BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
   The invention relates to a metal gasket to be disposed between, for example, a cylinder block and a cylinder head of an internal combustion engine. 
   In a metal gasket  100  disclosed in Japanese Patent Publication (KOKAI) No. 11-230355, as shown in  FIG. 7 , a gasket base plate  101  is provided with a plurality of annular beads  102  and  103  around a cylinder bore. When a cylinder head  104  and a cylinder block  105  are tightened, high surface pressures are generated at abutting portions between the cylinder head  104  and the annular bead  102  and between the cylinder block  105  and the annular beads  103  to improve the sealing ability thereat. 
   However, in the metal gasket  100 , a high surface pressure is generated at the abutting portion where the annular bead  102  or  103  abuts against the cylinder head  104  or the cylinder block  105 , and forms a pressure trace at the abutting surface. When the tightening of the cylinder head  104  and the cylinder block  105  is released for repair or inspection and the cylinder head  104  and the cylinder block  105  are tightened again with a new gasket  100  therebetween, the pressure trace does not necessarily match the annular bead  102  of the new metal gasket  100 . Therefore, there has been a problem that the sealing ability at the tightening portion is lowered. 
   In view of the above problems, the present invention has been made, and an object of the invention is to provide a metal gasket wherein a plurality of annular beads increases surface pressures at abutting portions, and a pressure trace is not formed on an abutting surface of the cylinder head. 
   Further objects and advantages of the invention will be apparent from the following description of the invention. 
   SUMMARY OF THE INVENTION 
   In order to solve the above problems, the present invention is to provide a metal gasket to be disposed between two connecting members where fluid flows therein to prevent the fluid from leaking outside. The metal gasket includes a gasket base plate having an opening through which the fluid passes and a plurality of annular beads formed in a wave shape over a predetermined area extending outwardly from a hole edge of the opening; and a shim for covering the annular bead portion to abut against at least one surface of the gasket base plate so that the gasket base plate and the shim are connected. 
   As a preferable embodiment of the metal gasket according to the present invention, the shims are provided on both sides of the gasket base plate. Also, the shims extend from the opening of the gasket base plate to the vicinity of an outer side of the bead located at the outermost position to cover the beads. It is preferable that the shims are fixed to the gasket base plate by a laser welding. 
   A preferable embodiment of the metal gasket according to the present invention is structured such that a relationship between a thickness t 1  of the gasket base plate and a thickness t 2  of the shim is t 1 &gt;t 2 , and a relationship between a hardness h 1  of the gasket base plate and a hardness h 2  of the shim is h 1 &lt;h 2 . 
   According to the metal gasket of the present invention having the structure described above, in the metal gasket wherein surface pressures at the abutting portions of a plurality of annular beads are increased, the surface pressures acting on the cylinder head or the cylinder block become uniform, so that a pressure trace is prevented from being formed on the cylinder head or the cylinder block. Also, the gasket base plate around the cylinder bore has a large thickness due to the shims to thereby improve the sealing ability. 
   According to the metal gasket of the present invention described above, while the surface pressures at the abutting portions of the annular beads are increased, the formation of the pressure trace on the abutting surface of the cylinder head can be prevented. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view showing a metal gasket according to a first embodiment of the invention; 
       FIG. 2  is an explanatory schematic sectional view of the metal gasket corresponding to a section taken along line  2 — 2  in  FIG. 1 ; 
       FIG. 3  is a sectional view corresponding to  FIG. 2  showing a modified example of the first embodiment; 
       FIG. 4(A)  is a conventional metal gasket for explaining a surface pressure generating condition, and  FIG. 4(B)  is a metal gasket of the first embodiment, respectively; 
       FIGS. 5(A) and 5(B)  are partial sectional views of the metal gasket at the same position as that shown in  FIG. 2  according to a second embodiment; 
       FIGS. 6(A) to 6(C)  are partial sectional views of the metal gasket at the same position as that shown in  FIG. 2  according to a third embodiment; and 
       FIG. 7  is a partial sectional view of the conventional metal gasket at the same position as that shown in  FIG. 2 . 
   


   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention will be explained in detail based on embodiments shown in the accompanying drawings. In the respective embodiments shown in the drawings, a metal gasket of the invention is applied to a contact portion of a cylinder block and a cylinder head of an engine. 
     FIGS. 1 and 2  show a first embodiment of a metal gasket  10  to be attached to a cylinder head attaching surface  2  of a cylinder block  1 . The metal gasket  10  is attached to a contact portion between the cylinder block  1  and a cylinder head  3  to prevent combustion gas in a cylinder bore  6  provided therein and engine cooling water or oil circulating therein from leaking outside. The metal gasket  10  is formed of, as clearly shown in  FIG. 2 , a gasket base plate  11 , and first and second shims  12  and  13  disposed on upper and lower surfaces of the gasket base plate  11 , respectively. Also, the metal gasket  10  includes openings (combustion chamber holes)  14  substantially corresponding to the cylinder bores  6  formed in the cylinder block, and also, fluid holes  7  of cooling water and oil and holes  8  for tightening bolts, as apparent from a flat surface shape shown in  FIG. 1 . 
   The gasket base plate  11  is formed of a metal plate with a thickness t 1  in the order of, for example, 0.2 to 0.3 mm, and includes multiple annular beads  16 ,  17  and  18  in a wave shape in a predetermined area extending outwardly from a hole edge of the opening  14 . The first shim  12  and second shim  13  are provided on upper and lower surfaces of the gasket base plate  11  for covering portions of the multiple annular beads  16 ,  17  and  18 . The shims  12  and  13  may have a structure wherein the respective shims surround each of the cylinder bores  6  and are integrally connected, as shown in the drawing; or a structure wherein the shims surrounding each of the cylinder bores  6  are independently formed in a ring shape. The shims  12  and  13  are formed of a metal plate having a thickness t 2  in the order of, for example, 30 to 100 μm, smaller than the thickness t 1  of the gasket base plate  11 , and having a hardness h 2  greater than the hardness h 1  of the gasket base plate  11 . Incidentally, the thickness t 1  of the gasket base plate  11  and the thickness t 2  of the shims  12  and  13  are not limited to the above-stated ranges. 
   The gasket base plate  11  and the first and second shims  12  and  13  are laser-welded at welding portions  19  on the upper and lower surfaces so as to surround the periphery of the annular bead  18  disposed at the outermost position from the opening  14  at an outer side end or in the vicinity thereof. Incidentally, including any embodiments described below, in a case that the shims surrounding the respective peripheries of the cylinder bores  6  are integrated as shown in  FIG. 1 , it is desirable that the shims are laser-welded to the gasket base plate for connecting the entire periphery surrounding the plural cylinder bores. However, a portion of the entire periphery may not be welded. Also, in a case that the shims are in a ring shape for surrounding each cylinder bore  6  individually, the whole periphery of each cylinder bore may be separately welded, or may be partially welded. Further, the welding may be replaced with other connecting methods. 
     FIG. 3  shows a modified example of a welding portion  19  in the first embodiment, wherein the shims on the upper surface and the lower surface may be laser-welded from only one surface of the gasket base plate  11 . 
   Next, an effect of the case in which the cylinder block  1  and the cylinder head  3  are tightened with the metal gasket  10  according to the first embodiment will be explained.  FIGS. 4(A) and 4(B)  are drawings for explaining a difference in operations and effects between the metal gasket  10  having the first and second shims  12  and  13  and a conventional metal gasket without these shims. In the metal gasket  10  of the first embodiment, as shown in  FIG. 4(B) , the annular beads  16 ,  17  and  18  (surface pressures Sa 1 ); portions  20  and  21  projecting downwardly relatively and formed by the annular beads  16 ,  17 , and  18 ; and angle portions  22 ,  23  at both ends of the beads contact the cylinder head and the cylinder block through the shims  12  and  13 ,  50  that surface pressures become uniform as compared with a case shown in  FIG. 4(A)  wherein surface pressures are directly applied to the cylinder head and the cylinder block from the annular beads  16 ,  17 , and  18  and the like. 
   In other words, when the surface pressures Sa 1  and Sb 1  in the first embodiment shown in  FIG. 4(B)  are compared with the surface pressures Sa 0  and Sb 0  in the conventional metal gasket shown in  FIG. 4(A)  at portions corresponding to the above respective portions, although respective integration values do not change, the surface pressures Sa 1  and Sb 1  are generally lowered by A and become flat. Thus, an extreme high surface pressure is eliminated, thereby preventing a pressure trace from being formed by the metal gasket  10  on the attaching surfaces  2  and  4  (refer to  FIG. 2 ) of the cylinder block and the cylinder head. Also, since the gasket base plate  11  is made thick due to the shims  12  and  13  around the cylinder bore, the sealing ability in the portion can be increased. 
     FIGS. 5(A) and 5(B)  show metal gaskets  30 A and  30 B according to the second embodiment of the present invention. Incidentally, in the second embodiment, only structures different from that of the first embodiment are explained, and other explanations are omitted. The metal gasket  30 A shown in  FIG. 5  (A) is structured such that the first shim  12  covers a portion of the upper surface of the gasket base plate  11  having the annular beads  16 ,  17  and  18 , and only the first shim  12  is laser-welded at the welding portion  19 . Also, the metal gasket  30 B shown in  FIG. 5(B)  is structured such that the second shim  13  covers a portion of the lower surface of the gasket base plate  11  same as that mentioned above, and only the second shim  13  is laser-welded at the welding portion  19 . 
   In the metal gaskets  30 A and  30 B, the shim  12  or  13  is welded only on a side where there is a risk of generating a pressure trace, or a side where a pressure trace is desired not to take place. For example, in an engine where the cylinder block  1  and the cylinder head  3  are formed of different materials, the shim  12  or  13  is welded to one having a lower hardness than the other. Thus, the cost can be reduced. 
     FIGS. 6(A) to 6(C)  show metal gaskets  40 A,  40 B, and  40 C according to the third embodiment of the present invention. In the third embodiment, only differences from those in the first embodiment are explained, and explanations of the other portions are omitted. The metal gasket  40 A shown in  FIG. 6(A)  is structured such that the second shim  13  is laser-welded on the lower surface of the gasket base plate  11 , and a third shim  42  covering the annular beads  17  and  18  is laser-welded to the upper surface. The metal gasket  40 B shown in  FIG. 6(B)  is structured such that the first shim  12  is laser-welded on the upper surface of the gasket base plate  11 , and a fourth shim  43  covering the lower surface portion corresponding only to the annular bead  16  is laser-welded in the vicinity of a hole edge of the opening  14 . Further, the metal gasket  40 C shown in  FIG. 6(C)  is structured such that the second shim  13  is laser-welded on the lower surface of the gasket base plate  11 , and a fifth shim  44  covering the upper surface portion corresponding to the annular beads  16  and  17  is laser-welded in the vicinity of the hole edge of the opening  14 . 
   In these metal gaskets, the shim  12  and  13  are welded only on portions where there is a risk of generating a pressure trace, or where the pressure trace is desired not to take place and, at the same time, the shim  42 ,  43  or  44  is selectively welded to a specific portion where there is a risk of generating the pressure trace, or where the pressure trace is desired not to take place. Thus, the surface pressure can be controlled at the required portions. Also, the necessary surface pressure can be obtained by the shims  42  and  43  or  44 , and each shim functions as a stopper for stopping a movement of the metal gasket  40 A,  40 B, or  40 C. The metal gaskets  40 A,  40 B, and  40 C according to the third embodiment are effective not only for an automobile but also for other various applications. 
   The metal gaskets according to the present invention are not limited to the above-described embodiments, and they can be modified within the scope of claims. For example, in any metal gaskets according to the above-described embodiments, a coating material can be applied on the outer surface side of the gasket base plate or the shim. Further, while the above embodiments direct to the metal gaskets used for an engine, the metal gaskets according to the present invention can be used for tightening portions in other applications. 
   The disclosure of Japanese Patent Application No. 2003-349326, filed on Oct. 8, 2003, is incorporated in the application. 
   While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.
A gasket is interposed at a joining portion between two members. The gasket includes a gasket base plate having an opening through which fluid flows and a plurality of annular beads formed in a wave shape over a predetermined area extending outwardly from a hole edge of the opening; and at least one shim covering only the annular beads. The shims are laser-welded at a welding portion to cover the beads. Thus, the metal gasket does not have a pressure trace on abutting surfaces of a cylinder head.