. The second longitudinal axis is offset from the first longitudinal axis. A stepped bore section is positioned between the first bore section and second bore section. The spark plug also includes a terminal stud at least partially disposed in the first bore section, a center electrode positioned at least partially in the second bore section, and a ground electrode extending with the insulator and terminating at a position axially beyond the second end of the insulator. 
     In another exemplary embodiment, there is provided an insulator for a spark plug, the insulator including a first end having a first opening, a second end having a second opening, and a bore extending from the first opening to the second opening. The bore includes a first section extending along a first longitudinal axis and configured to receive a terminal stud and a second section extending along a second longitudinal axis and configured to receive a center electrode. The second longitudinal axis is offset from the first longitudinal axis. 
     In still another exemplary embodiment, there is provided a spark plug including an outer shell and an insulator having a bore extending therethrough, the bore having a first bore section and a second bore section. The first bore section and the outer shell extend along a common longitudinal axis and the second bore section extends along a second longitudinal axis offset from the common longitudinal axis. The spark plug further includes a center electrode and a ground electrode. 
     The above described and other features are exemplified by the following figures and detailed description. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional side view of a spark plug according to an exemplary embodiment of the present invention; 
         FIG. 2  is a side view of the spark plug of  FIG. 1 ; 
         FIG. 3  is a front end view of the spark plug of  FIG. 1 ; 
         FIG. 4  is a cross-sectional side view of a spark plug according to another exemplary embodiment of the present invention; 
         FIG. 5  is a side view of the spark plug of  FIG. 4 ; and, 
         FIG. 6  is a front end view of the spark plug of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     A spark plug  5  in accordance with exemplary embodiments of the present disclosure includes an outer metal shell  10 , an insulator  20  at least partially surrounded by the outer shell  10 , and a center electrode  40  surrounded by the insulator  20 . The insulator  20  includes a bore  26  through which the center electrode  40 , a terminal stud  30 , and a resistor  60  are located. The center electrode  40  extends past the insulator  20 , and a ground electrode  50  extends from the outer shell  10  adjacent to where the center electrode  40  extends past the insulator  20 . A center longitudinal axis of the outer shell  10  is aligned along a first longitudinal axis A. The outer shell  10  is configured to include a threaded region  17  that enables at least the center electrode  40  and the ground electrode  50  of the spark plug  5  to be secured into an opening (not shown) in a combustion chamber of an engine (not shown). In illustrative embodiments, at least a portion of the bore  26  is not aligned with the first longitudinal axis A but instead is offset form the first longitudinal axis A along a second longitudinal axis B that may be parallel to the first longitudinal axis A. In other words, the portion of the bore that is not aligned with the first longitudinal axis A is not symmetrical about the longitudinal axis of the outer shell  10 , but instead, is symmetrical about the second longitudinal axis B. The offset of the portion of the bore  26  enables the insulator  20  to be formed of the same or increased amount of insulation material along the metal shell  10 , especially within the region where the ground electrode  50  is coupled to the metal shell  10 , thereby maintaining or increasing the dielectric strength of the insulator  20 . 
     As illustrated in  FIG. 1 , the outer shell  10  of the spark plug  5  includes a first end  15  and a second end  16  opposite the first end  15 . A first-diameter portion  11  is adjacent the first end  15 , a second-diameter portion  12  is adjacent the second end  16 , and a stepped-diameter portion  13  is positioned between the first-diameter portion  11  and the second-diameter portion  12 . The outer shell  10  defines the first longitudinal axis A along which the length of the outer shell  10  extends. A bore  14  extends through the length of the outer shell  10  from the first end  15  to the second end  16  and is also aligned along the first longitudinal axis A. The outer shell  10  may be manufactured using suitable processes. For example, the outer shell  10  may be machined or headed. 
     The first-diameter portion  11  includes the threaded region  17  extending radially outward from an outer surface of the outer shell  10 . The threaded region  17  may be used to threadably engage a corresponding threaded hole in an engine (not shown), thereby securably fixing the spark plug  5  to the engine. In an exemplary embodiment, the threaded region  17  is disposed toward a central region of the spark plug  5 , away from longitudinal ends of the spark plug  5 . The first-diameter portion  11  also includes an inner diameter defined by the bore  14  and a protrusion  18  extending radially inward, i.e., toward the first longitudinal axis A, as illustrated in  FIG. 1 . 
     The second-diameter portion  12  of the outer shell  10  includes an inner diameter also defined by the bore  14  and an outer diameter D  1 . In an exemplary embodiment, the outer diameter D 1  may be in the range of 7.5 mm-10 mm, but is not limited thereto. In an illustrative embodiment, the inner and outer diameters of the second-diameter portion  12  may be less than the inner and outer diameters of the first-diameter portion  11 . 
     The stepped-diameter portion  13  is positioned between the first-diameter portion  11  and the second-diameter portion  12  along the length of the outer shell  10 . The stepped-diameter portion  13  includes an inner diameter of varying size along at least part of a length of the stepped-diameter portion  13 . The stepped-diameter portion  13  also includes an outer diameter of varying size along at least part of the length of the stepped-diameter portion  13 . As illustrated in  FIG. 2 , the outer surface of the stepped-diameter portion  13  is configured to form a seat  13   a  of the spark plug  5  when the spark plug  5  is engaged in an threaded hole of the engine (not shown). 
     It should be understood that the first-diameter portion  11 , the second-diameter portion  12 , and the stepped-diameter portion  13  of the outer shell  10  are referenced for the purpose of assisting in the identification of various features in the exemplary embodiments of the present invention. The relative dimensions of the portions and their respective positions relative to the other features of the exemplary embodiments are not limited to the configurations shown in the illustrated exemplary embodiments. 
     The insulator  20  of the spark plug  5  includes a first end  21  and a second end  22  opposite the first end  21 . When the insulator  20  is located in the bore  14  of the outer shell  10 , the first end  21  and the second end  22  extend past the first end  15  and the second end  16  of the outer shell  10 , respectively. The insulator  20  further includes at least one groove  24   a  formed along an outer periphery thereof. The at least one groove  24   a  is configured to receive the protrusion  18  of the outer shell  10  when the insulator  20  is secured in the bore  14  of the outer shell  10 . 
     The insulator  20  includes a first section  23 , a second section  24 , and a third section  25 . In an exemplary embodiment, the first section  23  of the insulator  20  represents an outer surface of the spark plug  5  and extends from the first end  21  of the insulator  20  to a point where the insulator  20  is adjacent the first end  15  of the outer shell  10 . The second section  24  of the insulator  20  extends away from the first section  23  to a point where the insulator  20  is adjacent the second end  16  of the outer shell  10 . An outer diameter of the insulator  20  along the second section  24  generally corresponds to the respective inner diameters of the first-diameter portion  11 , second-diameter portion  12  and stepped-diameter portion  13  of the outer shell  10 , so that the second section  24  of the insulator  20  may be tightly received in the outer shell  10 . That is, the outer diameter of the insulator  20  along the second section  24  is generally tightly received in the bore  14  of the outer shell  10 . 
     In illustrative embodiments, and as seen in  FIG. 1 , a gap or opening  64  may be formed between the second section  24  and the outer shell  10  adjacent the second end  16  of the outer shell  10 . The second section  24  of the insulator  20  adjacent the opening  64  is reduced in diameter to form the opening  64  while maintaining the outer diameter of the second end  16  of the outer shell  10  at a constant diameter. The reduction in diameter of the insulator  20  means that the insulation material may be reduced along that portion of the insulator  20 . The opening  64  is configured to receive a portion of the ground electrode  50 , as further discussed below. 
     The third section  25  of the insulator  20  extends axially beyond the second end  16  of the outer shell  10  to the second end  22  of the insulator  20 . The third section  25  includes a tapered portion  25   a  where an outer diameter of the third section  25  is reduced. The outer diameter of the third section  25  generally represents an outer surface of the spark plug  5 . 
     It should be understood that the first section  23 , the second section  24 , and the third section  25  of the insulator  20  are referenced for the purpose of assisting in the identification of various features in the exemplary embodiments of the present invention. The relative dimensions of the sections and their respective positions relative to the other features of the exemplary embodiments are not limited to the configurations shown in the illustrated exemplary embodiments. 
     The first end  21  of the insulator  20  is formed to include a first opening  21   a , and the second end  22  of the insulator  20  is formed to include a second opening  22   a . A bore  26  extends through the insulator  20  from the first opening  21   a  to the second opening  22   a . The bore  26  includes a first-bore section  27 , a stepped-bore section  26   a , and a second-bore section  28 . The second-bore section  28  is located opposite of the first-bore section  27 , and the stepped-bore section  26   a  is located between the second-bore section  28  and the first-bore section  27 . The first-bore section  27 , the second-bore section  28 , and the stepped-bore section  26   a  are configured to receive the terminal stud  30  or the center electrode  40 , or both, as further discussed below. 
     The first-bore section  27  may be defined between the first opening  21   a  of the insulator  20  and the stepped-bore section  26   a  within the bore  26 . The first-bore section  27  extends generally coaxially with the first section  23  of the insulator  20  and the outer shell  10 . That is, the first-bore section  27  has a common longitudinal axis, the first longitudinal axis A, with the outer shell  10  and the first section  23  of the insulator  20 . The first-bore section  27  may also be symmetrical about the first longitudinal axis A. The first-bore section  27  may have a first bore diameter B 1 . 
     The second-bore section  28  may be defined between the stepped-bore section  26   a  of the bore  26  and the second opening  22   a  of the insulator. The second-bore section  28  has a diameter that is less than the diameter of the first-bore section  27  and extends along a second longitudinal axis B that is offset from first longitudinal axis A. The second-bore section  28  is therefore not symmetrical about the first longitudinal axis A. The second-bore section  28  may have a second bore diameter B 2 . In illustrative embodiments, the second bore diameter B 2  is less than the first bore diameter B 1 . 
     As the second-diameter portion  12  of the outer shell  10  near the second-bore section  28  is maintained at a constant diameter B 2  and the insulator  20  generally fits tightly against the outer shell  10 , the offset or shifted second longitudinal axis B ensures there is additional insulator material between the second-bore section  28  and a portion of the outer shell  10  and/or the ground electrode  50 . This insulator material traditionally would have been reduced due to the opening  64  for the ground electrode  50  in standard spark plugs. Accordingly, the insulator  20 , in the region of the second-bore section  28 , has an increased wall thickness along an axial side  34  of the spark plug  5 . Specifically, side  34  is located along the same side as opening  64  or ground electrode  50 . Thus, in the region of the second-bore section  28 , the wall thickness of the insulator  20  varies such that an increased wall thickness is provided along the side  34  that is opposite from which the second-bore section  28  is offset. Here, the term “increased” is used to describe the thickness of the material of the insulator  20  relative to a scenario where the first-bore section  27  and the second-bore section  28  extend coaxially with one another along the same first longitudinal axis A, as in standard spark plugs. 
     The increased thickness in the insulator  20 , which results from the features detailed above, increases the dielectric strength of the insulator  20 . As a result, the spark plug may handle a larger potential difference without failing, and thus, may be used in standard ignition systems using a higher voltage and including a standard spark gap G. 
     The insulator  20  may be formed of any suitable insulating material used in spark plugs. For example, the insulator  20  may be formed of a ceramic material. The insulator  20  may be manufactured using several suitable processes. For example, the insulator  20  may be made by injection molding or using an isostatic press and grind process. 
     In exemplary embodiments, the terminal stud  30  is inserted into the bore  26  of the insulator  20  and includes a head  31  and body  32 . The head  31  of the terminal stud  30  extends outwardly from the first section  23  of the insulator  20 , past the first opening  21   a  of the bore  26 . The body  32  extends within the first section  23  of the insulator  20  in the first-bore section  27 . In an exemplary embodiment, the body  32  terminates in the first-bore section  27 , as illustrated in  FIG. 1 . The terminal study  30  may generally be aligned with the first longitudinal axis A 
     The center electrode  40  also extends within the bore  26  of the insulator  20 . In an exemplary embodiment, the center electrode  40  includes a skirt  41  and a body  42 . The skirt  41  of the center electrode  40  may be positioned in the first-bore section  27  and the stepped-bore section  26   a . The skirt  41  may be offset from the second longitudinal axis B and may generally be aligned with the first longitudinal axis A. In illustrative embodiments, the body  42  of the center electrode  40  extends through the second-bore section  28  of the bore  26 . A head section  43  of the body  42  extends axially beyond the second end  22  of the insulator  20  through the second opening  22   a  of the bore  26 . 
     The ground electrode  50  extends along a portion of the length of the outer shell  10  and insulator  20  and projects beyond the insulator  20  and center electrode  40 . The ground electrode  50  may be at least partially received in a slot of the insulator  20 , as illustrated in  FIG. 1 . The ground electrode  50  includes a first segment  51 , a second segment  52 , and a third segment  53 . The ground electrode  50  may be formed in a specialized press or other suitable manufacturing process. 
     The first segment  51  of the ground electrode  50  extends generally along a portion of the length of the outer shell  10  and insulator  20 , as illustrated in  FIG. 1 . At least a portion of the first segment  51  may be positioned between the outer shell  10  and insulator  20  in the opening  64  to secure the ground electrode  50  to the outer shell  10 . The second segment  52  of the ground electrode  50  is positioned at one end of the first segment  51  and may form a curved portion of the ground electrode  50 . The third segment  53  of the ground electrode  50  is positioned at one end of the second segment  52  and extends in a direction generally perpendicular to at least a portion of the first segment  51  and the second longitudinal axis B. The third segment  53  extends across the first and second longitudinal axes A and B. The third segment  53  is spaced from the head section  43  of the center electrode  40 , thereby providing a spark gap G between the third segment  53  of the ground electrode  50  and the center electrode  40 . 
     A resistor  60  may be positioned in the bore  26  of the insulator  20  between the terminal stud  30  and the center electrode  40 . In an exemplary embodiment, the resistor  60  may be positioned in the first-bore section  27  and provides an electrical communication between the terminal stud  30  and the center electrode  40 . In exemplary embodiments, a first contact glass portion  61  and a second contact glass portion  62  are disposed adjacent to respective ends of the resistor  60 . The first contact glass portion  61  is disposed between the terminal stud  30  and the resistor  60 . The second contact glass portion  62  is disposed between the resistor  60  and the second contact glass portion  62 . 
     With reference to  FIGS. 4-6 , another exemplary embodiment of the present invention is provided. The spark plug  100  of this embodiment includes an outer shell  110 , an insulator  120 , a terminal stud  130 , a center electrode  140 , a ground electrode  150  and a resistor  160 . Unless otherwise described below, these parts generally correspond to the similar parts described in the exemplary embodiments above. 
     The outer shell  110  of spark plug  100  includes a first end  115  and second end  116 . In the spark plug  100  of this exemplary embodiment, the outer shell  110  is configured somewhat differently than described above with reference to  FIGS. 1-3 . Here, the outer shell  110  does not include a second diameter portion which corresponds to the second-diameter portion  12  described above. Rather, the outer shell  110  only comprises a first-diameter portion  111  adjacent the first end  115  and a stepped-diameter portion  113  adjacent the second end  116  of the outer shell  110 . The first-diameter portion  111  includes a threaded portion  117  formed on an outer periphery thereof and extending radially outward from the outer shell  110 . An inwardly projecting protrusion  118  is formed at the first end  115  of the outer shell  110 . The outer shell  110  extends along a first longitudinal axis A. A bore  114  extends through the length of the outer shell  110  from the first end  115  to the second end  116  and is also aligned along a first longitudinal axis A. 
     The insulator  120  of spark plug  100  includes a first end  121  and a second end  122  opposite the first end  121 . The first end  121  has a first opening  121   a  and the second end  122  has a second opening  122   a . When the insulator  120  is located in the bore  114  of the outer shell  110 , the first end  121  and the second end  122  extend past the first end  115  and the second end  16  of the outer shell  110 , respectively. The insulator  120  further includes at least one groove  124   a  formed along an outer periphery thereof. The at least one groove  124   a  is configured to receive the protrusion  118  of the outer shell  110 . 
     The insulator  120  also includes a first section  123 , a second section  124 , and a third section  125 . The first section  123  extends from the first end  121  of the insulator  120  to a point on the insulator  120  adjacent the first end  115  of the outer shell  110 , and generally represents an outer surface of the spark plug  100  in this region. The second section  124  extends from adjacent the first end  115  of the outer shell  110  to adjacent the second end  116  of the outer shell  110 . The second section  124  is disposed radially within the outer shell  110  and is generally tightly received in the bore  114  of the outer shell  110 . The third section  125  extends from the second end  116  of the outer shell  110  to the second end  122  of the insulator  120 . The outer diameter of the third section  125  generally represents an outer surface of the spark plug  100 . The third section  125  includes an outer diameter D 2 . In an exemplary embodiment, the outer diameter D 2  may be in the range of 7.5 mm-10 mm, but is not limited thereto. 
     The insulator  120  includes a bore  126  extending from the first opening  121   a  to the second opening  122   a  of the insulator  120 . The bore  126  includes a first-bore section  127 , a second-bore section  128 , and a stepped-bore section  126   a . The first-bore section  127  extends coaxially with the outer shell  110  along the first longitudinal axis A. The second-bore section  128  has a smaller inner diameter and outer diameter than the first-bore section  127 . In addition, the second-bore section  128  extends along a second longitudinal axis B. The second longitudinal axis B is offset from the first longitudinal axis A. In other words, the second-bore section  128  is symmetrical about the second longitudinal axis B, but not the first longitudinal axis A. The stepped-bore section  126   a  is positioned between the first-bore section  127  and the second-bore section  128  and has a diameter which varies along the length of the stepped-bore section  126   a  to taper from the first-bore section  127  to the second-bore section  128 . 
     The offset longitudinal axes A and B allow for a portion of the insulator  120  to be of increased thickness, and thus, increased dielectric strength. In addition, by not including an outer shell extending over the third section  125  of the insulator  120 , the diameter of the insulator may be increased to generally correspond to the diameter of a bore in an engine in which the spark plug  100  is to be installed, thereby further increasing the dielectric strength of the insulator  120 . 
     The terminal stud  130  is received in the bore  126  along the first opening  121   a . In the exemplary embodiment shown in  FIG. 4 , the terminal stud  130  includes a head portion  131  which extends outwardly from the first end  121  of the insulator  120  outside of the first opening  121   a . The terminal stud  130  also includes a body portion  132  which extends within the first-bore section  127  of the bore  126 . 
     The center electrode  140  is received within the bore  126  of the insulator  120 . In the exemplary embodiment shown in  FIG. 4 , the center electrode  140  includes a skirt  141  and a body  142 . In illustrative embodiments, the skirt  141  of the center electrode  140  may be positioned in the first-bore section  127  and the stepped-bore section  126   a . The skirt  141  is offset from the second longitudinal axis B (and not symmetrical about the second longitudinal axis B) and may generally be aligned with the first longitudinal axis A, as illustrated in  FIG. 4 . The body  142  of the center electrode  140  extends through the second-bore section  128  of the bore  126 . A head section  143  of the body  142  extends axially beyond the second end  122  of the insulator  120  through the second opening  122   a.    
     The ground electrode  150  extends along a portion of the outer periphery of the insulator  120 . In the exemplary embodiment shown in  FIG. 4 , the ground electrode  150  extends from the second end  116  of the outer shell  110  to a position beyond the second end  122  of the insulator  120 . The ground electrode  150  includes a first segment  151 , a second segment  152 , and a third segment  153 . 
     The first segment  151  extends generally along a portion of the length of the insulator  120 . In contrast to the exemplary embodiment of  FIGS. 1-3 , here, no portion of the first segment  151  is positioned between the outer shell  110  and the insulator  120 . Rather, the first segment  151  of the ground electrode  150  is positioned beyond the second end  116  of the outer shell  110 , along the outer periphery of the third section  125  of the insulator  120 . The second segment  152  is positioned at an end of the first segment  151  opposite from where the first segment  151  is coupled to the outer shell  110 . The second segment  152  may be a curved portion of the ground electrode  150 . 
     The third segment  153  is positioned at one end of the second segment  152  that is opposite of where the second segment  152  is connected to the first segment  151 . The third segment  153  may extend in a direction generally perpendicular to at least a portion of the first segment  151 . The third segment  153  extends across the first and second longitudinal axes A and B. The third segment  153  is spaced from the head section  143  of the center electrode  40 , thereby providing a spark gap G between the third segment  153  of the ground electrode  150  and the center electrode  140 . 
     The resistor  160  is positioned in the bore  26  of the insulator  120  between the terminal stud  130  and center electrode  140  and provides an electrical communication between the terminal stud  130  and the center electrode  140 . In exemplary embodiments, a first contact glass  161  and a second contact glass  162  are disposed adjacent to respective ends of the resistor  160 . In illustrative embodiments, the first contact glass  161  may be disposed between the terminal stud  130  and the resistor  160 . The second contact glass  162  may be disposed between the resistor  160  and the second contact glass  162 . 
       FIGS. 1-3  and  4 - 6  illustrate a spark plug  5 / 100  constructed in accordance with an exemplary embodiment of the present invention. In illustrative embodiments, the spark plug  5 / 100  may be a small diameter spark plug. A “small diameter” spark plug refers generally to a spark plug having a reduced diameter in a lower region, between the spark gap G and the threaded region  17 / 117  of the spark plug  5 / 100 , compared to conventional spark plugs. For example, a small diameter spark plug may have a diameter in the range of 7.5 mm-10 mm in the lower region of the spark plug. It is understood however, that in the exemplary embodiments, the diameter of the spark plug in this region is not limited to such range, and other diameters may be suitable depending on cost and manufacturing considerations. For example, a small diameter spark plug according to the exemplary embodiments described herein may have a diameter of less than 7.5 mm where costs and material handling considerations allow. In the exemplary embodiments below, this diameter is shown generally as D 1  and D 2  in figures. 
     In the exemplary embodiments detailed above and shown in the figures, by offsetting the second-bore section  28 / 128  from the first-bore section  27 / 127  and the first longitudinal axis A, and by including a threaded region  17 / 117  on the outer shell  10 / 110  in a central region of the spark plug  5 / 110  and a ground electrode  50 / 150  coupled to the outer shell  10 / 110 , the insulator  20 / 120  wall thickness may be maintained or increased, especially near the ground electrode  50 / 150 . As a result of maintaining or increasing the wall thickness, the dielectric strength may be maintained or increased as well. Accordingly, the spark plug  5 / 110  described herein may be used in standard ignition systems. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode for carrying out this invention, but that the invention will include all embodiments falling within the scope of the present application.