Spark plug and method for making it

Abstract

 A spark plug comprises a central electrode (101), an insulator (106) surrounding the central electrode, an outer shell (110) surrounding a portion of the insulator and a ground electrode (116) arranged to form a spark gap (117) with the protruding end (104) of the electrode (101). The shell is constructed so that the internal diameter of its end (110a) facing spark gap is larger than the external diameter of the insulator (106) at its widest point so that the insulator can be inserted into the shell (110) from the spark gap end. The insulator (106) and the central electrode (101) are designed so that the central electrode (101) can be inserted into the insulator (106) from the end of the insulator (106) which when in use, is nearest to the spark gap end of the shell (110). The assembly of the spark plug is a reversed assembly when compared to the assembly of conventional spark plugs.

Description

[0001] The present invention relates to a spark plug comprising a central electrode, an insulator surrounding the central electrode, an outer shell surrounding a portion of the insulator and at least one ground electrode arranged to form a spark gap with an end of the central electrode protruding from the insulator. The invention further relates to a method for manufacturing a spark plug of the thus defined type.

[0002] The manufacture, i.e. the assembly of the different components of a conventional spark plug and its associated disadvantages will be discussed in detail hereinbelow. It is desirable to provide a spark plug which is simpler to assemble and hence cheaper to manufacture. It is also desirable to improve the structure of conventional spark plugs with regard to their resistance to the pressure present in the combustion chamber of an engine.

[0003] Hitherto, spark plugs have been made in certain sizes to comply with current International Standards and to fit existing engines. There is now a trend for engine manufacturers to require smaller spark plugs to contribute towards improved engine design. Thus, it is desirable to provide a spark plug whose overall size is smaller than those known hitherto.

[0004] The spark plug according to the present invention is characterised in that the end of the shell facing the spark gap has an internal diameter which is greater than the outer diameter of the insulator at its widest point, whereby the insulator may be inserted into the shell from said end facing the spark gap i.e. from the bottom of the shell (bottom-up or reversed assembly).

[0005] The method of manufacturing a spark plug according to this invention is characterised in that the insulator is inserted into the shell from the end of the shell which faces the spark gap.

[0006] As a result of the spark plug construction according to this invention the shell may be provided with an inner annular seat facing the spark gap and the insulator may be shaped so as to abut against said seat on the shell. The abutment of the insulator against the seat on the shell is capable of resisting in all circumstances the pressure which acts from the combustion chamber against the insulator and guarantees that the insulator and central electrode as well as any other components located inside the shell (core assembly) are not accidentally ejected out of the shell by said pressure in the combustion chamber.

[0007] This feature of the spark plug according to the invention is a substantial improvement over conventional spark plugs wherein such accidental ejections of the core assembly could happen because of a different spark plug structure mandated by a different spark plug assembly (top-down or conventional assembly).

[0008] Indeed hitherto spark plugs have been assembled from the shell top end, i.e. from its outer end when in use on an engine. Thus in conventional spark plugs the seat on the shell and the shoulder on the insulator respectively face the opposite direction when compared to the spark plug according to the present invention and additional means are then required to resist the pressure in the combustion chamber. Experience has shown that because of constructional constraints due to the top-down assembly it is rather difficult to make said additional means sufficiently strong.

[0009] The seat on the shell of the spark plug according to the invention may be in the form of a substantially conical inner surface and the insulator may have a correspondingly shaped outer surface. However, in the preferred embodiment of the invention, the seat on the shell is shaped so as to abut against a shoulder provided on the insulator.

[0010] To simplify the assembly of the different components of the spark plug in the method according to the invention it is desirable for as many of the components as possible, such as for example the central electrode, to be inserted into the shell from its bottom end. This feature has further advantages which will be described hereinbelow.

[0011] It is well known that the heat rating of a spark plug mainly depends on the depth of an annular space between the insulator and the shell which is generally present in spark plugs and which is open to the spark gap. Hitherto variation of the size of the space has been achieved by different designs of insulator and shell. It has now been discovered that the heat rating of a plug can be varied by starting off with a plug having universal components and placing a heat conducting insert in a portion of the space. The size of the portion will determine the heat rating of the plug. The use of heat conducting inserts is described and claimed in our simultaneously filed copending application entitled "Spark Plugs With Universal Components". The use of inserts is particularly convenient in the method of manufacturing spark plugs according to the present invention since they are inserted into said space from the bottom end of the spark plug.

[0012] A non-suppressor spark plug may be constructed in accordance with the present invention using a cylindrical central electrode which fits snugly within and extends the full length of the insulator. Alternatively if a resistive element is required, a fired-in resistive seal may be provided, to be described in more detail hereinbelow.

[0013] The insulator used in the present invention is preferably considerably smaller in diameter, at least in its outer (top) portion, than that which would be used in a corresponding plug of the prior art. However, a standard sized plug for current applications may be constructed according to the invention either by using a standard sized insulator or by adding a collar around a small-diameter insulator to bring it up to standard size.

[0014] Other preferred features and advantages of the present invention will become apparent from the following exemplary description of several preferred embodiments thereof with reference to the accompanying drawings in which:

Figure 1 is an exploded view, partly in section showing the components of a spark plug of the prior art;

Figure 2 is a part sectional view of a spark plug assembled from the components of figure 1;

Figure 3 is an exploded view, partly in section showing the components of a spark plug according to a first embodiment of the present invention;

Figure 4 is a part sectional view of a spark plug assembled from the components of figure 3;

Figure 5 is an exploded view partly in section showing the components of a spark plug according to a second embodiment of this invention;

Figures 6a, 6b and 6c are part sectional views showing three spark plugs assembled from components illustrated in figure 5;

Figure 7 is an exploded view, partly in section showing the components of a spark plug according to a third embodiment of the invention;

Figure 8 is a part sectional view showing a spark plug assembled from the components of figure 7; and

Figure 9 is a part sectional view showing acore assembly of a spark plug according to a fourth embodiment of the invention.

[0015] In all of the drawings to be referred to below, spark plugs are illustrated with their "spark gap" or "inner" (bottom) ends lowermost and their "outer" (top) ends uppermost. In all of the views the shell, insulator and ground electrode are shown cut away to reveal the interior components.

[0016] Referring firstly to figures 1 and 2, a typical conventional spark plug includes a central electrode 1, a conductor 2 in the form of a rod, a spring 3, a suppressor 4, a terminal 5 and an insulator 6. These components comprise what is commonly referred to as the core assembly of the spark plug. The insulator has a central bore 7 which receives the other components 1, 2, 3, 4, 5, referred to above. The function of the components 1, 2, 3, 4 and 5 is well understood in the art and will not be described in detail herein.

[0017] The bore 7 of the insulator 6 is narrower at its lower end than at its upper end thus providing an upwardly facing seat 8. The central electrode 1 has a widened portion at its upper end shaped to engage the seat 8 whereby to locate the components axially in the bore 7. The central electrode 1 fits snugly in the narrow lower portion of the bore 7. As shown in figure 2 there is a space 9 between the conductor 2 and the inner surface of the bore 7 which is filled with a proprietary powder sealing material, generally indicated at 30 in figure 1, which in particular ensures that the conductor 2 and electrode 1 are not dislodged from their desired position. The terminal 5 has a narrow lower portion 5a having suitable dimensions to be received in the bore 7 of the insulator 6 as a snug fit. The upper portion 5b of the terminal 5 is wider and the two portions are joined by a downwardly facing shoulder 5c which rests on the upper end of the insulator 6 after assembly.

[0018] Thus the components of the core assembly are manufactured so that the items 1, 2, 3, 4, 5 can be inserted into the insulator only from the top, in particular because of the seat 8 and of the correspnding widened portion at the upper end of the central electrode 1.

[0019] The conventional spark plug of figures 1 and 2 further comprises a shell 10 which surrounds the lower end of the core assembly. The shell is so shaped that it provides a seat 12 which faces upwardly as shown in drawings, and which locates a corresponding shoulder 14 on the insulator 6. Thus, during manufacture, the insulator 6 is inserted into the shell 10 from the upper end of the shell as shown in the drawings. A gasket 15 is positioned between the seat 12 and the shoulder 13. It in particular prevents the ingress of gases from the combustion chamber of the engine. A ground electrode 16 is attached to the shell 10 and arranged to form a spark gap 17 with the end of the central electrode 1 which protrudes from the insulator 6.

[0020] The shoulder 13 on the insulator 6 is part of a flange 20 which defines the widest portion of the insulator 6. The upper portion of the shell 10 is shaped to surround the flange 20 so that there is a space 18 above the flange between the shell and the insulator. This space 18 is also filled with a proprietary powder 30 before the formation of a turnover flange 19 at the upper edge of the shell 10 which encloses the powder 30 to complete the spark plug assembly.

[0021] The aforedescribed conventional spark plug construction has a number of disadvantages. Thus the formation of the core assembly alone requires four so-called powder tamping steps and further two powder tamping steps are needed to assemble the core in the shell 10.

[0022] As described above, a particular problem in the manufacture of spark plugs is ensuring their resistance to the very high gas pressures which exist in the combustion chamber of an engine. This pressure acts on the end of the core assembly facing the spark gap, for example, and can, as already said above, cause the core assembly to be ejected from the shell 10 if the holding power of the turnover flange 19 and of the powder in the space 18 is not sufficiently strong.

[0023] As will be seen hereunder these disadvantages of conventional spark plugs are eliminated in the spark plug according to the invention, in particular because of the absence of proprietary powder (between the insulator and the shell) and of a turnover flange which often are weak points in conventional spark plugs.

[0024] A spark plug according to a first embodiment of the invention will now be described with reference to figures 3 and 4 of the drawings wherein a so-called "non-suppressor" plug is illustrated. In this plug the core assembly simply comprises a central electrode 101, an insulator 106 and a terminal 105.

[0025] The insulator 106 has a bore 107 of uniform cross section extending along its whole length. The central electrode 101 is in the shape of a nail having a shaft 103 and a head 104. The head 104 provides a seat 104a which abuts against the lower surface 106a of the insulator 106 in the assembled plug. During assembly the electrode 101 is inserted into the bore 107 of the insulator 106 from its lower end as illustrated.

[0026] The shaft 103 of the electrode 101 is longer than the bore 107 of the insulator 106 and protrudes beyond the upper end of the insulator 106 after assembly. The terminal 105 comprises a cylindrical collar which surrounds this protruding end of the electrode 101.

[0027] A gasket 109 is positioned between the terminal 105 and the insulator 106 and fits tightly around the shaft 103 of the electrode 101. The purpose of this gasket 109 is to act as a seal against any combustion gases which may escape from the combustion chamber between the shaft 103 and the insulator 106. Preferably in the assembled plug, the shaft 103 ends flush with the upper surface of the terminal 106.

[0028] The shell 110 has a first portion 110a shown lowermost in the drawings and a second portion 110b shown uppermost. The shell has a bore 111 along its length whose diameter is smaller in the second portion than in the first thus providing an annular seat 112 in the bore. The insulator 106 is provided with a flange 120 which provides an upwardly facing shoulder 113. According to the method of the present invention, during assembly the insulator is inserted into the shell from its lower end as shown until the shoulder 113 on the insulator abuts against the seat 112 of the shell. Thus, the internal diameter of the bore 111 in the portion 110a of the shell 110 must be larger than the external diameter of the flange 120, which is the widest portion of the insulator 106. Preferably a gasket 115 is positioned between the seat 112 and shoulder 113 to prevent the escape of gases from the combustion chamber. The shell is provided with a flange 119 on the outside of the lower portion 110a. This flange rests against the engine cylinder head when the plug is in use.

[0029] The insulator 106 is retained in the bore of the shell 110 by any suitable means. One example is a spring washer 114 which retains the insulator 106 against the seat 112. The washer is retained in an annular recess on the inner surface of the shell 110. Other examples of insulator retaining means will be described below.

[0030] A ground electrode 116 is attached to the lower end of the shell 110 and arranged to form a spark gap 117 with the head 104 of the central electrode 101, thus completing the assembly.

[0031] It will be appreciated that the spark plug of figures 3 and 4 can be assembled in several different ways according to the method of the invention. For example, the central electrode 101 can be inserted into the insulator 106 before or after the insulator 106 has been inserted in the shell 110. The terminal 105 and gasket 109 may be clamped in coaxial position so that the electrode 101 can be inserted through the insulator 106, gasket 109 and terminal 105 in one step. Alternatively, the gasket 109 and terminal 105 may be placed over the end of the central electrode 101 after it has been inserted through the insulator 106 and before or after the insulator has been inserted in the shell 110.

[0032] The ground electrode 116 can be attached to the shell 110 before the insulator is placed in the shell 110, provided it is left in the "straight" position illustrated in figure 3. Alternatively it can be secured to the shell 110 as a final step,either before or after being bent to the shape shown in figure 4.

[0033] It should be clear from the above that the spark plug described with reference to figure 3 and 4 is designed to be assembled by inserting the individual components into each other from the lower (bottom) or spark gap end, i.e. the end which is inside the combustion chamber when in use (bottom-up or reversed assembly). This is in contrast to conventional spark plugs which are designed to be assembled by inserting components into each other from the opposite end, i.e. the end which is outside the combustion chamber when in use (top-down or conventional assembly).

[0034] The spark plug construction illustrated in figures 3 and 4 has numerous advantages over the conventional construction. Firstly, it is considerably more resistant to the pressure in the combustion chamber of the engine. Indeed the insulator 106 is prevented from being forced out of the shell 110 because the shoulder 113 abuts against seat 112 in the shell. It is to be noted that the seat 112 in the shell and the corresponding shoulder 113 on the insulator can easily be made substantially larger than shown in the drawings. This new structure according to the invention is far more efficient than the combination of the proprietary powder and turnover flange used in prior art.

[0035] It should further be noted here that the wall of the shell 110 in the region of the small bore portion 110b is relatively thick compared to that of the remainder of the shell, so as to provide strengthening of this region. When compared to the turnover flange 19 (Fig. 2) of a conventional spark plug said wall is substantially thicker and thus is capable of resisting without failure the pressure in the combustion chamber of the engine.

[0036] In addition the shoulder 104a of the central electrode 101 abutting against the lower surface of the insulator 106 resists the pressure in the combustion chamber which tends to eject the electrode out of the insulator.

[0037] The means holding the insulator 106 within shell 110, such as the spring washer 114, may be relatively weak since, in use, the pressure from the combustion chamber will ensure that the insulator does not fall downwards out of the shell. The washer 114 simply has to ensure that the insulator is not dislodged during handling.

[0038] A further important advantage of the spark plug according to the invention over prior art spark plugs is the fact that it requires fewer manufacturing steps for its construction. This is mainly due to the fact that no proprietary powder is required between the insulator 106 and the shell 110 and thus no tamping steps are needed for this powder and there is no flange to be turned over.

[0039] Also, the spark plug of figures 3 and 4 can be made smaller. Comparing figures 2 and 4, it will be seen that the lower portions of the shells 10, 110 are substantially the same but the upper portion 110b of the shell 110 in figure 4 is considerably smaller in diameter. The shell 110 of figure 4 has been designed to fit the same engine as that of figure 2, hence the similar spark-end portions. However, for other applications, the spark-end portion can be made even smaller in diameter.

[0040] Furthermore, the insulator 106 of the spark plug of figure 4 is smaller in diameter than the insulator 6 of the plug of figure 2 over its entire length, except at the lowermost portion.

[0041] A non-suppressor plug can also be made considerably shorter in length than the plug shown in figure 4.

[0042] The provision of a head 104 on the central electrode 101 provides a larger spark surface than in the spark plug of figure 2. This is an advantageous feature since a large spark surface extends the life of the spark plug. A small spark surface is indeed worn away faster.

[0043] The diameter of the bore 111 of the lower portion of the shell 110 is larger than the diameter of the lower portion of the insulator and thus in the assembled plug an annular space 118 is present between the insulator and the shell. As is well known in the art, the depth X of this space can be varied to vary the heat rating of the plug since it defines a heat transfer path away from the spark gap 117 to the cylinder head of the engine. Hitherto, the depth of the annular space 118 has been varied by providing different insulators and/or shells for spark plugs of different heat ratings. As described in our copending patent application filed simultaneously herewith entitled "Spark Plugs with Universal Components", it is possible to change the heat rating of a conventional spark plug by filling a portion of the annular space with a heat conducting material making intimate contact with the insulator and the shell.

[0044] The heat rating of a spark plug according to the present invention can conveniently be modified in this way since the heat conducting material can be inserted into the open end of the annular space 118 before the ground electrode 116 is finally positioned.

[0045] As described in our co-pending application, the heat conducting material can be inserted in the form of a powder, or a suitably shaped cylinder or helical spring and stamped into place so that it makes intimate contact with the inner surface of the shell and the outer surface of the insulator. The heat conducting material is preferably a metal, such as for example copper.

[0046] Figure 5 and 6 illustrate a second embodiment of this invention corresponding to that of figures 3 and 4 except for the addition of an insert of heat conducting material. Like parts in figures 3, 4, 5 and 6 are indicated with like reference numerals.

[0047] Figure 6 illustrates three spark plugs which are identical except for the size of the insert, indicated at 130a, 130b, 130c and identical in all other respects to the spark plug of figure 4. For identical plugs, the size of the insert 130 determines the heat rating of the plug. Figure 5 is an exploded view corresponding to figure 3 with the addition of three alternative inserts 130a, 130b, 130c. It will be understood that figure 6 illustrates a range of plugs from a relatively "hot" plug in figure 6a (but not as hot as that of figure 4) to a "cold" plug in figure 6c. The heat conducting material also serves to retain the insulator in position within the shell.

[0048] As mentioned hereinabove, the insulator of the spark plug shown in figures 4 and 6 is smaller in diameter than the insulator of the conventional plugs. Clearly it is desirable to be able to construct spark plugs according to the present invention which will be suitable for existing boot-type spark plug connectors, for example, and this will require them to have the same external dimensions as a conventional plug. This can easily be achieved according to the present invention by using a differently sized insulator. Alternatively, a range of spark plugs may be constructed in accordance with the present invention using a universal insulator and a collar surrounding the upper or outer portion of the insulator, the collar having suitable dimensions to enable the plug to be used in its chosen application.

[0049] Figures 7 and 8 show respectively exploded and assembled cross sectional views of a spark plug provided with a collar as described above. Like parts in figures 7 and 8 to those described above are indicated with like reference numerals.

[0050] The spark plug of figures 7 and 8 is identical to the spark plug of figures 5 and 6b, except that the insulator 206 of figures 7 and 8 has a straight sided portion protruding upwards from the shell in contrast to the ribbed portion of the insulator 106 of figures 3, 4, 5 and 6. This portion is surrounded by a collar 230. The collar is shaped to fit snugly around the insulator 206 and has the same external dimensions as the portion of the insulator 6 protruding beyond the shell 10 in the spark plug of figure 2. It will be noted that the gasket 209 of the spark plug of figure 8 is larger than that of the previous figures so that it also seals any space between the collar 230 and insulator 206.

[0051] A range of spark plugs having different sized collars for different application could be manufactured according to this invention using a universal insulator 206. The collar is preferably made from the same material as the insulator 206.

[0052] All of the spark plugs described above with reference to figures 3 to 8 were non-suppressor plugs. However, it is a simple matter to incorporate a suppressor into a plug constructed in accordance with the present invention. Figure 9 illustrates a core assembly including a suppressor suitable for use in a spark plug of the present invention.

[0053] The illustrated core assembly, includes an insulator 306 having the same external dimensions as the insulators 106 and 206 described above. However, instead of having a uniform bore, the bore 307 of the insulator has a first portion 307a (shown lower most) of smaller diameter than a second portion 307b, thus providing a seat 311 therebetween whose function will be described below.

[0054] The core assembly includes a central electrode 301 having a shaft 303 and a head 304 which abuts against the lower surface of the insulator 306.The shaft 303 ends just above the seat 311 in the insulator 306. During assembly, the central electrode 301 is inserted into the insulator 306 from the lower or spark gap end.

[0055] The core assembly shown in figure 9 further comprises a suppressor seal 312 to be described in more detail below, a conductor 302, a terminal 305 and a gasket 309. The terminal 305 and conductor 302 may be integrally formed or the terminal 305 may comprise a collar fitted over the end of the conductor 302 as shown in dotted lines. The conductor 302 is inserted into the insulator 306 from above.

[0056] Prior to insertion of the conductor 302, a small quantity of suppressor glass-metal powder is tipped into the base 307b of the upper portion of the insulator so that it covers the end of the central electrode and fills the space between the central electrode and the bore. After insertion of the terminal 305 and conductor 302, the assembly is fired so as to cause the glass-metal powder to melt and form a semi-conductor. After subsequent cooling the semi-conductor seals any space between the central electrode and the insulator and provides a suppressorelement 312 for radio frequency interference suppression. This suppressor element 312 takes the place of the spring 3 and suppressor 4 used in the conventional spark plug of figures 1 and 2.

[0057] The gasket 309 serves to retain the terminal 305 and conductor 302 in place and seal the remaining space between the conductor and insulator 306. The gasket can be added to the assembly at any suitable time depending whether the terminal 305 and conductor 309 are integral or separate elements. If desired the terminal or terminal portion may have the same shape as that 5 of figure 2, so that it extends into the bore 307 to prevent any movement of the conductor 302.

Claims

1. A spark plug comprising a central electrode (101), an insulator (106) surrounding the central electrode, an outer shell (110) surrounding at least a portion of the insulator (106) and at least one ground electrode (116) arranged to form a spark gap (117) with the central electrode (101), characterised in that the end of the shell (110a) facing the spark gap has an internal diameter which is greater than the outer diameter of the insulator (106) at its widest point whereby the insulator (106) may be inserted into the shell (110) from said spark gap end.

2. A spark plug as claimed in claim 1 in which the shell (110) has a generally annular seat (112) facing the spark gap (117) and the insulator (106) is shaped so as to abut against the seat (112).

3. A spark plug as claimed in claim 2 in which a seal (115) is located between said seat (112) and said insulator (106,113).

4. A spark plug as claimed in claim 1, 2 or 3 in which the central electrode comprises a shaft (103) having a head (104) at the spark gap end which abuts against the surface of the insulator (106).

5. A spark plug as claimed in any preceding claim in which a portion of the insulator (206) extends beyond the shell (110) away from the spark gap (117) and is surrounded by a collar (230).

6. A spark plug as claimed in any preceding claim in which the central electrode (101) extends out of the insulator (106) in the direction away from the spark gap (117) and further comprising a terminal (105) in the form of a collar surrounding the protruding end of the central electrode (101).

7. A spark plug as claimed in any of claims 1 to 5 in which the central electrode (101) is connected to a terminal via a suppressor element.

8. A spark plug as claimed in claim 7 in which the suppressor element is in the form of a seal (312)covering the end of the electrode (301).

9. A spark plug as claimed in claim 7 or 8 in which a conductor (302) extends between the suppressor element and the terminal.

10. A spark plug as claimed in claim 9 in which the conductor (302) extends out of the insulator (306) in the direction away from the spark gap and further comprising a terminal (305) in the form of a collar surrounding the protruding end of the conductor.

11. A spark plug as claimed in claim 9 in which the terminal (305) is integral with the conductor (302).

12. A spark plug as claimed in any preceding claim in which there is an annular space (118) between the insulator (106) and the shell (110) which is open towards the spark gap (117) and further comprising means positioned within said space for retaining the insulator (106) in the shell (110).

13. A spark plug as claimed in claim 12 in which said means comprises an annular spring washer (114).

14. A spark plug as claimed in any preceding claim in which there is an annular space (118) between the insulator (106) and the shell (110) which is open towards the spark gap (117) a portion of which is filled with a heat conducting material (130) arranged to make intimate contact with the insulator (106) and the shell (110).

15. A spark plug as claimed in claim 14 in which the heat conducting material (130) comprises a powder.

16. A spark plug as claimed in claim 14 or 15 in which the heat conducting material (130) comprises a cylinder.

17. A spark plug as claimed in claim 14, 15 or 16 in which the heat conducting material (130) comprises a helical spring.

18. A spark plug as claimed in any of claims 14 to 17 in which the heat conducting material (130) is a metal.

19. A spark plug as claimed in any of claims 14 to 18 in which the heat conducting material (130) is copper.

20. A spark plug substantially as hereinbefore described with reference to any of figures 3 to 9 of the accompanying drawings.

21. A method of manufacturing a spark plug comprising inserting a central electrode (101) into the bore of a generally cylindrical insulator (106), providing the insulator with a shell (110) surrounding at least a portion of the insulator (106) and arranging a ground electrode to form a spark gap (117) with the central electrode (101), characterised by inserting the insulator (106) into the shell (110) from the end of the shell (110a) which faces the spark gap (117).

22. A method of manufacturing a spark plug as claimed in claim 21 further compromising inserting the central electrode (101) into the end of the insulator (106) which faces the spark gap (117).

23. A method of manufacturing a spark plug as claimed in claim 21 or 22 in which an annular space (118) is provided between the insulator (106) and the shell (110) which is open towards the spark gap (117), and in which heat conducting material (130) is inserted into the open end of said annular space (118).

24. A method of manufacturing a spark plug substantially as herein described with reference to the accompanying drawings.

Drawing