Abstract:
A threaded fastener is configured to reduce stress experienced at a female threaded portion of a hole formed in an engine block, when the threaded fastener is inserted therein, and when an axial load is applied thereto. In a particular application, a crankshaft is supported between a top supporting wall of a crankcase and a bottom supporting wall of a main cap. The inventive bolt includes a hollowed-out male threaded portion, which fits into a hole in the top supporting wall. The male threaded portion includes an elastically deformable low-rigidity portion, in which a cylindrical hole is concentrically formed. The maximum allowable stress of the top supporting wall is less than that of the bolt, due to material considerations. The low-rigidity portion is placed overlapping the female threaded portion in the axial direction, and since it is elastically deformable, it helps alleviate the stress experienced by the female threaded portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    The present application claims priority under 35 USC 119 based on Japanese patent application No. 2003-074841, filed Mar. 19, 2003.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a threaded fastener for insertion into a threaded hole to connect components of an internal combustion engine, and to an internal combustion engine in which the fastener is used. More particularly, the present invention relates to a threaded fastener including a cylindrical shaft with an elastically deformable low-rigidity portion adapted to withstand combustion-related axial loading.  
           [0004]    2. Description of the Background Art  
           [0005]    Many different designs for bolts and other threaded fasteners are known, and many such fasteners are commercially available. Conventional fasteners include bolts such as the one disclosed in, for example, Japanese Utility Model No. 3016308. In this known bolt used for fastening two members together, a tip portion of a threaded shaft is adapted to be screwed into a threaded through-hole formed in a receiving member. A hexagonal cavity is formed inside the threaded shaft of the bolt, and an Allen wrench, or similar tool, can be inserted into the cavity when needed.  
           [0006]    In the event that the bolt breaks off at the threaded shaft, the broken shaft may be unscrewed by inserting a suitable tool into the cavity from the opening of the threaded hole, thereby allowing removal of the broken shaft from the threaded hole.  
           [0007]    In the technology disclosed in Japanese Utility Model No. 3016308, the cavity is made in a hexagonal shape, in order to engagingly receive the tool therein. Further, in the reference, it is not clear whether or not an external force is applied to the member in which the threaded hole is formed, and the relationship between the allowable stress of the member to be fastened and the allowable stress of the bolt is not clear.  
           [0008]    Hence, the technology of the above reference does not take into account the reduction of a concentrated stress caused at the female threaded portion into which the tip portion of the threaded shaft is screwed, at the time when an axial load based on the external force is applied to the bolt. In addition, according to the teachings of this reference, the radial thickness of the threaded shaft having the cavity formed therein is not uniform in a peripheral direction.  
           [0009]    Further, as described in, for example, microfilm disclosed in Japanese Utility Model Application No. 63-39229 (Japanese Unexamined Utility Model Publication No. 1-143418), it is known that in the case of a bolt for fastening a member to which an external force is applied, when an axial load is applied to the bolt, based on the external force applied to the member, a concentrated stress is caused at a female threaded portion of a hole formed in the member into which the tip portion of the male threaded portion is screwed.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention has been made in view of the circumstances described above. It is an object of the present invention to provide an improved bolt designed for reducing a concentrated stress experienced at a female threaded portion of a hole formed in a member to be fastened, when an external force is applied thereto via application of an axial load to the bolt.  
           [0011]    It is another object of the present invention to maintain or enhance the strength of the constituent member which receives the bolt, in a case where the inventive bolt hereof is used in an internal combustion engine.  
           [0012]    In one illustrative embodiment of the invention, a specialized bolt is provided for insertion into a threaded hole formed in a constituent member of an internal combustion engine, to which a combustion load is axially applied via the bolt.  
           [0013]    A first aspect of the present invention relates to a bolt for fastening a first member to a second member, the bolt including a male threaded portion for insertion into a female threaded portion of a hole formed in the first member to be fastened. When using a bolt according to the first aspect, an axial load is applied to the bolt, based on an external force applied thereto, and the load is transferred to the first member via the bolt.  
           [0014]    In the bolt according to the first aspect hereof, the male threaded portion is provided with a low-rigidity portion in which a hollow cylindrical bore is formed, concentric with the center axis of the male threaded portion and substantially circular in a cross section. In use, the low-rigidity portion of the fastener contacts the first member at a position overlapping a screwed portion, where the male threaded portion is screwed into the female threaded portion in an axial direction.  
           [0015]    According to this embodiment of the present invention, in the low-rigidity portion of the male threaded portion in which the hollow portion is formed, its rigidity is reduced as compared with a case where the male threaded portion is solid. Hence, when the axial load is applied to the bolt on the basis of the external force applied to the first member to be fastened, the low-rigidity portion is elastically deformed with relative ease, so a load applied to the screw thread of a screwed end portion from a screw thread of the male threaded portion can be relieved.  
           [0016]    In addition, since the hollow portion is substantially circular in cross section, the load applied to the screw thread of the screwed end portion, from the screw thread of the male threaded portion, is relieved uniformly in the peripheral direction with respect to the screwed end portion.  
           [0017]    As a result, according to the first aspect of the present invention, the following effect is produced. That is, since the load which is applied to the screw thread of the screwed end portion of the threaded hole, from the screw thread of the bolt to which the axial load based on the external force applied to the first member to be fastened is relieved, a concentrated stress, which would otherwise be caused at a root-of-threaded portion formed in the root of thread of the screwed end portion, is reduced.  
           [0018]    Further, since the concentrated stress is reduced uniformly in the peripheral direction of the screw thread of the screwed end portion, the strength of the first member to be fastened can be easily maintained.  
           [0019]    The present invention, according to a second aspect hereof, provides an internal combustion engine in which a crankshaft is rotatably supported by a first bearing portion provided on a crankcase, and a second bearing portion fastened to the first bearing portion. The engine includes a bolt having a male threaded portion screwed into a female threaded portion of a threaded hole formed in the first bearing portion, wherein the allowable stress of the first bearing portion is less than the allowable stress of the bolt.  
           [0020]    During operation of the engine, an axial load based on a combustion load applied to the first bearing portion is transferred to the bolt, wherein the male threaded portion includes a low-rigidity portion, in which a hollow portion concentric with the center axis of the male threaded portion and substantially circular in cross section is formed. The hollow portion of the bolt is formed at a position overlapping the screwed end portion of a screwed portion where the male threaded portion is screwed into the female threaded portion in an axial direction.  
           [0021]    According to this second aspect of the present invention, rigidity is reduced in the low-rigidity portion of the bolt, and this low-rigidity portion is screwed into the threaded hole of the first bearing portion to which combustion load is applied through a crankcase. Hence, when the axial load, based on the combustion load, is applied to the bolt for fastening the second bearing portion to which the combustion load is applied via the first bearing portion and the crankshaft, the same operation is effected between the low-rigidity portion and the screwed end portion as in the first aspect hereof.  
           [0022]    Further, since the hollow portion is formed in the bolt, for relieving load applied to the screwed end portion from the male threaded portion, the hollow portion of the threaded hole does not need to be made larger, or can be made as small as practically feasible.  
           [0023]    As a result, according to the embodiment of the present invention according to the second aspect hereof, the following effect is produced. That is, since the load applied to the screw thread of the screwed end portion from the screw thread of the bolt is relieved, which axial load is based on the combustion load applied to the first bearing portion, the concentrated stress, which would otherwise have been caused at the root-of-threaded portion in the screwed end portion, is reduced.  
           [0024]    Further, since the concentrated stress is reduced uniformly in the peripheral direction of the male threaded portion, the strength of the female threads formed in the first bearing portion is easily maintained. Still further, in order to reduce the concentrated stress, the hollow portion of the threaded hole formed in the first bearing portion does not need to be made larger or can be made as small as possible, so the first bearing portion is maintained or increased in strength.  
           [0025]    Hence, the desired strength of the first bearing portion can be secured without increasing the size and weight of the first bearing portion, and by extension, the internal combustion engine can be made efficiently.  
           [0026]    The embodiment of the present invention according to a third aspect hereof is an internal combustion engine of the type in which a cylinder block is fastened to a crankcase rotatably supporting a crankshaft, with a bolt having a male threaded portion screwed into a female threaded portion in a threaded hole formed in the crankcase, wherein the allowable stress of the crankcase is less than the allowable stress of the bolt.  
           [0027]    During operation of the engine, an axial load based on an combustion load applied to the crankcase is transferred to the bolt. The male threaded portion of the bolt includes a low-rigidity portion, in which a hollow portion, concentric with the center axis of the male threaded portion and substantially circular in a cross section is formed. The hollow portion of the bolt is formed at a position overlapping the screwed end portion of a screwed portion where the male threaded portion is screwed into the female threaded portion in an axial direction.  
           [0028]    According to this invention, rigidity is reduced in the low-rigidity portion of the bolt screwed into the threaded hole of the crankcase to which combustion load is applied, as is the case with the bolt according to the first aspect hereof. When the axial load, based on the combustion load, is applied to the bolt for fastening the crankcase and the cylinder block, the same operation is effected, between the low-rigidity portion and the screwed end portion, as in the first aspect hereof.  
           [0029]    Further, the hollow portion is formed in the bolt for relieving the load applied to the screwed end portion from the male threaded portion, so the hollow portion of the threaded hole does not need to be made larger, or can be made as small as possible.  
           [0030]    As a result, according to third aspect of the invention, the following effect is produced. That is, since the load applied to the screw thread of the screwed end portion from the screw thread of the bolt to which the axial load based on the combustion load applied to the crankcase is relieved, the concentrated stress caused at the root-of-threaded portion in the screwed end portion is reduced.  
           [0031]    Further, since the concentrated stress is reduced uniformly in the peripheral direction of the male threaded portion, the strength of the crankcase is easily maintained. Still further, in order to reduce the concentrated stress, the hollow portion of the threaded hole formed in the crankcase does not need to be made larger or can be made as small as possible, and the strength of the crankcase maintained or increased.  
           [0032]    Hence, the desired strength of the crankcase can be secured without increasing the size and weight of the crankcase, and by extension, the internal combustion engine can be made efficiently.  
           [0033]    For a more complete understanding of the present invention, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like numbers refer to like parts. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIG. 1 is a longitudinal cross-sectional view of an internal combustion engine, in which crankshaft journal bolts in accordance with a first embodiment of the present invention are used.  
         [0035]    [0035]FIG. 2 is a partial transverse cross-sectional view of the engine of FIG. 1, taken along the line II-II in FIG. 1.  
         [0036]    [0036]FIG. 3 is an enlarged cross-sectional detail view of a selected portion of the engine of FIG. 2.  
         [0037]    [0037]FIG. 4 is a cross-sectional view, corresponding to FIG. 2, of an engine incorporating a bolt and socket according to a second embodiment of the invention.  
         [0038]    [0038]FIG. 5 is a partial cross sectional view of an internal combustion engine according to a third embodiment of the invention, in which stud bolts according to the invention are used. 
     
    
     DETAILED DESCRIPTION  
       [0039]    Several embodiments of the present invention will be described herein, with reference to FIGS. 1 through 5. The selected embodiments and working examples are intended to illustrate, rather than limit the present invention.  
         [0040]    FIGS.  1  to  3  are illustrations related to a first embodiment of the present invention. Referring to FIG. 1 and FIG. 2, an internal combustion engine E 1  is shown, in which journal bolts  40  (FIG. 2) are used, in accordance with a first aspect of the present invention. The engine E 1  is a DOHC in-line 4-cylinder 4-stroke internal combustion engine, and is adapted to be mounted on a motorcycle frame, or in a similar application.  
         [0041]    The internal combustion engine E 1  includes an engine body assembled from a plurality of constituent members, including a cylinder block  1  in which four cylinders  2  are integrally formed. The engine E 1  also includes a bottom half crankcase  3   b  joined to a top half crankcase  3   a  with a large number of bolts. The top half crankcase  3   a  consists primarily of a skirt portion which is a bottom portion of the cylinder block  1 .  
         [0042]    The internal combustion engine E 1  also includes a cylinder head  4 , joined to the top end portion of the cylinder block  1 , and a valve cover  5  joined to the top end portion of the cylinder head  4 .  
         [0043]    A piston  6  is fitted inside each cylinder  2 , in such a way that it can reciprocate therein, and a combustion chamber  7  is formed between the cylinder head  4  and the top of the piston  6 .  
         [0044]    A crankshaft  10  is received in a crank chamber  8  formed by a crankcase  3 , constructed of the top half crankcase  3   a  and the bottom half crankcase  3   b . The crankshaft  10  is rotatably supported by the crankcase  3 . The respective pistons  6  are each connected to the crankshaft  10  via the respective connecting rods  9 , and the reciprocating motions of the pistons  6  are converted into the rotary motion of the crankshaft  10 .  
         [0045]    On the other hand, for each combustion chamber  7 , the cylinder head  4  is provided with a pair of intake valves  12  that selectively opens or closes, a pair of intake ports  11  opening to the combustion chamber  7 , respectively, and a pair of exhaust valves (not shown) that opens or closes a pair of exhaust ports (not shown) opening to the combustion chamber  7 , respectively.  
         [0046]    The respective intake valves  12  and the exhaust valves are opened or closed at predetermined times in synchronization with the rotation of the crankshaft  10  by a valve system V 1  having an intake camshaft  14  and an exhaust camshaft (not shown) rotated at a rotary speed of ½ times the rotary speed of the crankshaft  10 , in operative association with the crankshaft  10  via a transmission mechanism having a timing chain  13 . This valve system V 1  is arranged in a valve system chamber  15  formed of the cylinder head  4  and the valve cover  5 .  
         [0047]    In each cylinder  2 , an air-fuel mixture formed in an intake unit (not shown) is sucked into the combustion chamber  7  from the intake port  11  through the pair of opened intake valves  12  in an intake stroke, in which the piston  6  is moved down. The air-fuel mixture in the cylinder  2  is then compressed by the piston  6 , moved up in a compression stroke, and is then ignited and burned by a spark generated by an ignition plug.  
         [0048]    After ignition, in an expansion stroke, the piston  6  is forced down by the pressure of rapidly expanding combustion gas. The downward movement of the piston rotates the crankshaft  10  via the connecting rod  9 . The combustion gas is discharged as exhaust gas through the exhaust port from the combustion chamber  7  past the pair of opened exhaust valves in an exhaust stroke and further is discharged to the outside via an exhaust system (not shown).  
         [0049]    The crankshaft  10  has a rotary center line L 1  located on a substantially horizontal plane P (FIG. 2) including the mating faces  16  of the top half crankcase  3   a  and the bottom half crankcase  3   b . The crankshaft  10  is rotatably supported at a plurality of journal portions  10   c , in this embodiment, five journal portions  10   c  by a bearing unit constructed of bearing portions B provided at intervals in a direction A 1  of the rotary center line L 1 , and of the same number as journal portions  10   c.    
         [0050]    A driven gear  17 , which is connected to and driven by a starting motor via a one-way clutch, and the rotor of an AC generator  18  are mounted in this order from the crankcase side on one end  10   a  of the crankshaft  10  protruding outside the crank chamber  8  on the one side of the crankcase  3 . The driven gear  17  and the AC generator  18  are received in a receiving chamber  20  formed of the crankcase  3  and a cover  19  joined to the crankcase  3 .  
         [0051]    A driving sprocket  21 , over which the timing chain  13  is looped, is mounted on the other end  10   b  of the crankshaft  10  protruding outside the crank chamber  8  on the other side of the crankcase  3 . The driving sprocket  21  is received in a transmission chamber  23  formed of the crankcase  3  and a cover  22  joined to the crankcase  3 .  
         [0052]    Each bearing portion B, which rotatably supports the journal portion  10   c  via a bearing metal  24  of a main bearing arranged on the outer periphery of the journal portion  10   c , includes a top journal supporting wall  31 , as a first bearing portion which is provided on the top half crankcase  3   a , and a bottom journal supporting wall  32 , as a second bearing portion which is provided on the bottom half crankcase  3   b.    
         [0053]    The bottom journal supporting wall  32  is fastened to the top journal supporting wall  31  with a pair of journal bolts  40 , arranged at positions between which the journal portion  10   c  is sandwiched. The top journal supporting wall  31  is integrally molded with the top half crankcase  3   a , and the bottom journal supporting wall  32  is integrally molded with the bottom half crankcase  3   b . A reference numeral  25  designates a pin for aligning and positioning both of the journal supporting walls  31 ,  32  relative to one another.  
         [0054]    As shown in FIG. 2, in each bearing portion B, each journal bolt  40  includes a head portion  41  and a shaft portion  42 . The head portion  41  is provided for abutting against a fastening face formed on the bottom surface of the bottom journal supporting wall  32 . The shaft portion  42  is passed through a through hole  33 , formed in the bottom journal supporting wall  32 , and is screwed into a female threaded hole  34  formed in the top journal supporting wall  31 .  
         [0055]    In order to reduce the weight of the internal combustion engine E 1 , the cylinder block  1  and the bottom half crankcase  3   b  are molded of a molding material made of a light metal base alloy, for example, aluminum base alloy. The allowable stresses of the cylinder block  1  and the bottom half crankcase  3   b  are set at a value less than the allowable stress of the journal bolt  40  molded of a different molding material, for example, made of an iron base alloy.  
         [0056]    Referring to FIG. 3 in combination with FIG. 2, the threaded hole  34  has a center axis L 2  orthogonal to the mating face  16 , and has a closed end defined by an innermost surface  37 . The threaded hole  34  also has a female threaded portion  35  where a female thread is formed, and a non-threaded portion  36  between the female threaded portion  35  and the innermost surface  37  of the threaded hole  34 .  
         [0057]    The shaft portion  42  of the journal bolt  40  has a center axis L 3 , orthogonal to the mating face  16 , and coinciding with the axis L 2  of the threaded hole  34 . The shaft portion  42  of the journal bolt  40  also has a cylindrical portion  43  and a male threaded portion  44  that is closer to the tip end surface  42   a  of the shaft portion  42  than the cylindrical portion  43  and in which a male thread is formed.  
         [0058]    In a state where the top journal supporting wall  31 , as a first member to be fastened, is fastened in each bearing portion B to the bottom journal supporting wall  32  as a second member to be fastened with the pair of journal bolts  40 , the male threaded portion  44  of each journal bolt  40  is screwed into the female threaded portion  35  of the threaded hole  34 , this portion being referred to as a screwed portion R. In this embodiment, the screwed portion R in the female threaded portion  35  is a portion of the female threaded portion  35 .  
         [0059]    As a result, a portion of the female threaded portion  35 , closer to the innermost surface  37  than the tip end surface  42   a  of the male threaded portion  44 , becomes a non-screwed portion  35   f  where the male threaded portion  44  is not screwed into the female threaded portion  35 . A hollow portion  38 , consisting of a space that is not occupied by the journal bolt  40 , is formed in the threaded hole  34  by the non-screwed portion  35   f  and the non-threaded portion  36 .  
         [0060]    The male threaded portion  44  has a hole  45  formed therein, defining a hollow portion in the tip of the male threaded portion  44 . The hole  45  is formed at a position overlapping the screwed end portion Ra of the screwed portion R in the female threaded portion  35 .  
         [0061]    In other words, a portion which forms a thread groove  35   a  in which one pitch of screw thread  44   a  at the tip of the male threaded portion  44  in the screwed portion R is screwed, and includes screw threads  35   b ,  35   c  which are located on both sides across the thread groove  35   a  in a direction of the center axis L 2  (which is the same direction as an axial direction A 2  to be described later). A root-of-threaded portion  35   e  forms a root of thread  35   d  in the direction A 2  of the center axis L 3 , that is, in an axial direction A 2 .  
         [0062]    The hole  45  is a cylindrical hole having a substantially circular cross-sectional shape, and a closed end defined by the innermost surface  37 . The hole  45  is formed concentrically with the center axis L 3  of the male threaded portion  44 , by boring from the tip end surface  42   a , casting, or forging.  
         [0063]    The hole  45  has a depth extending in the axial direction A 2  over the screwed end portion Ra. Further, a portion of the hole  45 , overlapping the screwed end portion Ra in the axial direction A 2 , has a tapered inner peripheral surface expanding toward the tip end surface  42   a , and hence, forms a large diameter portion  45   b  having a larger diameter than a cylindrical portion  45   a , which has a cylindrical inner peripheral surface.  
         [0064]    Hence, the thickness (thickness in the radial direction) of the male threaded portion  44 , at the large diameter portion  45   b , becomes less than the thickness of the cylindrical portion  45   a . Here, the length of the cylindrical portion  45   a  and the thickness of a thin portion  44   b   1  of a portion of the male threaded portion  44 , where the large diameter portion  45   b  is formed, are set at suitable values from the viewpoint of the degree of reduction in concentrated stress and securing a fastening strength.  
         [0065]    In the male threaded portion  44 , a portion where this hole  45  is formed is reduced in rigidity as compared with a solid portion where the hole  45  is not formed, and hence, forms a low-rigidity portion  44   b . When an axial load in the axial direction A 2  is applied to the journal bolt  40 , the low-rigidity portion  44   b  is more easily elastically deformed, in the axial direction A 2 , than the solid portion.  
         [0066]    Next, the operation and effect of the first embodiment constructed in the above manner will be described.  
         [0067]    When the internal combustion engine E 1  is operated, a combustion load, caused by the pressure of the expanding gases resulting from combustion of the air-fuel mixture in the combustion chamber  7 , is applied upwardly in FIG. 2 to the cylinder block  1  joined to the cylinder head  4 .  
         [0068]    At the same time, the combustion load is applied downwardly against the crankshaft  10 , rotatably supported by the respective bearing portions B via the pistons  6  and the connecting rods  9 , and further downward in FIG. 2 to the bottom half crankcase  3   b.    
         [0069]    At the respective bearing portions B, by the combustion load, an upward load is applied to the top journal supporting wall  31 , integral with the cylinder block  1 . A downward load is applied to the bottom journal supporting wall  32 , integral with the bottom half crankcase  3   b.    
         [0070]    On the basis of the combustion load, which is an external force applied to the top journal supporting wall  31 , the reactive force of the combustion load is applied downwardly, as an axial load, to the pair of journal bolts  40  that fasten the top journal supporting wall  31  and the bottom journal supporting wall  32  together. At the same time, on the basis of the combustion load applied to the bottom journal-supporting wall  32 , an axial load in the same direction as the above reactive force is applied to the pair of journal bolts  40 .  
         [0071]    For this reason, as shown in FIG. 3, in the female threaded portion  35 , an upward load Fa is applied to the screw thread  35   b , closer to the non-screwed portion  35   f  of the screwed end portion Ra, and a downward load Fb is applied to the screw thread  35   c  opposite the screw thread  35   b  across the screwed end portion Ra from the screw thread  44   a  of the male threaded portion  44 . As a result of the above-described forces, it will be understood that a concentrated stress is caused at the root-of-threaded portion  35   e  of the screwed end portion Ra.  
         [0072]    At this time, in the male threaded portion  44 , the low-rigidity portion  44   b  having the hole  45  which is formed concentrically with the center axis L 3  of the male threaded portion  44 , and has a circular cross section, is formed at the position where the male threaded portion  44  overlaps the screwed end portion Ra in the axial direction A 2 , and the screw thread  44   a  is formed at the low-rigidity portion  44   b.    
         [0073]    Hence, in the low-rigidity portion  44   b , rigidity is reduced as compared with a case where the male threaded portion  44  is solid. For this reason, when an axial load, based on the combustion load, is applied to the journal bolt  40 , the thinner low-rigidity portion  44   b  is elastically deformed with relative ease, to relieve the load Fb applied to the screw thread  35   c  of the screwed end portion Ra from the screw thread  44   a  of the male threaded portion  44 , thereby reducing the concentrated stress caused at the root-of-threaded portion  35   e  of the screwed end portion Ra.  
         [0074]    Further, since the hole  45  is substantially circular in cross section, the load Fb applied to the screw thread  35   c  of the screwed end portion Ra from the screw thread  44   a  of the male threaded portion  44  is uniformly relieved in the peripheral direction with respect to the screwed end portion Ra, to reduce the concentrated stress uniformly in the peripheral direction of the screw thread  35   c  of the screwed end portion Ra. Hence, this helps to maintain and preserve the strength of the top journal supporting wall  31 .  
         [0075]    Still further, the hole  45  that is formed in the journal bolt  40 , provides a means for relieving the load Fb applied to the screwed end portion Ra from the male threaded portion  44 . Hence, the hollow portion  38  of the threaded hole  34 , which is formed in the top journal supporting wall  31 , having less allowable stress than the allowable stress of the journal bolt  40  so as to reduce the concentrated stress, does not need to be enlarged, and/or can be made as small as possible.  
         [0076]    For this reason, this can prevent a decrease in the strength of the top journal supporting wall  31  or enhance the strength and hence can secure the desired strength of the top journal supporting wall  31  without increasing the size and weight of the top journal supporting wall  31  and by extension the internal combustion engine E 1 .  
         [0077]    In addition, since the screw thread  44   a  is formed at the thin portion  44   b   1  of the low-rigidity portion  44   b , the screw thread  44   a  is more easily elastically deformed. Moreover, since the other portion of the low-rigidity portion  44   b  is thicker than the thin portion  44   b   1 , the other portion can relieve the load Fb applied to the screw thread  44   a  of the male threaded portion  44 , and at the same time, secure a desired fastening strength at the screwed portion other than the screwed end portion Ra.  
         [0078]    Next, a second embodiment of the present invention will be described with reference to FIG. 4. This second embodiment is different from the first embodiment of the internal combustion engine E 1 , having a cylinder center axis L 4  orthogonal to a substantially horizontal plane P including the mating face  16 , in that the second embodiment is an internal combustion engine E 2  having a cylinder axis L 4  inclined at an angle with respect to the horizontal plane P, and otherwise has the fundamentally same construction as in the first embodiment.  
         [0079]    Hence, the like or corresponding parts of the parts in the first embodiment are designated by the like reference symbols and their concrete descriptions will be omitted. Also this second embodiment can produce the same operation and effect as the first embodiment.  
         [0080]    Next, a third embodiment of the present invention will be described with reference to FIG. 5. This third embodiment is an internal combustion engine E 3  in which the same hole  45  as in the first embodiment is formed in a stud bolt for fastening a cylinder block  52  to a crankcase  51 . For this reason, the same reference symbols as in the first embodiment will be used. If necessary, FIG. 3 is also referred to in combination.  
         [0081]    The internal combustion engine E 3  in which stud bolts  40   1 ,  40   2  in accordance with the invention are used is an OHV type single cylinder 4-stroke internal combustion engine, provided to be mounted on a vehicle such as a motorcycle.  
         [0082]    The internal combustion engine E 3  is provided with an engine body constructed of constituent parts of the crankcase  51 , the cylinder block  52  and a cylinder head  53  that are mounted in sequence on the top end of the crankcase  51 , and a valve cover  54 , joined to the cylinder head  53  with bolts or other suitable fasteners.  
         [0083]    The crankcase  51 , the cylinder block  52 , and the cylinder head  53  are fastened into one piece with four stud bolts  40   1 ,  40   2  (two of them are shown in FIG. 5).  
         [0084]    As is the case with the first embodiment, a piston is fitted in the cylinder block  52  made of a single cylinder in such a way that it can reciprocate, and a combustion chamber is formed between the cylinder head  53  and the piston. A crankshaft  55  received in a crank chamber formed of the crankcase  51  is rotatably supported by the crankcase  51 . The piston is connected to the crankshaft  55  via a connecting rod, and the reciprocating motion of the piston is converted into the rotary motion of the crankshaft  55 .  
         [0085]    The cylinder head  53  is mounted with an intake valve and an exhaust valve which open or close an intake port and an exhaust port  53   a  communicating with the combustion chamber, respectively. These intake and exhaust valves are opened or closed at predetermined timings in synchronization with the rotation of the crankshaft  55  by a valve system having a rocker arm received in a valve system chamber  56  formed of the cylinder head  53  and the head cover  54 .  
         [0086]    The rocker arm is supported by a rocker shaft held by a holder  57  fixed to the cylinder head  53  in such a way as to swing and is swung by a push rod operated by a camshaft rotating in operative association with the crankshaft  55 .  
         [0087]    An air-fuel mixture formed in the intake unit is sucked into the combustion chamber from the intake port through the opened intake valve in an intake stroke in which the piston is moved down. The air-fuel mixture sucked into the cylinder block  52  is compressed by the piston moved up in a compression stroke and then is ignited and burned by an ignition plug.  
         [0088]    The piston is forced down by the pressure of combustion gas in an expansion stroke rotates the crankshaft  55  via the connecting rod. The combustion gas is discharged in an exhaust stroke as exhaust gas through the opened exhaust valve from the combustion chamber to the exhaust port  53   a  and then is discharged to the outside via an exhaust unit.  
         [0089]    The crankcase  51  that rotatably supports the crankshaft  55  via a pair of main bearings each formed of a ball bearing (one main bearing  58  is shown in FIG. 5) is a so-called left/right divided crankcase constructed of a pair of half crankcases which have a mating face including the crankshaft  55  and orthogonal to the rotary center line L 1  of the crankshaft  55  (one half crankcase  51  a is shown in FIG. 5).  
         [0090]    The crank chamber receives the crankshaft  55  and a transmission having a main shaft connected to and driven by the crankshaft  55  via a starting clutch and a transmission clutch.  
         [0091]    Four stud bolts  40   1 ,  40   2  include a pair of first stud bolts  40   1 , for fastening the cylinder block  52  and the cylinder head  53  together to the crankcase  51 , and a pair of second stud bolts  40   2  for fastening the cylinder block  52 , the cylinder head  53 , and the holder  57  together to the crankcase  51 .  
         [0092]    Each of the stud bolts  40   1 ,  40   2  has male threaded portions  44 ,  46  formed at the bottom end portion and the top end portion which are both end portions of its shaft portion  42  and the bottom end portion is screwed into the threaded hole  34  formed in the crankcase  51 .  
         [0093]    One of the first stud bolts  40   1  and one of the second stud bolts  40   2  are screwed into the threaded holes  34  formed in one half crankcase  51   a  and the other first stud bolt  40   1  and the other second stud bolt  40   2  are screwed into the threaded holes  34  formed in the other half crankcase  51   b.    
         [0094]    Each first stud bolt  40 , is passed through the through holes  60 ,  62  which are formed in the cylinder block  52  and the cylinder head  53 , respectively, and a nut  65  is screwed on its male threaded portion  46  protruding upward from the top end surface of the cylinder head  53 . Each second stud bolt  40   2  is passed through the through holes  61 ,  63   64  which are formed in the cylinder block  52 , the cylinder head  53 , and the holder  57 , respectively, and a nut  66  is screwed on its male threaded portion  46  protruding upward from the top end surface of the cylinder holder  57 .  
         [0095]    Here, in order to reduce the weight of the internal combustion engine E 3 , the cylinder block  52 , the crankcase  51 , and the cylinder head  53  are molded of a molding material made of a light metal base alloy, for example, an aluminum base alloy. The allowable stresses of the cylinder block  52 , the crankcase  51 , and the cylinder head  53  are set at less values than the allowable stresses of the respective stud bolts  40   1 ,  40   2  molded of a molding material, for example, made of an iron base alloy.  
         [0096]    The threaded hole  34  having the center axis L 2  orthogonal to the mating face  67  of the crankcase  51  and the cylinder block  52  and having a closed end has a female threaded portion  35  where a female thread is formed and a non-threaded portion  36  between the female threaded portion  35  and the innermost surface  37 . Each of the respective stud bolts  40   1 ,  40   2  has the center axis L 3  orthogonal to the mating face  67 .  
         [0097]    Referring to FIG. 3 in combination, in a state where the crankcase  51  as a first member to be fastened is fastened to the cylinder block  52  as a second member to be fastened with four stud bolts  40   1 ,  40   2 , the male threaded portions  44  of the respective stud bolts  40   1 ,  40   2  are screwed into the female threaded portions  35  of the threaded holes  34  in the screwed portions R.  
         [0098]    Here, the screwed portion R, the non-screwed portion  35   f  and the hollow portion  38  are the same as those in the first embodiment. Further, as is the case with the first embodiment, in the male threaded portion  44 , the hole  45  constructing a hollow portion in the male threaded portion is formed at a position overlapping the screwed end portion Ra of the female threaded portion  35  in the direction of the center axis L 3 , that is, in the axial direction A 2  to form the low-rigidity portion  44   b.    
         [0099]    According to this third embodiment of the present invention, as to the respective stud bolts  40   1 ,  40   2 , the same following operation and effect as the journal bolt  40  in the first embodiment can be produced.  
         [0100]    That is, when the internal combustion engine E 3  is operated, the combustion load produced by the combustion of the air-fuel mixture in the combustion chamber is applied upward in FIG. 5 to the cylinder head  53 . At the same time, the combustion load is applied downward in FIG. 5 to the crankshaft  55  via the piston and the connecting rod and further to the crankcase  51 .  
         [0101]    On the basis of the combustion load which is an external force applied to the crankcase  51 , its reactive force is applied as an axial load to the four stud bolts  40   1 ,  40   2  that fasten the cylinder block  52  and the crankcase  51  into one piece. At the same time, on the basis of the combustion load applied to the cylinder head  53 , an axial load in the same direction as the reactive force is applied to the respective first stud bolt  40   1  and on the basis of the combustion load applied to the holder  57  via the cylinder head  53 , an axial load in the same direction as the reactive force is applied to the respective second stud bolts  40   2 .  
         [0102]    At this time, as to the concentrated stress caused in the root-of-threaded portion  35   e  of the screwed end portion Ra, the same operation and effect as in the first embodiment are produced between the female threaded portion  35  of each of the threaded holes  34  formed in the crankcase  51  and the male threaded portion  44  of each of the stud bolts  40   1 ,  40   2 .  
         [0103]    Referring to reference symbols in the parentheses in FIG. 3 in combination, in the male threaded portion  44 , the low-rigidity portion  44   b  having the hole  45  which is formed concentrically with the center axis L 3  of the male threaded portion  44  and has a circular cross section is formed at the position where the male threaded portion  44  overlaps the screwed end portion Ra in the axial direction A 2  and the screw thread  44   a  is formed at the low-rigidity portion  44   b.    
         [0104]    Hence, in the low-rigidity portion  44   b , rigidity is reduced as compared with a case where the male threaded portion  44  is solid. For this reason, when an axial load based on the combustion load is applied to the stud bolts  40   1 ,  40   2 , the low-rigidity portion  44   b  is elastically deformed with relative ease to relieve the load applied to the screw thread  35   c  of the screwed end portion Ra from the screw thread  44   a  of the male threaded portion  44 , thereby reducing the concentrated stress caused at the root-of-threaded portion  35   e  of the screwed end portion Ra.  
         [0105]    Further, since the hole  45  is circular in cross section, the load applied to the screw thread  35   c  of the screwed end portion Ra from the screw thread  44   a  of the male threaded portion  44  is uniformly relieved in the peripheral direction with respect to the screwed end portion Ra to reduce the concentrated stress uniformly in the peripheral direction of the screw thread  35   c  of the screwed end portion Ra. Hence, this facilitates the strength designing of the crankcase  51 .  
         [0106]    Still further, the hole  45 , which is means for relieving the load applied to the screwed end portion Ra from the male threaded portion  44  is formed in each of the stud bolts  40   1 ,  40   2 . Hence, the hollow portion  38  of the threaded hole  34  which is formed in the crankcase  51  having less allowable stress than the allowable stresses of the stud bolts  40   1 ,  40   2  so as to reduce the concentrated stress does not need to be enlarged or can be made as small as possible.  
         [0107]    For this reason, this can maintain the strength of the crankcase  51  or enhance the strength of the crankcase  51  and hence can secure the desired strength of the crankcase  51  without increasing the size and weight of the crankcase  51  and by extension the internal combustion engine E 3 .  
         [0108]    A modified construction in an embodiment in which a portion of the embodiments described above is modified will be described.  
         [0109]    While the second bearing portion is constructed of the bottom journal supporting wall integrally formed with the bottom crankcase in the first embodiment, the second bearing potion may be bearing caps which are parts separate from each other.  
         [0110]    While the crankcase is divided into left and right parts in the third embodiment, the crankcase may be divided by a plane including the rotary center line of the crankshaft and orthogonal to the cylinder axis, in other words, may be divided into upper and lower parts.  
         [0111]    While the bolts in accordance with the invention fasten the bearing portions of the crankshaft of the internal combustion engine or the crankcase to the cylinder block, the bolts may fasten two arbitrary members to be constructed which construct the internal combustion engine and further may fasten two arbitrary members to be fastened of the internal combustion engine.  
         [0112]    Although the present invention has been described herein with respect to a limited number of presently preferred embodiments, the foregoing description is intended to be illustrative, and not restrictive. Those skilled in the art will realize that many modifications of the preferred embodiments could be made which would be operable. All such modifications, which are within the scope of the claims, are intended to be within the scope and spirit of the present invention.