US3256457A - Spark plug with insulator nose spaced from center electrode - Google Patents

Description

June 14, 1966 M. A. BRETSCH SPARK PLUG WITH INSULATOR NOSE SPACED FROM CENTER ELECTRODE Filed July 13, 1961 INVENTOR. MICHAEL A. BRETSCH ATTORNEYS United States Patent 3,256,457 SPARK PLUG WiTH FLYSULATOR NOSE SPACED FROM CENTER ELECTRGDE Michael A. Bretsch, Toledo, Ohio, assignor to Champion Spark Plug Company, Toledo, Ohio, a corporation of Delaware Filed July 13, 1961, Ser. N 123,878 Claims. (Cl. 313-141) This invention relates broadly to spark plugs, and, more particularly, to ceramic insulated spark plug assemblies highly resistant to insulator cracking caused by thermal gradients brought about during normal service use thereof, and to methods of producing such assemblies.

In the use of ceramic insulated spark plugs for high output internal combustion engines, including aircraft engines, substantial difliculty has been encountered because the insulators have had a tendency to crack at the nose portion. The nose portion is that part of the spark plug insulator surrounding the center electrode that extends inwardly towards the firing end from the seat on'which the insulator rests when the spark plug is assembled. When the nose portion cracks the resulting discontinuity often causes a drastic temperature rise, with the result that the tip of the spark plug may run so hot as to cause pro-ignition in the associated cylinder and a possible failure of the engine. Pre-ignition is a serious difiiculty in a high output internal combustion engine. If such an engine continues operation in pre-ignition for more than a few seconds, the extreme heat generated will damage the spark plug, and the engine. If pre-ignition does not occur after formation of a crack in the nose portion of a spark plug insulator, the crack may become a complete fracture and a part of the insulator may be dislodged. The dislodged part may itself cause severe mechanical damage in the combustion chamber.

In addition to spark plug failure caused by pre-ignition resulting from nose cracking as described above, pre-ignition with resulting spark plug failure often results when the engine output surpasses that the particular spark plug can withstand. When a spark plug is operated in a high output engine, and the output of that engine is gradually increased, an engine output is reached at which that particular spark plug causes pre-ignition.

The indicated means effective pressure of an engine at which a spark plug, when operating within the engine, will cause pre-ignition, is commonly denominated the I.M.E.P. rating of that spark plug. It has been found that the I.M.E.P. rating of a spark plug can be increased by shortening the nose thereof, but that such expedient is relatively undesirable because such plugs are particularly subject to carbon fouling when operated at low output. It has also been found that the I.M.E.P. rating of a spark plug can be effectively increased without.

deleteriously affecting the susceptibility of the plug to foulding by casting a silver electrode part in a bore through the nose portion of the spark plug insulator. Such expedient, however, increases the susceptibility of the insulator to nose cracking and, therefore, is not desirable. As a consequence, an acute problem exists in previously known spark plug designs: that of finding a spark plug having a high I.M.E.P. rating which is not subject to substantial carbon fouling, while at the same time is capable of operation in high output engines without a tendency to crack at the nose portion.

It has now been discovered, and the instant invention is based upon such discovery, that the advantages of the cast in place, silver. electrode structure, can be achieved, and the tendency for the cast silver to cause nose cracking substantially eliminated or at least mini- Patented June 14, 1966 ice mized by casting a metal having a high thermal conductivity, for example silver, into a metallic sleeve or tube having a relatively low thermal conductivity in comparison with the cast metal, such sleeve or tube being disposed in at least a part of the bore through the nose of the insulator. In a specific. instance, excellent results have been achieved where a split nickel sleeve, before casting of thesilver, has been expanded against the wall of the insulator bore; in such case, after casting of the silver or other high thermal conductivity metal, and because of the contraction of such metal during cooling thereof from a molten condition, an air gap is established between the Wall of the insulator bore and the metallic sleeve, with the result that the sleeve fits loosely within the insulator bore, and, in fact, may even slide therein when the assembly is tipped. It has been found that, in service, the nickel tube or sleeve, constituting a material having a low thermal conductivity relative to that of the cast silver body, and the gas filled gap be tween the exterior of the tube and the insulator nose bore wall, constitute thermal barriers to the transfer of heat between the nose of the insulator and the electrode part, whereby as a consequence of the thermal barriers, stressing of the nose bore wall by the cast silver body to an extent sufiicient to cause insulator cracking is prevented, but, at the same time, heat transfer between the two is sutiiciently high that the spark plug has a high I.M.E.P. rating.

It is, therefore, an object of the invention to provide an improved spark plug assembly.

It is a further object of the invention to provide such an assembly which will operate satisfactorily in engines running under a high load, or at a high I.M.E.P. Without insulator nose cracking.

More particularly, it is a further object of the invention to provide a spark plug assembly which includes a body of a material having a high thermal conductivity, at least part of which body is within a bore of the insulator, and means disposed within an annular space between the body and the wall of the insulator defining the nose bore, which means is effective to decrease the rate of heat transfer between the body and the nose bore wall.

It is a still further object of the invention to provide a method for producing such a spark plug assembly.

Other objects and advantages will in part be apparent and will in part appear hereinafter.

For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and attached drawing, illustrating a preferred embodiment thereof, in which:

FIG. 1 is a vertical sectional view of a spark plug assembly embodying the present invention; and

FIG. 2 is a horizontal sectional view taken along the line 22 of FIG. 1.

According to the invention, an improved spark plug assembly is provided. Such assembly, as shown in the drawings, comprises an appropriate ceramic insulator indicated generally at 11. The insulator is engaged in and supported by a metal shell indicated generally at 12 which is threaded at 13 for engagement in the combustion chamber of an engine. The insulator 11 has a nose portion 14 extending from its firing end to an intermediate area indicated generally at 15 which is operatively associated with the metal shell 12 for conductive heat transfer between the two. The insulator 11 has a central bore indicated generally at 16 and formed in steps as indicated at 17 and 18. The step 18 is provided in the bore through an intermediate insulator portion 19 so that the bore diameter defined by a nose bore wall 20 is less adjacent the firing end of the insulator than through the intermediate portion. A split metallic tube or sleeve 21, which in the specific embodiment is nickel, is disposed in the smaller diameter insulator bore portion, adjacent the insulator firing end, between the nose bore wall 20 and an electrode part indicated generally at 22. The electrode part 22 includes a firing tip 23, the lower extremity of which constitutes a sparking surface operatively associated with a ground electrode 24 which is structurally integral with the metal shell 12. The electrode part 22 also includes a body 25 of cast metal, which in the particular embodiment is silver. The body 25 is in both mechanical and electrical contact with the firing tip 23, and also with an upper electrode part 26. The upper electrode part 26 extends upwardly through a suitable gastight seal 27 and makes electrical contact with a resistor part 28 which is urged thereagainst by a spring 29 that is confined within a cap member 30. The nickel sleeve 21 is positioned in closely adjacent, spaced relationship relative to the wall of the insulator defining the nose bore. For optimum plug performance the spacing is kept to a minimum, and preferably is not greater than about 0.001". Excellent results have been achieved when this spacing has been approximately 0.0005". This space constitutes a gas-filled gap between the exterior of the sleeve and the insulator nose-bore wall 20.

The assembly illustrated in'the drawing can be readily produced by inserting the split nickel tube or sleeve 21 into the bore of the insulator in the position shown in FIG. 1. The sleeve 21 preferably has an exterior diameter slightly greater than the interior diameter of the bore in which it is inserted so that it is compressed slightly when in position. After insertion, the sleeve is turned outward at its upper end to conform to the stepped insulator bore at this point, and to form a mechanical lock preventing the downward movement of the sleeve. The firing tip 23, preferably made of platinum or other corrosion resistant metal, is then dropped into the sleeve with one end protruding through the hole in the sleeve bottom. A rod of silver is next positioned above the firing tip with a washer 31 and the upper electrode part 26 positioned thereabove. The resulting assembly is then placed in a suitable furnace and heated to a temperature sufficiently high to fuse the silver rod (melting point 960 C.), and thereafter cooled sufficiently slowly to prevent cracking. The silver, upon fusion, in effect forms a brazing-type of bond with the nickel sleeve so as to become integral therewith, no substantial alloying, however, taking place between the metals. Upon cooling, the silver, of course, contracts, and pulls the split nickel sleeve into a smaller diameter whereby the air filled gap is formed between the outer diameter of the sleeve and the insulator nose bore wall. The gas-tight seal 27 is then formed in any suitable way; the insulator is inserted in the metal shell 12, with suitable gasketing, and the shell is crimped into engaging and supporting relationship relative to the insulator; and the remainder of the assembly is completed in a conventional manner.

It will be apparent from the above description that a metallic sleeve, having a lower thermal conductivity than the cast-in-place electrode part, is disposed between the cast metallic body and the wall of the insulator which defines the bore thru the nose thereof, or the nose bore wall. It will be appreciated that the sleeve or tube material, by virtue of the difference in thermal conductivity, constitutes a thermal barrier between the cast electrode part and the ceramic insulator. In addition to the sleeve or tube itself serving as a thermal barrier between the cast electrode part and the ceramic insulator, the air gap, formed by contraction of the silver as described, serves as a further barrier to the transmission of heat between the cast electrode part and the insulator. However, neither of the thermal barriers above described is of sufficient magnitude to lower the I.M.E.P. rating appreciably below that obtained with a conventional cast silver electrode assembly without such barriers. In other words, even though the thermal barrier resulting from the sleeve and air gap in spark plug assemblies produced in accordance with the invention is effective substantially to prevent insulator nose cracking, still the sleeve material has a sufliciently hi h thermal conductivity that heat transfer between the electrode and insulator is high, and the spark plug assembly has a high I.M.E.P. rating.

It has been found that substantially equivalent results can be achieved by what may be denominated a reversal of the air gap relative to the split sleeve. For example, the interior of the sleeve can first be oxidized to prevent adhesion of silver or the like thereto, and can then be positioned in the insulator bore as described, with a suitable bonding material disposed between its exterior wall and the insulator bore. This bonding material can be silver, or any suitable ceramic bonding material. In this instance, when the assembly is heated, with silver in posito flow into the split nickel sleeve upon melting, the sleeve is bonded to the insulator bore but the oxide coating on the interior of the tube prevents adhesion with the silver therein, with the result that contraction upon cooling provides an air gap to act as a thermal barrier between the body of silver and the nickel sleeve. By carrying out the procedure which has been described in this paragraph except that the silver or other bonding material is omitted from between the exterior of the nickel sleeve and the insulator bore, a still further modified result can be achieved: a limited thermal barrier between the exterior of the nickel sleeve and the insulator bore and a greater thermal barrier between the silver body and the interior of the nickel sleeve.

The sleeve or tube is also effective to prevent direct contact between the cast body and the nose bore wall, and, therefore, when the assembly is heated in service, prevents stressing to an extent which would be sufiicient to cause insulator cracking, of the nose bore wall by the expanded cast body.

As will be readily appreciated, the sleeve material, in addition to having a suitable thermal conductivity, must possess a certain refractory or mold-like property which enables a metal having a high thermal conductivity and forming the electrode part, for example, silver, to be cast therein without causing either a chemical or physical change in the structure of the sleeve material. For example, the sleeve material must have a considerably higher melting point thanthe cast metal, and must be inert'with respect to such cast metal at the temperatures employed for casting so that substantially no interface alloying, which would tend to materially lower the thermal conductivity of the cast electrode part, takes place. As previously indicated, nickel has been found to be especially well suited for use as the sleeve material. In addition to nickel, other specific materials that may be mentioned as particularly well adapted for use as the sleeve material are certain nickel alloys such as D Nickel percent nickel, 4.5 percent manganese) and Inconel (76 percent nickel, 13 percent chromium, 7 percent iron and other minor constituents, as well as chrominum, molybdenum, and iron). In general, high temperature metals and alloys having a thermal conductivity not greater than C.G.S. are preferred. Thermal conductivity as used herein and in the appended claims, is defined as the quantity of heat in calories which, at room temperature or approximately 18 C. to 22 C., is transmitted per second through a plate one centimeter thick across an area of one square centimeter when the temperature difference is 1 C.

As has hereinbefore been indicated, the cast material must be one that possesses an extremely high thermal conductivity. In addition to silver, excellent results may be obtained with aluminum, copper, and gold, employed as the cast electrode part. In this respect, it has been found that the cast material must have a thermal conductivity of at least 0.5. Therefore, any metal or alloy capable of being cast under normal conditions, and having a thermal conductivity, as measured in the above manner, of at least 0.5 is suitable for use as the cast electrode material in the present invention.

Itwill be appreciated that an insulator can stand a certain magnitude of thermal stress in service without cracking, but that the magnitude of thermal stress which causes cracking depends upon the composition of the insulator. For example, certain insulators containing large proportions of BeO are capable of withstanding much higher thermal stresses, without cracking, than are more conventional insulators containing at least 90 per cent of A1 0 High alumina insulators, however, are considerably more resistant to cracking when subjected to thermal stress than are still other insulators that have been used or might be used. In its essential details, the instant invention contemplates a thermal barrier (the sleeve or tube and air gap) disposed between a body of a material having a high thermal conductivity (for example, silver) and an insulator nose bore wall. Ideally, the magnitude of the thermal barrier should be just sufiicient to prevent cracking of the particular insulator by thermal stress encountered in service. An assembly, including such a thermal barrier, has a maximum I.M.E.P. rating, for a given design, for the particular insulator material. However, a lesser thermal barrier would suffice to prevent nose cracking of an insulator capable of withstanding a high thermal stress, and a greater thermal barrier would be required to prevent nose cracking of an insulator which could withstand only a smaller stress. Accordingly, an assembly according to the invention must include a thermal barrier of sufiicient magnitude to prevent nose cracking, but the magnitude thereof cannot be defined numerically because it depends upon other factors.

It will be apparent from the foregoing description of the method for producing a spark plug assembly according to the invention that a new and useful method improvernent as well as a new and useful spark plug assembly is provided. Such method is for producing a spark plug assembly comprising an insulator that is to be engaged and supported by a metal shell, the insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end to an intermediate insulator area adapted for operative association with the shell for conductive heat transfer between the shell and the sleeve or tube, the nose having a bore defined by a nose bore wall, and a terminal portion having a bore axially aligned with the nose bore.

The method includes the steps of forming at least a portion of an electrodepart by casting a metal or metal alloy having a thermal conductivity of at least 0.5 within the insulator nose bore, and additionally supporting in the insulator nose bore, prior to the casting step, a split metallic sleeve or tube of a material having a relatively low thermal conductivity in comparison with the cast electrode part, which sleeve or tube is effective substantially to prevent direct contact between the insulator nose bore wall and the cast material, both during casting and subsequently, and which is also effective to space the cast body from the nose bore wall a'distance sufficient that the insulator does not crack under service conditions when in the combustion chamber of an internal combustion engine operated from an idle condition to a high-load condition at which pro-ignition occurs. The spacing provided should also be sufiiciently small that the cast body is effective to increase the I.M.E.P. rating of a spark plug assembled from the insulator structure.

While it is believed that the more advantageous embodiments of the invention have been described, it is apparent that many modifications and variations can be made in the specific construction, arrangement, or form of the parts, and in specific procedure discussed without departing from the spirit and scope of the present invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be 6. within the purview and scope of the invention asdefined by the appended claims.

What I claim:

1. A spark plug assembly comprising a metal shell which is threaded for engagement in the combustion chamber of an engine, an insulator engaged and supported by said shell, said insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end to an intermediate insulator area operatively associated with said shell for conductive heat transfer between the two, said nose having a bore defined by a nose bore wall, and the terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having a sparking surface in the vicinity of the firing end of said insulator, said electrode part including a cast body of a material having a thermal conductivity of at least 0.5, at least a part of said body being within the nose bore and in closely adjacent, spaced relationship to the nose bore wall, and means disposed within an annular space between said body and the nose bore wall, structurally integral with said body and out of contact with and spaced from said nose bore wall by no more than about 0.001 inch, said means comprising a material having a relatively low thermal conductivity in comparison with that of said body and being effective to decrease the rate of heat transfer between said body and the nose bore wall in an amount sutficient to prevent stressing of the nose bore wall by said body. when the assembly is heated in service, to an extent sufiicient to cause insulator cracking.

2. A spark plug assembly comprising a metal shell which is threaded for engagement in the combustion chamberof an engine, an insulator engaged and sup-ported by said shell, said insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end to an intermediate insulator area operativelv associated with said shell for conductive heat transfer between the two, the nose having a bore defined by a nose bore wall, and the terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having a sparking surface at the firing end of said insulator, said electrode part including a cast body of a material having a thermal conductivity of at least 0.5, at least a part of said body being within the nose bore and in closely adjacent, spaced relationship to the nose bore wall, and a tube disposed within an annular space between said body and the nose bore wall, said tube being out of contact with. and spaced from the nose bore wall by no more than about 0.001 inch and being structurally integral with said body, .and being comprised of a material having a relatively low thermal conductivity in comparison with that of said body, whereby, when the assembly is heated in service, stressing of the nose bore wall by said body to an extent sufiicient to cause insulator cracking is prevented.

3. A spark plug assembly comprising a metal shell which is threaded for engagement in the combustion chamber of an engine, an insulator engaged in and supported by said shell, said insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end to an intermediate insulator area operatively associated with said shell for conductive heat transfer between the two, said nose having a bore defined by a nose bore wall, and the terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having a sparking surface at the firing end of said insulator, said electrode part including a cast silver body, at least a part of said body being in the nose bore and in closely adjacent spaced relationship to the nose bore wall, and means disposed within an annular space between said body and the nose bore wall, in substantially continuous contact with one and out of contact with and spaced not more than about 0.001 inch from the other, said means comprising a material having a lower thermal conductivity than said cast silver body and being effective to decrease the rate of heattransfer between said body and the nose bore wall in an amount sufficient to prevent stressing of the nose bore wall by said body, when the assembly is heated in service, to an extent sufficient to cause insulator cracking.

4. A spark plug assembly comprising a metal shell which is threaded for engagement in the combustion chamber of an engine, an insulator engaged and supported by said shell, said insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end into an intermediate insulator area operatively associated with said shell for conductive heat transfer between the two, said nose having a bore defined by a nose bore .wall, and the terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having a sparking surface at the firing end of said insulator, said electrode part including a cast copper body, at least a part of said body being within the nose bore and in closely adjacent, spaced relationship to the nose bore wall, and means disposed within an annular space between said body and the nose bore wall, structurally integral with said body and out of contact with and spaced from said nose bore wall by approximately 0.001 inch, said means comprising a material having a lower thermal conductivity than said east copper body and being effective to decrease the rate of heat transfer between said body and the nose bore wall in an amount sufiicent to prevent stressing of the nose bore wall by said body, when the assembly is heated in service, to an extent sufficient to cause insulator cracking.

5. A spark plug assembly comprising a metal shell which is threaded for engagement in the combustion chamber of an engine, an insulator engaged and supported by said shell, said insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end into a intermediate insulator area operatively associated with said shell for conductive heat transfer between the two, said nose having a bore defined by a nose bore -wall, and the terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having a sparking surface at the firing end of said insulator, said electrode pant including a cast gold body, at least a pant of said body being within the nose bore and in closely adjacent, spaced relationship to the nose bore wall, and means disposed within an annular space between said body and .the nose bore wall, structurally integral with said body and out of contact with and spaced from said nose bore wall by approximately 0.001 inch, said means comprising a material having a lower thermal conductivity than said cast gold body and being effective to decrease the rate of heat transfer between said body and the nose bore wall in an amount sufficient to prevent stressing of the nose bore :wall by said body, when the assembly is heated in service, to an extent sufiicient to causeinsulator cracking.

6. A spark plug assembly compising a metal shell which is threaded for engagement in the combustion chamber of an engine, an insulator engaged and supported by said shell, said insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end into an intermediate insulator area operatively associated with said shell for conductive heat transfer between the two, said nose having a bore defined by a nose bore wall, and the terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having a sparking surface at the firing end of said insulator, said electrode part including a cast aluminum body, at least a part of said body being within the nose bore and in closely adjacent, spaced relationship to the nose bore wall, and means disposed w-ithin an annular space between said body and the nose bore wall, structurally integral with said body and out of contact with and spaced from said nose bore wall by approximately 0.001 inch, said means comprising a material having a lower thermal conductivity than said cast aluminum body and being effective to decrease the rate of heat transfer between said body and the nose bore wall in an amount sufficient to prevent stressing of the nose bore wall by said body, when the assembly is heated in service, to an extent sufficient to cause insulator cracking.

7. A spark plug assembly comprising a metal shell which is threaded for engagement in the combustion chamber of an engine, an insulator engaged and supponted by said she-ll, said insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end to an intermediate insulator area operatively associated with said shell for conductive heat transfer between the two, said nose having a bore defined by a nose bore wall, and a terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having a sparking surface at the firing end of said insulator, said electrode part including a cast body of a material having a thermal conductivity of at least 0.5, at least a part of said body being within the nose bore and in closely adjacent spaced relationship to the nose bore wall, and a nickel sleeve disposed within an annular space between said body and the nose bore wall, out of contact with and spaced from the nose bore wall by not more than approximately 0.001 inch and structurally integral with said body, whereby when the assembly is heated in service, stressing of the nose bore wall by said body to an extent sufficient to cause insulator cracking is prevented.

8. A spark plug assembly comprising a metal shell which is threaded for engagement in the combustion chamber of an engine, an insulator engaged and supported by said shell, said insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end to an intermediate insulator area operatively associated with said shell for conductive heat transfer between the two, said nose having a bore defined by a nose bore wall, and a terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having .a sparking surface at the firing end of said insulator, said electrode part including a cast body of a material having a thermal conductivity of at least 0.5, at least a part of said body being with-in the nose bore and in closely adjacent spaced relationship to the nose bore wall, and a sleeve disposed within an annular space bet-ween said body and the nose bore wall, out of contact with and spaced from one by not more than approximately 0.001 inch and structurally integral with the other, said sleeve comprising a metal selected from the group consisting of nickel, nickel alloys, iron, molybdenum and chromium, .whereby when the assembly is heated in service, stressing of the nose bore wall by said body to an extent sufficient to cause insulator crack-ing is prevented.

9. A spark plug assembly comprising a metal shell which is threaded for engagement in the combustion chamber of an engine, an insulator engaged in and supported by said shell, said insulator having a terminal portion and a nose including a firing end, the nose extending from the firing end .to an intermediate insulator area operatively associated with said shell for conductive heat transfer between the two, said nose having a bore defined by a nose bore wall, and the terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having a sparking surface at the firing end of said insulator, said electrode part including a cast silver body, at least a pant of said body being in the nose bore and in closely adjacent spaced rela- .tionship to the nose bore wall, and means disposed within an annular space betwen said body and the nose bore wall, structurally integral with said body and out of contact with and spaced no more than approximately 0.001 inch from said nose bore wall, said means comprising a material having a lower thermal conductivity than said cast silver body and being effective to decrease the rate of heat transfer betwen said body and the nose bore wall in an amount sufficient to prevent stressing of the nose bore Wall by said body, when the assembly is heated in service, to an extent suflicient to cause insulator crackmg.

10. A spark plug assembly comprising a metal shell which is threaded for engagement in the combustion chamber of an engine, an insulator engaged and supported by said shell, said insulator having a terminal portion and a nose including a firing end, the nose eX tending from the firing end to an intermediate insulator area operatively associated with said shell for conductive heat transfer between the two, said nose having a bore defined by a nose bore wall, and a terminal portion having a bore axially aligned with the nose bore, an electrode part disposed in the nose bore and having a sparking surface at the firing end of said insulator, said electrode part including a cast body of a material having a thermal conductivity of at least 0.5, at least a part of said body being within the nose bore and in closely adjacent spaced relationship .to the nose bore wall, and a sleeve disposed within an annular space between said body and the nose bore Wall, out of contact with and spaced no more than approximately 0.001 inch from the nose bore wall and structurally integral with said body, said sleeve compris- 10 ing a metal selected from the group consisting of nickel, nickel alloys, iron, molybdenum and chromium, whereiby when the assembly is heated in service, stressing of the nose bore wall by said body to an extent sufiicient to cause insulator cracking is prevented.

References Cited by the Examiner UNITED STATES PATENTS 2,081,500 5/ 19 37 Nowosielski 313- X 2,226,415 12/ 1940 Kapp 313142 2,296,033 9/ 1942 Heller 31311.5 2,360,287 10/ 1944 Smith 2925 1 2 2,717,438 9/ 1955 Schwartzwolder et al. 29-25 .12 2,871,388 1/1959 Adair 313-141 FOREIGN PATENTS 885,466 5/ 1943 France. 886,455 7/ 1943 France. 558,977 1/ 1944 Great Britain.

GEORGE N. WEST BY, Primary Examiner.

JOHN W. HUCKERT, Examiner.

V. LAFRA'NCHI, Assistant Examiner.

Claims (1)

1. A SPARK PLUG ASSEMBLY COMPRISING A METAL SHELL WHICH IS THREADED FOR ENGAGEMENT IN THE COMBUSTION CHAMBER OF AN ENGINE, AN INSULATOR ENGAGED AND SUPPORTED BY SAID SHELL, SAID INSULATOR HAVING A TERMINAL PORTION AND A NOSE INCLUDING A FIRING END, THE NOSE EXTENDING FROM THE FIRING TO AN INTERMEDIATE INSULATOR AREA OPERATIVELY ASSOCIATED WITH SAID SHELL FOR CONDUCTIVE HEAT TRANSFER BETWEEN THE TWO, SAID NOSE HAVING A BORE DEFINED BY A NOSE BORE WALL, AND THE TERMINAL PORTION HAVING A BORE AXIALLY ALIGNED WITH THE NOSE BORE, AND ELECTRODE PART DISPOSED IN THE NOSE BORE AND HAVING A SPARKING SURFACE IN THE VICINITY OF THE FIRING END OF SAID INSULATOR, SAID ELECTRODE PART INCLUDING A CAST BODY OF A MATERIAL HAVING A THERMAL CONDUCTIVITY OF AT LEAST 0.5, AT LEAST A PART OF SAID BODY BEING WITHIN THE NOSE BORE AND IN CLOSELY ADJACENT, SPACED RELATIONSHIP TO THE NOSE BORE WALL, AND

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