Method for producing a cylinder head

Abstract: A method is provided for producing a cylinder head unit (11) of an internal combustion engine, comprising steps of casting a cylinder head unit (11) having valve openings (13a,14a), providing said valve openings (13a,14a) with valve seats by fastening valve seat blanks and applying a finishing treatment to said valve openings (13a,14a) and said valve seats (19). Further, during casting of the cylinder head (11) manufacturing reference surfaces are formed thereon and the position for said valve seat blanks on said valve openings is determined on the basis of indexing on said manufacturing reference surfaces. Then said valve seat blanks are metallurgically bonded to said valve openings.

Description: The present invention relates to a method for producing a cylinder head unit of an internal combustion engine, comprising the steps of casting a cylinder head having valve openings, providing said valve openings with valve seats by fastening valve seat blanks and applying a finishing treatment to said valve openings and said valve seats, wherein during casting of the cylinder head manufacturing reference surfaces are formed thereon. In the past, the cylinder head units for engines were formed by casting aluminum alloy materials, and valve seats were attached to the valve face surfaces for the intake valves and exhaust valves. Since these valve seats would be exposed to high heat as they made repeated contact with the air intake and exhaust valves, they were formed from a sintered ferrous metal that has excellent resistance to wear and high temperatures. As is shown in Figure 18, they were pressed into a retaining hole formed in the openings on the combustion chamber side of the cylinder head for the air intake ports and exhaust ports to unitize them with the cylinder head unit. Figure 18 is an enlarged sectional view of a conventional valve seat that was press-fitted into the cylinder head. In the figure, 1 is the cylinder head unit, 2 is the press-fitted type of valve seat, 3 is the retaining hole for the valve seat. The retaining hole 3 for the valve seat was formed by machining around the port opening area of the cylinder head. Conventional press-fitted type valve seats 2 were installed by first making the retaining holes 3 as described above in the cylinder head unit 1, and then by heating the cylinder head unit 1 in a furnace to cause the diameter of the retaining holes 3 to increase, and then pressing-in the valve seats. To wit, the valves seats would be attached to the cylinder head unit by shrink fitting. When this type of valve seat 2 is fitted into the cylinder head unit 1 under high temperature conditions, the valve guides (not shown) that support the air intake and exhaust valves are also pressed into their retaining holes. These valve guide retaining holes are cut at the same time as the retaining holes 3 using a machining process. The problem with using press fitting for the valve seats is that it is not possible to simplify the production process or to shorten the production time. The reasons are first, the retaining holes 3 for the press fitting must be positioned with high accuracy to position the valve seats, and the machining processing takes too much time. Second is the fact that a heating furnace has to be employed in a heating process for affixing the valve seats 2 to the cylinder head unit. Third, there must be a washing process using a water washing device prior to the heating process to prevent any cuttings generated during the machining process from getting into the foregoing heating furnace. US-A-3,935,679 discloses a grinding fixture for grinding cylinder head castings, in particular, for automatic grinding of cylinder head castings for internal combustion engines, using reference surfaces established during casting in the combustion chamber cavities. Said grinding fixture supports cylinder head casting during automatic grinding of the casting and is adapted to level the casting and to hold the casting in the leveled condition, so that the casting can be passed through an automatic grinding mechanism to grind off parting lines, flash, flow pins and gating from the upper surface and side surfaces of the casting. Accordingly, It Is an objective of the present invention to improve a method for producing a cylinder head unit of an internal combustion engine as indicated above so as to facilitate the simplification of the producing process and the accurate determination of the positioning of the valve seat length and the valve openings. According to the present invention, this objective is solved by a method for producing a cylinder head unit as indicated above in that a position for said valve seat blanks on said valve openings is determined on the basis of indexing on said manufacturing reference surfaces, wherein said position for said valve seat blanks is dependent from an axial direction of valve guide holes drilled after casting, metallurgically bonding said valve seat blanks to said valve openings is carried out by applying electricity to said cylinder head unit through an electrode, a guide rod is advanced coaxially aligned with said electrode, such that said guide rod enters said valve guide hole and simultaneously guides said electrode for matching a pushing direction with said axis of a valve, wherein said electrode is in direct contact with said valve seat blanks electrically, but isolated from said cylinder head and said guide hole. According to a preferred embodiment said manufacturing reference surfaces define three directions comprising a first end surface on the side of said cylinder head facing a cylinder body, a second surface parallel to said first surface and a third surface being perpendicular to both the first and second surfaces. According to another preferred embodiment, said valve guide holes are drilled prior to or after the bonding process of said valve seat blanks. According to another preferred embodiment the pressing force and/or said electricity are applied according to a predetermined pattern. Since the magnitude of sinking of the valve seat base material into the opening may be measured continuously during the whole bonding process it is also possible that said magnitude of sinking of the valve seat base material into the opening may be controlled. According to a further preferred embodiment the valve seat blank is pressed around the port openings and electrical resistance is used to heat the zone where the two are in pressure contact; this causes the temperature to rise at the surfaces where the two are in pressure contact, causing the atoms to become mutually dispersed. As a result, a layer of co-crystalline alloy is formed between the metal material of the valve seat and the component metal materials of the cylinder head unit. Since the liquid phase transition temperature of this co-crystalline alloy layer is low, the foregoing resistance heating causes this transition to a liquid phase, and while applying pressure to the valve seat blank, a plastic flow is generated that causes it, along with some stock metal from the cylinder head, to be squeezed out from the contact zone in which there is a mutual dispersing of the atoms from the sintered alloy of the valve seat blank and the metal of the cylinder head, which, in this condition, causes the valve seat blank to become embedded around the port opening and attached to the cylinder head. According to still another preferred embodiment, in attaching the valve seat material to the cylinder head, compared to the conventional case where valve seats are press-fitted into the cylinder head, any machining of the cylinder head is superfluous, and in addition, a heating furnace to heat up the cylinder head for the purpose of attaching the valve seats also is superfluous. Since the press-fitted type valve seats required retaining holes formed to high accuracy in the first place, no especial guide holes are needed to perform the indexing for the press fitting or to regulate the pressing direction. However, with the attachment method of this invention which does not utilize a press fitting structure, some sort of guide is necessary to determine the position for the valve seats and to apply pressure upon the valve seat blank in the axial direction. According to a further preferred embodiment, in attaching the valve seat blank to the cylinder head, when contrasted to the press fitting of the valve seat, there is no need for the boring of special guide holes for the valve seats prior to their attachment. In addition, the heating process for the cylinder head that utilizes a heating furnace, and the washing process needed to remove foreign matter therefrom prior to heating have been eliminated. According to a further preferred embodiment in attaching the valve seat blank to the cylinder head unit, special guide holes for the valve seats need not be drilled prior to their attachment. In addition, the heating process for the cylinder head that utilizes a heating furnace, and the washing process needed to remove foreign matter therefrom prior to beating have been eliminated. Further, in attaching the valve seat blank to the cylinder head unit, the indexing for the attachment openings and the boring of the foregoing holes for component parts is performed on the basis of common manufacturing reference surfaces, which means that the positioning relationships between the two remain unaffected by casting tolerances. Other preferred embodiments of the present invention are laid down in further dependent claims. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein: Figure 1 is a sectional view of the valve seat area of a cylinder head with the valve seat attached by the manufacturing method of this invention; Figure 2 is a sectional view showing the alignment of the valve seat blank with the port openings, containing only a partial enlargement of the valve seat blank and the cylinder head; Figure 3 is a front view of a press apparatus used to implement the method of this invention for manufacturing cylinder heads; Figure 4 is a side view of a press apparatus used to implement the method of this invention for manufacturing cylinder heads. Figure 5 is a sectional view showing the electrode in contact with the valve seat blank; Figure 6 is a graph showing the pressure application pattern, current value pattern, and amount of embedding; Figure 7 is a sectional view showing the formation of the layer of alloy from the metal stock from the covering film on the valve seat blank and the cylinder head metal stock, Figure 8 is a sectional view showing the plastic flow created in the cylinder head unity Figure 9 is a sectional view showing the embedding of the valve seat blank into the cylinder head unit; Figure 10 is a sectional view showing the valve seat after final finishing; Figure 11 is a top view showing an example of shield utilization; Figure 12 is a block diagram used to explain the method of manufacturing cylinder heads according to the third invention; Figure 13 is a block diagram used to explain the method of manufacturing cylinder heads wherein the nock pin holes are used for the positioning of the valve seat blanks on attachment openings formed by casting; Figure 14 is a block diagram used to explain the method of manufacturing cylinder heads according to the first invention; Figure 15 is a block diagram used to explain the method of manufacturing cylinder heads according to the second invention; Figure 16 is a block diagram used to explain the method of manufacturing cylinder heads where the attachment openings for the valve seat blanks have been formed by casting, nock pin holes being used to index the position of the valve seat blanks; Figure 17 is a sectional view showing other examples where the shape in the contact zone has been modified: in (a) and (b) a ridge has only been put on the valve seat blank, and in (c) the ridge only appears on the cylinder head unit; and Figure 18 is an enlarged sectional view of the valve seat area showing a prior art press fitting of the valve seat into a cylinder head. In describing an embodiment of this invention, the method for manufacturing cylinder heads of the third preferred embodiment, which involves the most processing steps, will be explained first, with reference to Figures 1 through 12. Figure 1 is a sectional view of the valve seat area of a cylinder head employing valve seats attached using the manufacturing method of this invention; Figure 2 is a sectional view showing the valve seat blank positioned over the port opening, in this figure only a part of the cylinder head and valve seat are shown in the enlargement. Figure 3 shows a front view of a press device which is used in implementing this invention's cylinder head manufacturing method; Figure 4 is the same shown in a side view; and Figure 5 is a sectional view showing the electrode in contact with the valve seat material. Figure 6 is a graph which shows the pressure pattern, the electrical current values and the degree of embedding. Figure 7 is a sectional view showing the formation of the alloy layer from the stock metal of the cylinder head and the metal film covering the valve seat blank; Figure 8 is a sectional view showing the onset of the plastic flow of the metal from the cylinder head stock; Figure 9 is a sectional view showing the embedding of the valve seat blank into the cylinder head; Figure 10 is a sectional view showing the finishing of the valve seat, Figure 11 is a top view showing a shield utilization example; and Figure 12 is a block diagram that explains the process steps for manufacturing a cylinder head according to the first embodiment. In these figures, 11 represents the cylinder head unit of a four-cycle engine; this cylinder head unit 11 was formed by casting an aluminum alloy material. Formed in it is a downward-opening dome-shaped concave zone 12 that forms a combustion chamber and an air intake port 13 that opens at one end of this concave zone 12, and an exhaust port 14. Also formed when casting this cylinder head unit 11 are manufacturing reference surfaces that define three directions, they are composed of the cylinder body end surface, a parallel surface, and another that is perpendicular to the two. These manufacturing reference surfaces can be formed, for example, on the concave zone 12 that forms the foregoing combustion chambers and on the inner wall of the cam chain chamber (not shown). The manufacturing reference surface formed on the foregoing concave area can be formed by partially notching upward into the cylinder head in the area of the concave zone 12, near the spark plug hole (not shown). When forming the manufacturing reference surface on the inner wall surface of the chain chamber, etc., one may form it by partially notching the inner wall of the chain chamber, just as was done for the foregoing concave zone 12. The air intake valve 17 and exhaust valve 18 are attached in the upper wall areas of the foregoing air intake port 13 and exhaust port 14 by valve guides 15, 16, and valve seats 19 are attached around the openings to both ports 13, 14. The foregoing valve guides 15, 16 have been press fitted into valve guide retainer holes 11a formed by machining the cylinder head unit 11. The retainer holes 11a for the valve guides are formed in a manner such that their axial lines C coincide with the axial lines of the openings 13a and 14a of the air intake port 13 and the exhaust port 14, respectively. The machining of the valve guide retainer holes with respect to the foregoing openings 13a, 14a is performed by indexing on the manufacturing reference surfaces that were formed during the casting of the cylinder head unit 11. The valve seat 19 shown in Figure 1 is a ring-shaped piece of valve seat blank that was attached to the cylinder head 11 by the method of this invention, and subsequently machined to its finished dimensions. In Figure 2, the foregoing valve seat blank bears the reference number 20. The valve seat blank 20 is composed of a ring-shaped sintered ferrous alloy 21 , for example, with a copper film 22 covering its surface. As is shown in Figure 2, the valve seat blank 20 is positioned over the openings 13a, 14a for the air intake port 13 and exhaust port 14, the latter being of a shape such that a part of their outer circumferential surfaces lies adjacent to the inside of these openings 13a, 14a. In Figure 2, the bottom surface of the cylinder head unit 11 (the surface containing the concave areas 12 forming the combustion chambers) is facing upward. To explain in further detail, the outside circumferential surface of the valve seats 20 assumes an increasingly smaller outside diameter as it gets closer to the side facing the cylinder head unit, thereby forming a sloping surface. Moreover, the bottom surface 20b of the valve seat blank 20 is sloped increasingly toward the cylinder head unit as its axis is approached. This means that the outside circumferential surface 20a and the bottom surface 20b join to form a convex curved surface. In Figure 2, this convex surface has been labeled 20c. In the position of the foregoing openings 13a, 14a that lies opposite the foregoing convex curved surface 20c is formed a ridge area 23 which partially narrows the inside diameter of the intake and exhaust ports 13, 14. This ridge area 23 is formed during the post-casting machining of the cylinder head unit 1. This ridge area 23 comprises the attachment opening used in this invention. To wit, by placing this valve seat blank 20 over the foregoing openings 13a, 14a as shown in Figure 2, the convex curved surface 20c on the seats comes into contact with the ridge area 23 on the cylinder head unit 11. The inside circumferential surface of the valve seat blank 20 is composed of sloped surface 20d which slopes in a manner such that the inside diameter of the valve seat blank 20 diminishes the closer to the cylinder head unit 11, and an axially extending surface 20e which extends from the sloped surface 20d parallel to the axial direction. The press device 24 shown in Figures 3 and 4 is used to join the above-structured valve seat blanks 20 to the cylinder head unit 11 at the aforementioned openings 13a, 14a. A lower platen 26 is affixed to the lower area of the frame 25 of the press apparatus 24 and, positioned above this lower platen, is a movable upper platen 27 which can be freely moved up and down. The lower end of a rod 28a composed of the working end of a cylinder apparatus 28, which is disposed in the vertical direction with respect to the axial line of the top of the press frame, is attached to this upper platen 27. The foregoing lower platen 26 and the upper platen 27 are respectively attached through electrically conducting materials 26a, 27a to an electric power supply (not shown) which supplies them with electricity. The conducting materials 27a connected to the upper platen 27 are structured to deform with the rise/fall operation of the upper platen 27, or to rise and fall with it. Also, in this embodiment, the upper platen 27 forms the anode while the lower platen is the cathode. Also attached to the top of the frame that supports the foregoing cylinder apparatus 28 is a laser displacement meter comprising a reflector 29 which reflects laser light to measure the displacement of the upper platen 27. In attaching the valve seat blank 20 using this press device, first the lower electrode 31 is affixed atop the foregoing lower platen 26, and then the cylinder head unit 11 is rested atop this lower electrode 31. At this time, the concave areas 12 that form the combustion chambers in the cylinder head 11 are facing upward and the axial line of the port opening to which the valve seat blank 20 will be attached aligned with the axial line of the rod 28a of the foregoing cylinder apparatus 28. Next, as is shown in Figure 5, a guide rod 32 is inserted into the retaining hole for the valve guide 15,16 for the port to which the valve seat blank 20 is to be attached, from the side that has the concave areas 12 that form the combustion chambers. This guide rod 32 is composed of a round metal rod 32a which is covered by a layer of insulating material 32b such as alumina. The positioning is determined and retained by a stopper 32c that fits into the valve guide retaining hole 11a. In this embodiment, the method used to form the foregoing layer of insulating material 32b was flame spraying of alumina or other ceramic material onto a round rod 32a and then polishing to finish. After that, the valve seat blank 20 is placed over the port opening and then the upper electrode 33 is rested atop the valve seat blank 20. This upper electrode 33 has a guide hole 33a into which fits the foregoing guide rod 32. Its lower end has a tapered surface 33b that closely fits the foregoing sloped surface 20d (Figure 2) of the valve seat material, and an axially extending surface 33c which fits tightly around the axially extending surface 20e of the valve seat blank 20 to keep it in position. There is also a magnet 33d affixed to the lower end of the upper electrode 33 which can magnetically hold and release the valve seat blank 20. To wit, by fitting the foregoing guide rod 32 into the foregoing guide hole 33a, the upper electrode 33 is aligned coaxially with the port opening for the cylinder head 11; the above mentioned tapered surface 33b and circumferential surface 33c are in close contact with the valve seat blank 20, thereby positioning it for a coaxial fitting to the port opening. After resting the upper electrode 33 on the valve seat blank 20, the top electrode 33 is rotated and a check is made to assure that the valve seat blank 20 is correctly fitted therein. Then, the cylinder apparatus 28 drives the upper platen 27 downward so that the foregoing upper electrode is in tight contact. At this time the lower surface of the upper platen 27 is parallel to the top surface of the upper electrode 33. Next, the foregoing cylinder apparatus 28 drives the upper platen 27 downward to hold the foregoing valve seat blank 20 using the upper electrode 33 at a constant pressure against the cylinder head unit 11. At this time, due to the regulation of the movement of the upper electrode 33 by the guide rod 32, the direction of this pressure being applied to the valve seat blank 20 coincides with the axial direction of the port openings 13a, 14a. Because of this, the pressure on the valve seat blank 20 is in line with the axial direction of the port openings 13a, 14a. The pressing force is varied as shown by the solid line showing the pressing force pattern in Figure 6. To wit, there is a first pressing force P1, a constant and relatively low level of force that is applied initially, and following that, a relatively higher constant second pressing force P2 is applied for the duration of the operation. After the application of the first pressing force P1, and when the upper platen 27 has stabilized, a reading is taken using the foregoing laser displacement meter 30 of the distance to the reflector, and that distance is recorded as the initial descent position of the upper platen 27. Then, after the amount of time T1 that is shown in Figure 6 after the application of the first pressing force P1, a voltage is applied to the foregoing upper platen 27 and lower platen 26 which creates a current flow between the platens, to wit, through the upper electrode 33, the valve seat blank 20, the cylinder head unit 11 and the lower electrode 31. At this time, the direction of the current flow is from the upper electrode 33 toward the cylinder head unit 11. The broken line in Figure 6 shows the current values, which are varied according to a current value pattern. To wit, after increasing the current value, the current value is dropped to near zero, then it is increased again, all the while applying the above described pressing force, and then returned to zero. At this time, the convex surface 20c on the valve seat blank 20 is resting on the ridge area 23 on the cylinder unit 11 as is shown in Figure 2. Since the surface area where the two are in contact is exceedingly small, there is a great deal of electrical resistance to the flow of current described above, causing the area of their contact to heat up. This heat is conducted to the entire contact interface between the valve seat blank 20 and the cylinder head unit 11. Thusly, as the temperature at the contact interface between the valve seat blank 20 and the cylinder head unit 11 increases, the atomic movement in the two metal materials pressing against each other in solid phase (the copper of the cooper film 22 and the aluminum of the cylinder head unit) becomes very active, causing atoms from each to be dispersed in the other. The degree to which the aluminum oxide film formed on the surface of the cylinder head unit 11 inhibits the dispersion of the atoms is not clear. By means of the above described mutual dispersing of atoms, the composition in the area of the interface is a co-crystalline alloy consisting of copper from the copper film 22 and aluminum alloy from the cylinder head unit 11. Figure 7 shows a model of the interface area at this time. In Figure 7, the area marked by A is that where there has been a mutual dispersion of atoms and the formation of the foregoing co-crystalline alloy. When the temperature at the foregoing interface rises further and the atoms exhibiting this dispersion phenomenon become more active and a part of the foregoing co-crystalline alloy enters the liquid phase, this co-crystalline alloy layer grows, and the interface between the solid and liquid phase expands with it. While this co-crystalline layer continues liquefying, a plastic flow (plastic deformation) is initiated in the aluminum alloy of the cylinder head unit 11 that lies adjacent as it is further heated, and as it is pressed by the valve seat blank 20. This plastic flow exhibits approximate vertical symmetry as shown in Figure 7, centered around the area of initial contact, so that, as shown in Figure 8, in conjunction with the liquefied co-crystalline alloy mentioned above, the plastic flow is pushed to the outside the contact zone. In Figure 8, the area wherein the co-crystalline alloy has been pushed out is labeled B. Also, because a part of the copper film 22 on this valve seat blank 20 has been converted into co-crystalline alloy and expelled from the contact zone, a part of the ring-shaped unit 21 is in contact with the aluminum alloy, and in this area, there is also a dispersion of atoms between them. The area in which this dispersion takes place is shown by C in Figure 8. Thus, with a part of the co-crystalline alloy being eliminated from the contact zone and a plastic flow occurring in the aluminum alloy, at time T2 shown in Figure 6, the valve seat blank 20 begins to become embedded into the cylinder head unit 11. The application of pressure is increased from the time that this valve seat blank 20 starts to embed until time T3 shown in Figure 6 is reached; this is the second pressing force P2 that was described above. With the increase of the press force, the plastic flow of the aluminum alloy increases, and the amount of co-crystalline alloy that is expelled also increases. As a result, in the un-reacted areas of the contact zone lies the co-crystalline alloy composed of copper and aluminum alloy, and as described above, this phenomenon repeats to further liquefy and eliminate the co-crystalline alloy. In addition, the sintered ferrous alloy composing the ring 21 has its atoms mutually disperse with the aluminum alloy at their increasingly broadening interface. From the time when the second press force P2 is applied until time T4 in Figure 6, the current flow is dropped once to zero, and then it is elevated back to its original value. The dropping of the current value restrains the heating temporarily, which restrains the plastic flow, and as shown in Figure 6, there is a temporary decline in the degree to which the valve seat blank 20 embeds. The reason for temporarily lowering the current value is to prevent the aluminum alloy from heating to the point where it becomes molten. Then, after the current value is increased again, as was described above, it is gradually decreased to zero during the T5 to T6 time frame. It is of course the case that so long as the current is flowing, and even after it has been cut, the above described reaction continues until such time as the temperature drops to the point when the reaction is impossible. The embedding of the sintered ferrous valve seat blank 20 into the aluminum alloy proceeds at the same time that the phenomenon of the co-crystalline alloy layer is created → liquefied → expelled by the plastic flow, and this results, as shown in Figure 9, by the approximately entire outside circumferential surface of the valve seat blank 20 becoming embedded in the cylinder head unit 11. At about the time that this embedding halts, as shown in Figure 6 (at time T7), the pressing by the cylinder apparatus 28 is halted, and the upper platen 27 is raised, after a determination has been made using the laser displacement meter 30 that measures the distance from the reflector 29 that the platen 27 has reached its final position, and then the cylinder head unit 11 is removed from the press device 24. The average current values and the total time of current application were determined using the completion of the whole process as a basis. Next, the amount of embedding of the valve seat blank 20 may be determined by computing the difference between the position from where the upper platen 27 began its descent to its final position. Should this value not lie within a pre-determined tolerance D (see Figure 6), then the attachment would be regarded as defective. In actual practice, the above described tolerance D was from about 0.5 to 2 mm in this example. While the tolerance depends upon the material used for the cylinder head unit 11, normally, it should range between about 1 mm and 1.5 mm. A determination is also made for those cylinder heads that pass the above described embedding evaluation of whether or not the average current value and the total current application time were within the tolerance values, and if so, production lots that have passed the pull-out inspection are sent for the final finishing of the valve seat materials. The above mentioned pull-out inspection is performed, for example, on each production lot of valve seats. After they have been joined, as shown in Figure 9, tensile force is applied to the valve seat blank 20 to try to pull it away from the cylinder head unit. To describe this in more detail, a jig is used to hold the inside circumferential edge of the bottom surface 20b (Figure 2) of the valve seat material, and a tensile force testing device (not shown) pulls upon this jig in the aforementioned direction; the test is passed if the force required to separate the valve seat blank 20 from the cylinder head unit 11 exceeds a predetermined load level. It is also possible to perform the same retention tests while heating or to perform heat shock testing, these in addition to the simple separation test for the valve seat blank 20 that was described above. The heat retention test would involve heating the cylinder head 11, as shown in Figure 9, in a furnace with exposure to the atmosphere to 300° C for from 24 to 200 hours, cooling, and then performing the foregoing separation test. The heat shock experiment could be performed on the cylinder head 11, as shown in Figure 9, by 10 repetitions of heating it in a furnace to 300° C with exposure to the atmosphere, removing it from the furnace, and immediately cooling to 0° C by immersing in ice water, and then checking the valve seat blank 20 for separation or for any cracks before subjecting it to the foregoing separation test. The final finishing of the valve seats shown in Figure 9 may be performed by machining away the unneeded areas, for example, to the shape shown in Figure 10. This final finishing removes the unneeded areas and the copper film from the ring 21, but it leaves the valve seats 19 attached to the cylinder head unit 11 by the atom dispersion zone C that is shown in Figure 10. The metallurgical bond formed between the aluminum alloy of cylinder head 11 formed in the above described manner and sintered ferrous alloy of the valve seat 19 is essentially different from a mechanical bond and non-continuous bond that lacks its atom dispersion. Further, this method also differs metallurgically from the welding method wherein electrical resistance heating is applied at the interface of two materials to cause their localized melting, and then the current is cut to allow the liquid phases to cool and harden. To wit, the metallurgical bond that was obtained with the cylinder head in this embodiment did not leave a residual molten reaction layer, but derives from the formation of a continues structure by mutual dispersion of atoms at the interface between both materials. In attaching the valve seat blank 20 to the cylinder head unit 11 as described above, in cases where the effects of the magnetic field generated by the application of current cause the co-crystallized alloy to be expelled in a specific direction from the junction area, it is preferable to position a shield 34 adjacent to the upper electrode 33 as is shown in Figure 11. Figure 11 is a top view showing the case where a shield is employed. That figure shows the upper electrode positioned against the valve seat blank 20 and the cylinder head unit 11. Detailed descriptions of the other parts will be omitted since they are the same as or similar to the ones described in Figures 1 through 10. The shield 34 used in this embodiment is formed from a vertically split cylinder made of ferrous ferromagnetic material, and the crown area of the outside circumferential surface points toward the frame 25 of the press apparatus 23. The direction for the frame 25 position is shown by the arrow in that figure. Thus, by employing this type of a shield 34, it is possible to control the direction and the magnitude of the magnetic field that arises from the application of current, and as a result, to control the area from the junction where the co-crystalline alloy is expelled. Figure 12 shows the manufacturing procedure used to join the valve seat blanks 20 to the cylinder head 11 using the above described method. In Figure 12, as shown in step 101 the manufacturing reference surfaces are formed during the casting of the cylinder head unit 11. After that, in step 102, the convex shaped areas 23 are formed as the junction opening around the port openings 13a, 14a. In step 103, the retaining holes 11a for the valve guides are formed. When these machining processes are performed, the foregoing manufacturing reference surfaces are used to perform the indexing. Next, during step 104, the valve seat blank 20 is positioned. At this time, the cylinder head unit 11, after having undergone the machining process, is affixed in the foregoing press apparatus 24, then, as described above, the guide rod 32 and the upper electrode 33 are used to position the valve seat blanks 20 over the port openings 13a, 14a. As indicated by a previous step 105, it is also possible to use a washing process prior to this positioning operating in order to remove any cuttings that were generated during the machining. This cleaning process can be implemented by water washing, or by compressed air cleaning of the cylinder head unit. After that, in step 106, the valve seat blank 20 is pressed against the cylinder head unit 11 by the press apparatus 24, and current is based between the valve seat blank 20 and the cylinder head unit 11. This process joins the valve seat blank 20 to the cylinder head unit 11. As was described above, after attaching the valve seat blanks, in step 107, the valve guides 15, 16 are press fitted into the valve guide retaining holes 11a in the cylinder head unit. In this embodiment, the valve guides 15, 16 are inserted cold, but in the case of using a shrink fit method as in the prior art, a heating process 108 can be applied prior to the press fitting process 107 for the valve guides. In such a heating process 108, the cylinder head unit 11 would be heated in a furnace, and the unheated valve guides 15, 16 would be inserted and then held in place by the shrinkage from cooling. After the press fitting of the valve guides 15, 16, step 109 involves the rough machining of the valve seat blanks; these, along with the valve guides 15, 16, are subsequently finished off into valve seats 19 in the next step 110. The subsequent processes involve seating the intake and exhaust valves 17, 18 into the valve seats 19 and inspecting for sealing, and machining the camshaft bearing areas. This machining of the bearing can be performed after the finishing of the valve seats 19 and valve guides 15, 16, or, it can be performed either before or after the foregoing step 103 where the retainer holes for the valve guides are machined, or step 107 where the valve guides are inserted. It is also possible to reverse the above described order for attaching the valve seat blanks 20 and inserting the valve guides 15, 16. In that case, after the step 103 in Figure 12 where the valve guide retaining holes were prepared, the valve guides 15, 16 would be pressed in (step 107) and then after that, the valve seat blanks 20 would be positioned (step 104), and the valve seat blanks 20 would be pressed, and current passed through them (step 106). It is also possible to insert a washing process 105, or a heating process 108 involving either cylinder head unit 11 heating, or cooling of the valve guides 15, 16 prior to the press fitting of the valve guides 15, 16. When this type of procedure is used to manufacture cylinder heads, the valve guide retaining holes 11a can be used for the positioning of the valve seat blanks 20 over the openings for attachments, so that, compared with the method that uses press fitting for the valve seats, it is not necessary to drill any special guide holes for the valve seats prior to attaching them. This method also renders unnecessary any use of a furnace to heat the cylinder head unit for the purpose of attaching the valve seats, as it does any need to remove machining cuttings prior to loading the cylinder head 11 into the heating furnace. Further, since the machining of the cylinder head involving preparing the junction openings where the valve seat blanks will be attached, and boring retaining holes for the valve guides is performed using common manufacturing reference surfaces, the positional relationships between the two remain unaffected by casting tolerances. In the above described embodiment, the positioning of the valve seat blanks 20 on the port openings 13a, 14a was performed using the retaining holes for the valve guides, but it is also possible to use as a positioning guide the holes used for the assembly of the cylinder head. For example, it is possible to use the nock pin holes (not shown) which hold nock pins (not shown) that facilitate the positioning of the cylinder head unit 11 on the cylinder body (not shown). Figure 13 shows the production steps involved when these nock pin holes are used for indexing the position and pressing of the valve seat blanks 20. Figure 13 is a block diagram showing the procedure for cylinder head manufacturing for the case when the nock pin holes are utilized for the positioning of the valve seat blanks after the machining process. In the figure, explanation will be omitted for those reference numbers that are the same or similar to those used in Figure 1 through 12. When the nock pin holes are used for positioning, following the machining of the openings for attaching the valve seats in step 102, the nock pin hole machining is performed in step 120. Then, in steps 104 and 106 the valve seat blanks 20 are positioned, and the valve seat blanks 20 are pressed and current is applied. At this time, the nock pin holes are utilized to determine the position of the valve seat blanks 20 around the opening and the direction of pressure application. After attaching the valve seat blanks to the cylinder head 11 in this manner, in step 103, the machining is done to make the valve guide retainer holes 11a. The positioning is also performed for this machining process based on indexing relative to the nock pin holes. This process is followed by the valve guide press fitting process in step 107, the valve seat blank rough finishing operation in step 109, and by the valve seat blank final finishing process in step 110. As described above, even when the nock pin holes are used for positioning, it is possible to reverse the order of the valve seat attachment and the press fitting of the valve guides. In other words, in this case, after the nock pin hole machining is accomplished in step 120 in Figure 13, then the valve guide retention hole machining process in step 103 → the valve guide press fitting process in step 107 → the valve seat blank positioning process in step 104 → the valve seat blank pressing and current application in step 106 → the valve seat rough finishing process in step 109 → and the valve seat final finishing in step 110 would be used to complete the process. The same types of effects as in Example 1 are realizable when this structure is employed. A method of manufacturing cylinder heads conforming to a third embodiment will now be explained with reference to Figure 14. Figure 14 is a block diagram that will be used to explain the manufacturing processed for cylinder heads according to this third embodiment. In the figure, detailed explanation will be omitted for those reference numbers that are the same as or similar to the ones used in Figures 1 through 13. As is shown by step 201 in Figure 14 for this method for manufacturing cylinder heads, manufacturing reference surfaces are cast into the cylinder head unit 11, and the openings for the attachment of the valve seat blanks 20 are formed. After casting, in step 202, the positioning of the valve seat blanks over the attachment openings is based upon indexing relative to the manufacturing reference surfaces. Next in step 203 the pressing and the current passage is performed on the valve seat blanks 20. In this process, the direction of the pressing of the valve seat blanks is set to be in the axial direction of the intake and exhaust valves 17, 18, on the basis of indexing on the manufacturing reference surfaces. Thus, after attaching the valve seat blanks 20 to the cylinder head unit 11 in this manner, in step 103 the retaining holes for the valve guides are bored → in step 107 the valve guides are press fitted → in step 109 the valve seat blanks are rough finished → and in step 110 the valve seat blanks receive their final finish processing to complete the process. When a cylinder head is manufactured according to this procedure, the junction openings where the valve seat blanks 20 are attached are formed during casting, and the manufacturing reference surfaces that were also formed during casting are used for positioning the valve seat blanks 20 over the foregoing junction openings. Therefore, compared to the conventional press fitting of valve seats, there is no machining needed on the cylinder head prior to the attachment of the valve seats. In addition, there is no need to heat up the cylinder head in a furnace for the purpose of attaching the valve seats, and moreover, because the foregoing two processes are not needed, neither is the washing process. As with the previous example, it is possible to include the same type of heating process 114 just prior to the press fitting of the valve guides. The method of manufacturing cylinder heads according to a fourth embodiment will be detailed below with reference to Figure 15. Figure 15 is a block diagram that will be used to explain the method of manufacturing cylinder heads Detailed explanation will be omitted for those reference numbers that refer to the same or similar parts used in the foregoing Figures 1 through 14. As is shown by step 201 in Figure 15, the method of cylinder head manufacturing forms manufacturing reference surfaces during the casting of the cylinder head unit, at which time the junction openings for the joining the valve seat blanks 20 are also formed. After the casting, first, the valve guide retainer holes 11a are bored (step 103). After that, it is the same procedure as shown in Figure 12 for Example 1. As it was with that example, it would also be possible to reverse the valve seat blank joining process and the valve guide press fitting process. When manufacturing cylinder heads using this procedure, the junction openings where the valve seat blanks 20 will be attached are formed by casting, and the valve guide retainer holes 11a are used for positioning the valve seat blanks 20 over the foregoing junction openings. Therefore, compared to the case wherein the valve seats are press fitted, the method does not require the boring of a special valve seat hole prior to attaching the valve seat. In addition, it is possible to eliminate the heating process in the furnace for the cylinder heads, as well as the washing process that was required before loading the cylinder heads into that heating furnace. In Example 4, the valve guide retainer holes 11a were used as an index in the positioning of the valve seat blanks 20, but it would be possible as well to use the holes in the cylinder head which facilitate its assembly for the same purpose. For example, it is possible to use the nock pin holes (not shown) which hold nock pins (not shown) that facilitate the positioning of the cylinder head unit 11 on the cylinder body (not shown). Figure 16 shows the production steps involved when these nock pin holes are used for indexing the position and pressing of the valve seat blanks 20. Figure 16 is a block diagram showing the cylinder head manufacturing process when the nock pin holes are used for positioning the valve seat blanks 20 at the junction openings that were formed during casting. Detailed explanation will be omitted for those reference numbers that refer to the same or similar parts used in the foregoing Figures 1 through 15. When the nocking holes are used to determine the positioning, as is shown in step 201 of Figure 16, when casting the cylinder head unit, the manufacturing reference surfaces and the junction openings where the valve seat blanks 20 will be attached are formed during the casting process. After casting, first the nock pin holes are bored (step 120). After that, the steps proceed in the same way as shown for Example 2 in Figure 13. As before, it would also be possible to reverse the valve seat blank attachment process and the valve guide press fitting process. Even with this structure, the effects realized are on a part with those in the example shown in Figure 15. In the above described examples both the valve seat blanks 20 and the cylinder head port openings 13a, 14a has convex areas (convex curved surface 20c and ridge area 23) formed upon them. The examples featured these convex zones as being pressed against each other, but it is not absolutely necessary to form convex areas on both parts; they may be on one or the other parts. This example is shown in Figure 17, (a) through (c). Irrespective of whether the junction openings where the valve seat blanks 20 are attached were formed during casting or by a machining process, it is possible to modify the shapes involved as described below. Figure 17 is a sectional view showing other embodiments where the shapes at the pressure area have been changed. In (a) and (b), examples are shown where there is a convex area formed on the valve seat blank; in (c) the example is for a projection formed only on the cylinder head. In this figure, explanation will be omitted for reference numbers that indicate the same or similar parts in the preceding Figures 1-10. In the example shown in Figure 17(a), the valve seat blank 30 has approximately the same shape as used in the previous examples, but there is a flat sloped surface formed around the port openings 13a, 14a upon which the valve seat blank 20 makes contact. In (b) of the same figure, the outer circumferential surface of the valve seat blank 20 is structured to fit into the port openings 13a, 14a. Also, the inside circumferential surface of the valve seat blank 20 differs from that described in the foregoing embodiments, it has an axially extending surface 20e that extends through the entire thickness of the valve seat blank 20. In (c) of the same figure, the valve seat blank has a flat sloping surface that comes into contact with the ridge 23 on the cylinder head unit 11. As can be seen from the examples shown in Figure 17 (a) - (c), it is possible to obtain the same effect irrespective of whether the ridge is placed on the valve seat blank 20 or on the cylinder head unit 11. As was shown in the examples, when a ridge is placed on both, it facilitates the generation of a plastic flow of the metal making up the cylinder head unit 11, and increases, relatively, the amount of that flow, thereby enabling the area of the contact surface with the valve seat blank 20 to be expanded for a higher strength junction. In the foregoing examples, the material used to make the cylinder head was AC4C stock, the valve seat blanks 20 were composed of sintered ferrous alloy in ring shapes 21 that were immersed in molten copper to create a film of copper covering them, but such could have been formed by electroplating, and the materials used and the method of forming the covering film are not limited to those described in the embodiments. For example, the material used to comprise the cylinder head unit 11 could be any of the materials conventionally used for engine cylinder heads such as AC4B stock, AC2B stock, etc. Any type of sintered ferrous alloy can be used to make the ring shaped members 21, and there is no great difference whether they are coated by immersion in molten copper or electroplated. The metal used to coat the ring shaped members 21 can be any that will form co-crystalline metal with the stock of the cylinder head unit 11. In selecting these materials and the film forming method, the most cost effective method should be chosen from the perspective of producing cylinder heads as an industrial product. Also, the degree to which the valve seat blank 2 was embedded in the foregoing examples was detected in the final stage of the process, but the displacement could be continuously measured while pressing, and then whether or not it was within the permissible tolerance could be checked periodically. By so doing, it is possible to eliminate defects and save the time that would be wasted in producing them. As explained above, the method of manufacturing cylinder heads according to the first invention involves forming during the casting process manufacturing reference surfaces which define three intersecting directions, and additionally, junction openings around the port openings where the valve seat blanks will be installed, and then after casting, by using the foregoing manufacturing reference surfaces to index the position of the valve seat blanks, which are composed of sintered ferrous rings coated with a metal film, and set them in place, and to again use the manufacturing reference surfaces as an index to apply pressure to the valve seat blanks in a direction that coincides with the axial direction of the intake/exhaust valves, and then, by heating the contact zone between the cylinder head and the valve seats using electrical resistance, causing the temperature to increase at the contact interface where the two are being pressed together, and causing the atoms to mutually disperse at that interface. As a result, a co-crystalline alloy layer will be formed from the metal that coats the valve seat blanks and the cylinder head stock metal. Since this co-crystalline alloy converts to a liquid phase at a low temperature, the foregoing resistance heating converts it into a liquid phase, and the valve seat blank, being pressed and heated, causes a plastic flow to cause this metal, along with the stock metal from the cylinder head, to be expelled from the junction. This results in contact between the sintered alloy of the valve seat blank and the metal stock of the cylinder head, with their atoms mutually dispersing, and in this state, the valve seat blank is embedded into the port opening of the cylinder head unit and attached thereto. In joining the valve seat blanks to the cylinder head unit, no machining of the cylinder head is required prior to the attaching operation; this compared to the conventional method where valve seats are pressed in which does required initial machining of the cylinder head. In addition, the use of a heating furnace for the purpose of heating the cylinder head for seat insertion is not needed, nor is a wash process needed prior to those steps. Because of this, compared to the conventional press fitting of the valve seats into the cylinder head, it is possible to eliminate three production steps from the production process. This is especially advantageous since it was impossible to continue the processing of the cylinder heads that had been heated in a furnace in the conventional method until they had cooled off. Accordingly, not only does the method greatly simplify the manufacturing process but it can shorten the manufacturing time as well. Furthermore, since the heating furnaces used conventionally would hold and heat a plurality of cylinder head units at the same time, a great deal of space was required for situating the furnace. By eliminating it, it is possible to make more efficient utilization of factory space. Since the elimination of the heating furnace also allows the shortening of the conveyor line for the cylinder heads, compared to the prior art, it is possible to shorten the transport time involved between machining processes. When casting cylinder heads, manufacturing reference surfaces that define three intersecting directions are formed and the openings around the ports where the valve seat blanks make contact are shaped, then after casting, the manufacturing reference surfaces are used to determine the machining position for making the various holes needed in the cylinder head structure, and then, these holes are used for indexing the position of the valve seat blanks composed of sintered ferrous alloy rings covered by a metal film and then are further used to align the pressing direction upon the valve seat blanks to match the axial direction of the valve axes for the intake and exhaust valves; next, electrical resistance heating is used to heat the contact zone between the cylinder head unit and the valve seat blank, so that the temperature at the contact interface between the two increases and the atoms mutually disperse. As a result, the metal material comprising the coating film on the valve seat blanks forms a co-crystalline layer with the metal stock from the cylinder head. Since this co-crystalline alloy converts to a liquid phase at a low temperature, the foregoing resistance heating converts it to a liquid phase, and the valve seat blank, being pressed and heated, causes a plastic flow to cause this metal, along with the stock metal from the cylinder head, to be expelled from the junction. This results in contact between the sintered alloy of the valve seat blank and the metal stock of the cylinder head, with their atoms mutually dispersing, and in this state, the valve seat blank is embedded into the port opening of the cylinder head unit and attached thereto. No especial guide hole is needed, compared to the case for press-fitted valve seats when a guide hole must be formed to a high degree of precision prior to pressing. However, the attachment method of this invention does not require this press fitting structure, no guide at all is needed to accurately position the valve seat blanks or to apply the pressure in a direction aligned axially with the valve seat blank. In attaching the valve seat blanks to the cylinder head unit, there is no requirement to first make a special valve guide hole, as there is in the press-fitting method, before attaching the valve seat blanks. Also not needed are a heating process using a furnace to heat the cylinder head units for the purpose of fitting the valve seats, or a washing process to prepare the cylinder head units for the heating furnace. Because of this, compared to the case where the valve seats are press fitted into the cylinder head, it is possible to simplify the production process and shorten the manufacturing time. In particular, the conventional method that employed the heating process did not allow cylinder head processing during the interval over which they were cooling down, so the elimination of this heating process can cause a dramatic simplification of the production process and a shortening of the production time. Because the conventional heating furnaces would accommodate a plurality of cylinder heads and heat them all at once, the device required a great deal of space, and eliminating it allows more effective utilization of factory space. Further, the elimination of the heating furnace enables the shortening of the conveyor line for the cylinder heads, thereby shortening the conveyance time between machining devices. Further, by using the structural holes that are formed in the cylinder head by necessity as guides for the positioning of the valve seats and the application of pressure, one hole can serve multiple purposes, thereby bringing about a cost advantage. The method of manufacturing cylinder heads involves forming, during the casting of the cylinder head, working references surfaces that define three directions and intersect, and then, after casting, determining the positions for machining the port opening and the various component holes in the cylinder head on the basis of these manufacturing reference surfaces, and then shaping the area of the port openings that makes contact with the valve seat blanks and boring the foregoing component holes, and next utilizing the foregoing holes to determine the position for the placement of the valve seat blanks, which are composed of sintered ferrous alloy rings covered with a metal film, and then, using the holes again as a reference for the application of pressure in the axial direction of the intake and exhaust valves, followed by the heating of the contact zone between the cylinder head unit and the valve seat blank using electrical resistance heating which causes the atoms at the interface between the two to mutually disperse at the compressed junction. As a result, the metal from the coating film on the valve seat blanks forms a co-crystalline alloy layer with the stock metal from the cylinder head. Since this co-crystalline alloy converts to a liquid phase at a low temperature, the foregoing resistance heating converts it to a liquid phase, and the valve seat blank, which is being pressed and heated, causes a plastic flow to cause this metal, along with the stock metal from the cylinder head, to be expelled from the junction. This results in contact between the sintered alloy of the valve seat blank and the metal stock of the cylinder head, with their atoms mutually dispersing, and in this state, the valve seat blank is embedded into the port opening of the cylinder head unit and attached thereto. When attaching the valve seat blanks to the cylinder head, it is not necessary to have a special process beforehand to bore guide holes for the valve seats prior to their attachment. In addition, it is possible to eliminate the heating furnace and the heating process that was used conventionally to attach the valve seats, and eliminate the washing process that was needed prior to loading the cylinder head units into the furnace. Because of this, compared to the case where the valve seat blanks were press fitted into the cylinder head, it is possible to simplify the production process and shorten the production time. Especially since the time required in th conventional process for heating and cooling the cylinder heads has been eliminated, it is possible to dramatically simplify the production process and shorten the production time through the elimination of this heating process. Further, since a plurality of cylinder heads were loaded into the heating furnace and heated at one time in the prior art, the furnace required a great deal of space, and eliminating allows the more efficient use of factory space. Further, because of the lack of a heating furnace, it is possible to shorten the conveyor lines for the cylinder heads between the machining stations, thereby shortening the conveyance time. In addition, the attachment openings where the valve seat blanks are attached to the cylinder head and the foregoing component holes were positioned on the basis of common manufacturing reference surfaces, and accordingly, the positional relationship between the two remains unaffected by casting tolerances. Since this makes it possible to use the component holes as a reference in accurately positioning the attachment openings, it is possible to attach valve seat blanks with a high degree of precision.

Claims: Method for producing a cylinder head unit of an internal combustion engine, comprising the steps of: casting a cylinder head (11) having valve openings (13a, 14a), providing said valve openings (13a, 14a) with valve seats (19) by fastening valve seat blanks (20) and applying a finishing treatment to said valve openings (13a, 14a) and said valve seats (19), wherein during casting of the cylinder head (11) manufacturing reference surfaces are formed thereon, characterized in that a position for said valve seat blanks (20) on said valve openings (13a, 14a) is determined on the basis of indexing on said manufacturing reference surfaces, wherein said position for said valve seat blanks (20) is dependent from an axial direction (C) of valve guide holes (11 a) drilled after casting, metallurgically bonding said valve seat blanks (20) to said valve openings (13a, 14a) is carried out by applying electricity to said cylinder head unit (11) through an electrode (33), a guide rod (32) is advanced coaxially aligned with said electrode (33), such that said guide rod (32) enters said valve guide hole (11a) and simultaneously guides said electrode (33) for matching a pushing direction with said axis (C) of a valve (17, 18), wherein said electrode (33) is in direct contact with said valve seat blanks (20) electrically, but isolated from said cylinder head (11) and said guide hole (11a). Method according to claim 1, characterized in that said manufacturing reference surfaces define three directions comprising a first end surface on the side of said cylinder head (11) facing a cylinder body, a second surface parallel to said first surface and a third surface being perpendicular to both the first and second surfaces. Method according to claim 1 or 2, characterized in that the position for said valve seat blanks (20) is dependent on knock-pin holes drilled after casting. Method according to at least one of the claims 1 to 3, characterized in that valve guides (15, 16) are inserted into the respective valve guide holes (11 a) prior to or after the bonding process of said valve seat blanks (20). Method according to at least one of the claims 1 to 4, characterized in that joining recesses for said valve seat blanks (20) are formed in said valve openings (13a, 14a) during casting or shaped after casting said cylinder head (11). Method according to at least one of the claims 1 to 5, characterized in that said valve guide holes (11a) are drilled prior to or after the bonding process of said valve seat blanks (20). Method according to at least one of the claims 1 to 6, characterized in that said metallurgical bonding of said valve seat blanks (20) comprises: (a) placing a valve seat base material (20) onto a surface of said openings (13a, 14a) of said cylinder head unit (11), and (b) pushing an electrode (33) against the end face of said valve seat base material (20) opposite to said cylinder head unit (11) with a pushing direction matched with an axis (C) of said intake or exhaust valve (17, 18), whereby said electrode being adapted to apply electricity to said cylinder head unit (11) through said valve seat base material (20). Method according to at least one of the claims 1 to 7, characterized in that either the electrode (33) or the cylinder head unit (11) or both are moved towards each other. Method according to at least one of the preceding claims 1 to 8, characterized in that the pressing force and/or said electricity are applied according to a predetermined pattern. Method according to at least one of the preceding claims 7 to 9, characterized in that in step (a) said valve seat base material (20) and said opening (13a, 14a) contact each other along a circumferential line and that this line of contact is provided by a convex portion (20c) of said valve seat base material (20) and/or a convex portion (23) of said opening (13a, 14a). Method according to at least one of the preceding claims 7 to 10, characterized in that during step (a) said electrode magnetically attracts said valve seat base material (20) for placing said valve seat base material (20) on the surface of said valve opening (13a, 14a). Method according to at least one of the preceding claims 7 to 11, characterized in that after steps (a) and/or (b) the electrode is rotated for checking whether the valve seat base material (20) is fitted correctly. Method according to at least one of the preceding claims 9 to 12, characterized in that the pattern of applying the pressing force comprises a first pushing force (P1) which is applied at an early stage of the bonding process and then a second pushing force (P2) which is applied with a certain higher value until bonding is completed. Method according to claim 13, characterized in that the pattern of applying the electricity starts when a time has lapsed after application of the first pushing force (P1), wherein a value of the electricity is first increased then decreased close to zero and thereafter increased again before reduced to zero during a time in which said second pushing force (P2) is still applied. Method according to claim 13 or 14, characterized in that the second pushing force (P2) is applied when it is recognized that the valve seat base material (20) has begun to sink. Method according to at least one of the preceding claims 7 to 15, characterized in that the direction and the magnitude of a magnetic flux in the magnetic field caused by energization is controlled to control the direction of the eutectic alloy removed from the bonnet portions. Method according to at least one of the preceding claims 7 to 16, characterized in that the magnitude of sinking of the valve seat base material (20) into the opening (13a, 14a) is measured continuously during the whole bonding process. Method according to claim 17, characterized in that said magnitude of sinking of the valve seat base material (20) into the opening (13a, 14a) is controlled, in particular on the basis of said measured sinking value. Method according to at least one of the preceding claims 7 to 18, characterized in that said valve seat base material (20) is made of an Fe-based sinter alloy being provided with a coating (22) of a metal or metal alloy being capable of forming an eutectic alloy with that cylinder head unit (11). Method according to at least one of the preceding claims 7 to 19, characterized in that the material of said cylinder head unit (11) is selected out of the group consisting of AC4C, AC4B and AC2B as set forth in the Japanese Industrial Standard (JIS). Method according to at least one of the preceding claims 7 to 20, characterized in that after step (b) a sampling test is carried out by applying a tensile force to the bonded valve seat base material (20).

Inventors: ADACHI SHUHEI, AMANO JUNKICHI, SAKAI HIROYUKI, ADACHI, SHUHEI, AMANO, JUNKICHI, SAKAI, HIROYUKI
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1996
Main classification: F01L3
IPC codes: F02F1, F01L3