Abstract:
A new type of plug incorporating a normally closed valve for draining oil from an oil pan of a car engine, wherein the normally closed valve is incorporated in a through hole provided as an oil draining path in a bolt type member fixed to the oil pan, an oil draining dedicated jig is assembled to the plug so as to open the normally closed valve in order to drain the oil contained in the oil pan by sucking it with a negative pressure, a movable valve disc member installed in the through hole is installed so that it is engaged with a valve seat provided near the outer end part of the through hole by a coiled spring so as to form the normally closed valve, and the tip end of the plug is made flush generally with the internal surface of the oil pan so as to prevent any structure from projecting toward the inside.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part application of patent application Ser. No. 09/647,806 filed on Oct. 19, 2000 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an oil drain plug for use with the oil pan of automobile engines. 
     Conventionally, automobile engines are provided with a mechanism for circulating lubricating oil (oil) through predetermined portions in order to allow the moving parts to move smoothly. The oil will be degraded over time with use to lose the lubricating performance desired. In addition, the oil will be contaminated by particles caused by wearing or sludge. Accordingly, the degraded oil is replaced at regular intervals (in general, at regular distances of travel about 3,000 to 5,000 km) from the viewpoint of preventing damage to the engine body and the like. 
       FIG. 5  is an explanatory view illustrating an example of an ordinary conventional engine having a configuration for replacing the degraded oil. Oil  100  in an oil pan  102  disposed under the crank chamber is supplied to predetermined portions by means of an oil pump  101 . The oil is pressurized and fed by the oil pump  101 . Then, the pressure of the oil is regulated by a regulating valve  103 . Subsequently, foreign matter such as metal particles, carbon impurities, and sludge, which contaminate the oil are removed to clean the oil via a filter  104 . Finally, the oil lubricates each of the portions of the engine and then returns to the oil pan  102  again. Incidentally, the oil pan  102  is generally filled with oil under normal conditions. 
     As described above, contaminants in the oil are filtered by the filter  104  in general in the course of circulation thereof, however, there is a limit to removing foreign matter by means of the filter. In addition, the degradation of the oil itself with time cannot be avoided. Accordingly, the oil is replaced in the following manner. A plug (drain cock)  106  is first removed from the oil drain hole (drain hole)  105 , which is provided in the bottom portion of the oil pan  102  and usually closed tightly, to drain the degraded oil by gravity. Then, the oil drain hole  105  is closed by the plug and new oil is poured from the upper portion of the oil pan  102 . 
     However, the aforementioned oil replacement is rather cumbersome in that the plug is first removed from the oil drain hole  105  and the plug is threaded into the hole again after the oil has been drained. In addition, it cannot be avoided for the operator to be smeared with the oil. 
     Accordingly, it is also practiced to drain the oil from the upper portion of the engine. For example, this includes a method for vacuuming the degraded oil with an elongated suction tube inserted in the attachment hole for an oil gauge, which is provided for checking the amount of the engine oil and contamination thereof. Also included is a method with the suction tube inserted in the oil filler port (these methods are hereinafter referred to as the “upper drainage methods”). 
     However, the upper drainage methods can provide no means for checking whether the distal end of the tube has reached the bottom portion of the oil pan since the tube is inserted from above the crankshaft or other complicated mechanisms that are accommodated in a narrow limited space. For this reason or another, it is difficult to completely drain the degraded oil. Thus, it may not be found that the degraded oil has not been replaced properly until the refilled oil is checked. In an extreme case, the refilled oil may be replaced again. In addition, the upper drainage methods have not been employed widely because the methods cannot provide means for readily eliminating metal particles and the like accumulated on the bottom portion of the oil pan. 
     On the other hand, the method for draining the degraded oil from the bottom of the pan allows complete drainage of oil. By making use of the advantage thereof, improved methods have been suggested to solve the problem of smearing the operator with oil. For example, these methods include an oil replacement method with a permanent fixed plug having a normally closed poppet valve built therein (U.S. Pat. No. 4,745,894 and the corresponding Japanese Patent Publication No. 04-48987). Also included is an oil replacement method with a permanent fixed plug having a normally closed ball valve built therein (Japanese Patent Publication No. 08-170782). 
     These methods remove the existing oil drain plug  106  of a simple configuration shown in FIG.  5  and substitute for the closing plug  106  a plug (not shown) which has a normally closed valve built therein and is fixed to the oil drain hole  105 . At the time of oil replacement, a jig dedicated to opening the built-in valve is attached to the plug to suck the oil  100  in the oil pan  102  under a negative pressure. Then, after the oil has been drained, the jig is removed from the plug and the built-in valve is returned to the closed position to allow new oil to be filled in the oil pan. 
     This method allows the oil  100  to be forcedly sucked and drained by a suction device, thereby providing a quick draining. Moreover, apart from the first plug replacement, the plug needs not to be removed nor threaded into the oil drain hole again. Furthermore, the method provides an advantage that the operator can drain the oil without being smeared therewith and the surrounding area can be prevented from being covered therewith. 
     However, when the inventor actually tried draining oil using the plug having the normally closed valve built therein according to the aforementioned prior-art method, it was found that the plug could be incorporated into only limited types of automobiles due to some problems that some automobiles were not adapted to have the plug attached thereto. 
     Accordingly, a further detailed study was made and it was found that these problems were originated from slight displacements in various mechanisms in the oil pan, which was designed in accordance with specifications different depending on the type of automobiles and which did not always require to be assembled with high accuracy. 
     That is, the plug having the normally closed valve built therein, disclosed in the aforementioned Japanese Patent Publication No. 08-170782, is adapted to allow oil to flow in from the periphery of the cylinder portion that accommodates the ball valve constituting the normally closed valve. Thus, the ball valve and the portion for accommodating a spring for biasing the ball valve to the normally closed position are attached to the oil pan so as to protrude into the oil pan for a certain length. In this case, presence of a structure on the axial line of the oil drain hole in the oil pan would cause the plug to be insufficiently threaded into the hole, thus resulting in defective attachment. Such a structure includes a baffle plate for preventing the oil from waving during travel, a separator, and a strainer for drawing up oil. In some cases, the plug cannot be attached to the automobile at all due to the design specification thereof. In addition, although the design specification allows the attachment of the plug, a displacement of a few millimeters occurring during the manufacture of the oil pan may make it impossible to attach the plug thereto. 
     In the case of the plug having the normally closed valve built therein, disclosed in U.S. Pat. No. 4,745,894, the distal end of the plug attached to the oil pan in the normally closed position is placed generally on the same plane as the bottom surface of the oil pan. Thus, this does not raise such a problem that the plug cannot be attached to the oil pan even in the presence of a structure on the axial line of the oil drain hole as described above. However, the plug according to this method is adapted to move the valve body disposed at an edge portion of the plug inside the oil pan into the oil pan to disengage the valve body from the valve seat, thus opening the valve to drain the oil. Thus, the presence of the structure mentioned above would lead to a drawback that the plug can be attached to the oil drain hole but the valve cannot be opened. Accordingly, it cannot be found that the plug cannot be opened until the oil is drained next time. This presents more detrimental effects in practice. 
     The present invention was developed to improve the plug having a normally closed valve built therein, which requires no removal of the plug and prevents the operator from being smeared with the oil and provides a quick draining, as described above. An object of the present invention is to provide the improved plug having a normally closed valve built therein for draining engine oil, the plug being made available to any automobiles without being affected by a slight displacement caused by the type of automobiles or the assembly of mechanisms in the oil pan. 
     Another object of the present invention is to provide an oil drain plug which protrudes less downwardly from the bottom of the oil pan to reduce the possibility of damaging the plug due to rubbing between the bottom of the automobile and the ground. 
     Still another object of the present invention is to provide an oil drain plug of a simple configuration with good mass-productivity to facilitate assembly, thereby providing an inexpensive plug. 
     SUMMARY OF THE INVENTION 
     The aforementioned objects are achieved by the oil drain plug according to the present invention. 
     The oil drain plug according to the present invention achieves the aforementioned objects. The engine oil drain plug according to the first aspect comprises a cylindrical bolt-type member having, on one end side thereof, a screw axis portion with a male screw formed thereon to be fixedly threaded into an oil drain hole provided on an oil pan of an internal combustion engine; a hexagonal bolt head portion, on the other end side, located outside the oil pan when the plug is fixedly threaded into the oil drain hole; an axial oil drain passage penetrating from the screw axis portion on the one end side to drain oil; and an annular valve seat provided adjacent said bolt head portion in the oil drain passage. The plug also includes a movable valve body member having a valve body portion, disposed in the oil drain passage of said cylindrical bolt-type member, for engaging said valve seat to close the oil drain passage, and being disposed so as to be movable in the oil drain passage to open the oil drain passage by displacing the valve body portion from the valve seat toward the inside of the oil pan. The plug also includes a spring member, extendedly interposed between a secured portion adjacent said one end side of the valve body and the movable valve body member, for biasing the movable valve body member to a normally closed position to allow the valve body portion thereof to be engaged with the valve seat. The oil drain plug is arranged to displace said movable valve body member against the bias of said spring member for biasing the same to the normally closed position to allow the valve body portion to be separated from the valve seat, thereby causing said oil drain passage to be opened to drain oil, wherein a distal end of said screw axis portion is provided generally on the same plane as an inner wall surface of the oil pan when said plug is fixedly threaded into the oil drain hole. 
     In the aforementioned configuration, the surface for engaging the valve body portion of the movable valve body member with the valve seat is preferably provided with a configuration for ensuring fluid-tight sealing to strictly prevent oil leakage. For example, a seal ring may be interposed which provides durability such as oil resistance and weather resistance. In general, the seal ring is incorporated into the valve body portion of the movable valve body member. 
     It is preferable to provide a displacement guide for the movable valve body member so as to stabilize the displacement of the movable valve body member and ensure a tight press fitting, to ensure against leakage of oil from said valve. For example, it is recommended to employ a configuration in which the movable valve body member is provided with a cylindrical portion integrated with the valve body portion and a guide portion acts as the enclosure for the cylindrical portion to displace in the axial direction on the inner wall of the cylindrical bolt-type member. 
     Incidentally, the statement that the distal end of said screw axis portion is provided generally on the same plane as the inner wall surface of the oil pan does not mean that the distal end is exactly located on the same plane in the foregoing configuration. This is only intended to avoid an aforementioned prior-art problem that structures in the oil pan interfere the attachment of the plug or the opening of the valve. Thus, it is not excluded that the distal end is adapted to slightly protrude inwardly from the inner wall surface of the oil pan without causing the foregoing problem to occur. 
     According to the present invention, the plug having a normally closed valve built therein according to the present invention can be attached to any type of automobiles with no problem in the same way as a simple plug which, without causing any particular problem to occur, is removed from the oil pan for replacing the oil and is attached thereto again after the oil has been drained. Moreover, the normally closed valve is opened by the displacement of the movable valve body member in the plug, whereby the valve can be opened without being affected by any structure present in the oil pan. Thus, the problem that a prior-art plug having a normally closed valve built therein was only available to a particular type of automobiles can be eliminated and the object of the present invention to allow a plug to be suitably applied to any type of automobiles can be achieved. 
     In the present invention according to the second aspect, in the previously mentioned aspects, the head portion of the cylindrical bolt-type member comprises a jig attachment holding portion for displacing the movable valve body member against the bias applied thereto in order to maintain the oil drain passage to an open position. 
     According to the invention, the open passage of the built-in normally closed valve can be maintained by the attachment of said jig, thus taking the load off the operator and particularly facilitating forced draining of oil by a negative-pressure suction device. 
     In the present invention according to the third aspect, as mentioned in each of the prior aspects, the annular valve seat provided adjacent the head portion in the oil drain passage of the cylindrical bolt-type member is formed by the calk-like sealing a valve seat member receives separate from said cylindrical bolt-type member to an end portion of said cylindrical bolt-type member. 
     The invention provides an advantage that the valve seat, which is critical to ensure prevention of oil leakage when the built-in normally closed valve is closed, can be formed from a member that is designed independently of and separately from the bolt-type member in material, shape, etc. 
     In the invention according to the first aspect, in each of the aforementioned aspects, the cylindrical bolt-type member is adapted to allow a spring member accommodating portion for normally biasing the movable valve body member to the closed position, a portion for accommodating the movable valve body member, and a portion for fixing the valve seat forming member to gradually increase in diameter, from one end portion toward the other end portion inside the oil pan. This gradual increase in diameter is necessary in the inner wall of the valve body member, for the reason that the wall thickness will become too thin at point X, thereby causing when the invention is installed into the oil pan and torque is applied the possibility of cracking. On the other hand, if the inner wall of the valve body member does not employ a gradual opening and the length of the bold-shape component protruding from the oil pan becomes long to compensate for the thin wall the possibility of rubbing the ground when the car is in motion. According to this invention, the spring for biasing the built-in valve to the normally closed position, the movable valve body member, and the valve seat forming member are disposed in the oil drain passage of the cylindrical bolt-type member in that order. The plug according to the present invention can be characterized by the press fitting of the end portion on the other end of the cylindrical bolt-type member so as to fix the valve seat forming member. This facilitates the manufacturing of the oil drain plug, thus improving productivity and providing the plug at lower costs than were possible with prior inventions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view illustrating an oil drain plug according to an embodiment of the present invention attached to an oil drain hole of an oil pan, in which an oil drain dedicated jig is attached thereto for draining oil; 
       FIGS.  2 ( a )- 2 ( c ) are explanatory views illustrating the oil drain plug according to the aforementioned embodiment, wherein FIG.  2 ( a ) is a longitudinal sectional view thereof, FIG.  2 ( b ) is a perspective view showing the movable valve body member of the plug, and FIG.  2 ( c ) is a perspective view showing the valve seat forming member; 
         FIG. 3  is a longitudinal sectional view illustrating the dedicated jig for opening the built-in normally closed valve of the oil drain plug according to the aforementioned embodiment; 
         FIG. 4  is a longitudinal sectional view illustrating a cap employed for protecting the oil drain plug according to the aforementioned embodiment under usual service conditions other than at the oil replacement. 
         FIG. 5  is a view illustrating a prior art method for draining oil from the oil pan of an engine to which the plug according to the present invention is applied. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be explained below in more detail with reference to the drawings. 
     The embodiment is illustrated in  FIGS. 1  to  4 .  FIG. 1  illustrates an oil drain plug according to the present embodiment attached to an oil pan, in which an oil drain dedicated jig is attached thereto for draining oil. FIGS.  2 ( a )- 2 ( c ) show explanatory views illustrating the oil drain plug according to the embodiment.  FIG. 3  illustrates the dedicated jig for opening the normally closed valve of the oil drain plug according to the embodiment.  FIG. 4  illustrates a cap employed for protecting the oil drain plug under usual service conditions other than the oil replacement. 
     Referring to  FIG. 1 , an oil pan is represented at  102  and an oil drain hole (female screw hole) is represented at  105  which is provided in the bottom portion of the oil pan  102 . These are adapted in the same way as those of FIG.  5 . The oil drain hole  105  is usually closed tightly by means of a simple closing plug (refer to reference numeral  106  of  FIG. 5 ) which has no normally closed valve built therein. However, to use an oil drain plug  1  according to this embodiment, the aforementioned closing plug  106  is removed and then the oil drain plug  1  is threaded into the oil drain hole as later described. Reference numeral  2  designates the oil filled in the oil pan  102 . 
     The oil drain plug  1  according to this embodiment is detailed in  FIGS. 1 and 2 . The oil drain plug  1  according to this embodiment includes a cylindrical bolt-type member  11  having a screw axis portion  111  on one end thereof, on which a male screw  1111  is formed; a head portion  112  communicating with the screw axis portion  111  and provided with a larger diameter on the other end; and a through-hole  113  stepped from the screw axis portion  111  to the head portion  112  (the step being larger in diameter at the bolt head portion). The oil drain plug  1  also includes a valve seat forming member  12  secured adjacent an opening on the head portion  112  side in the through-hole  113 , a movable valve body member  13  disposed in the through-hole  113  so formed as to constitute an open/close valve in cooperation with a valve seat  121  formed on the valve seat forming member  12 , and a coil spring  14  for biasing the movable valve body member  13  toward the valve seat  121 . The through-hole  113  serves as an oil drain passage. 
     Now, each of the aforementioned members constituting the oil drain plug will be explained. As shown in FIG.  2 ( a ), the cylindrical bolt-type member  11  has the screw axis portion  111  and the head portion  112 , as mentioned above. The through-hole  113  that penetrates the cylindrical bolt-type member  11  in the axial direction is provided with the following configuration from one end portion on the screw axis portion  111  side to the other end portion on the head portion  112  side. That is, there are formed in sequence the following components: an abutment seat  1131  provided as a short flange inwardly oriented adjacent the screw axis portion  111  side at the end portion of the coil spring  14 ; a cylindrical inner wall surface  1132 , subsequent to the abutment seat  1131 , extending generally up to an intermediate portion of the cylindrical bolt-type member  11  in the axial direction thereof, serving to accommodate the coil spring  14 , and slidably guiding the movable valve body member  13  in the axial direction thereof; a large diameter cylindrical surface  1134 , subsequent to the cylindrical inner wall surface  1132  and extending toward the aforementioned head portion  112  via a tapered portion  1133 ; a tapered portion  1136 , subsequent to the large-diameter cylindrical surface  1134 , for affixing with a press fitting the valve seat forming member  12  provided adjacent the head portion  112  via a seal ring incorporating stepped portion  1135 ; and a projection  1137  for affixing with a press fitting the valve seat forming member formed on the head portion  112 . 
     In addition, the axial dimension of the screw axis portion  111  is determined such that the distal end of the screw axis portion  111  lies generally in the same plane as the bottom surface of the oil pan  102  when the screw axis portion  111  is threaded fixedly into the oil drain hole (female screw hole)  105  of the oil pan  102  with a packing  3  disposed on the shoulder portion  1121  of the head portion  112 . The head portion  112  of the plug  1  is provided, on the outer circumference thereof, with a circumferential groove  1122  which is used for fixedly holding an oil drain dedicated jig when attached thereto as later described and for fixedly holding a protective cap  4  under normal service conditions as later described as well. The head portion  112  is also provided with a guide surface  1123  having a circular cross section for incorporating the dedicated jig and the protective cap  4  to the circumferential groove  1122  and a hexagonal bolt head portion  1124 , subsequent to the screw axis portion  111  and the shoulder portion  1121  of the head portion, for threading the plug into the oil drain hole. 
     Now, the valve seat forming member  12  in the above configuration is explained, which is detailed in FIGS.  2 ( a ) and  2 ( c ). That is, the valve seat-forming member  12  has a cylindrical (a ring-shaped) configuration, which is relatively short in the axial direction. As for the outer circumferential shape thereof, the member  12  is provided with a tapered outer circumferential surface  122  located adjacent the end portion on the head portion  112  side and coincident with the tapered portion  1136  for the press fitting of the cylindrical bolt-type member  11 . The member  12  is also provided, subsequent to the foregoing, with a seal ring incorporating stepped portion  123  that pairs up with the seal ring incorporating stepped portion  1135  and an outer circumference surface  124  extending axially for the fitting of the large diameter cylindrical surface  1134 . As for the inner circumferential shape, a cylindrical inner circumference surface  125  is positioned at the end portion side of the head portion  112  to define a passage for draining the oil. Subsequently, a tapered surface is provided which gradually enlarges the diameter of the passage so as to lead to the outer circumference surface  124 . Thus, the tapered surface that gradually enlarges the diameter of the passage forms the valve seat  121 . This gradual increase in diameter is necessary in the inner wall of the valve body member, for the reason that the wall thickness will become too thin at point X, thereby causing when the invention is installed into the oil pan and torque is applied the possibility of cracking. On the other hand, if the inner wall of the valve body member does not employ a gradual opening and the length of the bolt-shape component protruding from the oil pan becomes long to compensate for the thin wall the possibility of rubbing the ground when the car is in motion. Then, the valve seat forming member  12  is incorporated from the head portion  112  side of the bolt-type member  11  to allow the tapered outer circumferential surface  122  thereof to engage the fixedly calking tapered portion  1136 . Thereafter, the projection  1137  for fixedly calking the valve seat forming member is calked to allow the valve seat forming member  12  to be fixed to the bolt-type member  11 . 
     Now, the movable valve body member  13  will be explained in the aforementioned configuration. The member  13  is illustrated in FIGS.  2 ( a ) and ( b ). That is, the movable valve body member  13  according to this embodiment is adapted to have a cylinder portion  131 , which serves as an oil drain passage formed as the end portion on the screw axis portion  111  side, and valve body portion  132  formed as the end portion on the head portion  112  side. The cylinder portion  131  is provided with three oil passage holes  1311 , communicating between the interior and exterior thereof, equally spaced apart around the circumference thereof, and each provided with a diameter suitable for draining oil. 
     In addition, the cylinder portion  131  is adapted to have an outer diameter so as to be slidably guided and displaced in the axial direction on the cylindrical inner wall surface  1132  of the cylindrical bolt-type member  11 . 
     In addition, the valve body portion  132  is formed in the shape of an inverted trapezoid as shown in FIG.  2 ( a ) in this embodiment, and is provided with a circumferential groove  1321  on the tapered outer circumference surface thereof. The circumferential groove  1321  is provided with a seal ring  6  which effects oil-tight sealing when seated on the valve seat  121 . 
     Now, the method for assembling the oil drain plug  1  according to this embodiment configured as described above and the method for fixedly attaching the plug to the oil pan  102  are explained. 
     First, the coil spring  14  is fit into the cylindrical bolt-type member  11 , to which the projection  1137  is not affixed with a press fitting, from the large opening on the head side along the cylindrical inner wall surface  1132  of the through-hole  113  such that the distal end of the coil spring  14  engages the abutment seat  1131 . Then the movable valve body member  13 , into which the seal ring  6  has been incorporated, is fit into the cylindrical bolt-type member  11  such that the distal end of the cylinder portion  131  engages the end portion of the coil spring  14 . Thus the movable valve body member  13  is held with the coil spring  14  being compressed. Subsequently, a seal ring  5  is incorporated into the stepped portion  1135 , which is to accommodate the seal ring  5 , and thereafter the valve seat forming member  12  is fit into the through-hole  113  such that the tapered outer circumferential surface  122  engages the tapered portion  1136  of the through-hole  113 . Then, the projection  1137  is affixed with a press fitting to the valve seat forming member  12  and the movable valve body member  13  is released. Thus the assembling procedure is completed. 
     As described above, the oil drain plug  1  according to this embodiment can be assembled extremely easily and provides good mass-productivity. Furthermore, the plug  1  requires less number of parts to be used and can be made available in the market at low cost. In addition, the valve seat forming member  12  can be centered by the engagement of the tapered portion thereof with the hexagonal cylindrical bolt-type member  11 , thereby providing highly accurate positioning. This allows the seal ring  5  to prevent oil leakage and ensures the engagement of the seal ring  6  of the movable valve body member  13  with the valve seat  121  to prevent oil leakage, thus providing a dependable plug without oil leakage. 
     The plug  1  according to this embodiment assembled as described above can be fixedly incorporated into the oil pan  102  by removing the existing simple plug  106  (refer to  FIG. 6 ) attached to the oil drain hole  105  of the oil pan  102  and thereafter threading the screw axis portion  111  into the oil drain hole  105  via the packing  3 . At this time, the hexagonal bolt head portion  1124  provided on the head portion of  112  of the cylindrical bolt-type member  11  is used for the threading with a threading jig, such as a spanner. 
     For the plug  1  fixedly assembled as described above, the protective cap  4  shown in  FIG. 4  is fitted to the end portion of the head portion  112  of the bolt-type member  11  under normal service conditions (other than at the time of draining oil). The protective cap  4  employed in this embodiment is made of rubber and is adapted to have an radial projection  43 , provided on part of a barrel portion inner wall  42  of a main body  41  having a circular cross section and fitting the guide surface  1123  having a circular cross section of the valve seat forming member  12 , for fitting the circumferential groove  1122  of the bolt-type member  12 . However, the material and configuration of the cap are not limited to this embodiment so long as the cap can be securely attached to the end portion of the bolt-type member  12 . 
     Now, described is a preferable method for draining oil from the oil pan  102  into which the oil drain plug  1  according to this embodiment is fixedly incorporated as described above. 
     The preferable method is realized using an oil drain dedicated jig  7  shown in FIG.  3  and the method for draining oil is shown in FIG.  1 . 
     The oil drain dedicated jig  7  employed in this embodiment can be realized by a coupling that allows a fluid to start and stop flowing by attaching and detaching a general socket and plug. That is, the jig can be adapted to house a normally closed valve in one of the socket or the plug to allow a fluid to stop flowing by closing the normally closed valve when the socket and the plug are disengaged from each other, whereas allowing the fluid to start flowing by opening the normally closed valve when the socket and the plug are fittingly attached to each other. 
     That is, the oil drain dedicated jig  7  that is used as a socket to constitute a coupling by pairing with the plug  1  according to this embodiment includes a plug fitting portion  71 , large in diameter, located on one side, and having an inner wall surface  711  of a circular cross section so as to fit to the guide surface  1123  of the head portion  112  of the plug  1 . The jig  7  also comprises a coupler connecting portion  72 , located on the other side, for allowing a hose  8  for sucking oil to be connected thereto. Thus, an axial through-hole leading from the plug fitting portion  71  to the coupler connecting portion  72  is formed as an oil drain passage  73 . 
     In addition, the aforementioned plug fitting portion  71  includes an attachment and detachment switching mechanism. The mechanism is provided with ball accommodating holes  712  the accommodating diameter of which gradually increases outwardly in the radial direction so that balls are prevented from projecting more than a certain distance inwardly in the radial direction and the balls are allowed to displace outwardly. The mechanism allows the ball accommodating holes  712  and a plurality of balls  713  accommodated in the ball accommodating hole  712  to be equally spaced along the circumference thereof. The mechanism is further provided with an annular cylindrical ball displacement restricting slider  714  for restricting the outward radial displacement of the balls to two stages. The ball displacement restricting slider  714  is pressed against one end portion with a coil spring  715  provided on the outer circumference of the plug fitting portion  71  and is retained by means of an engagement ring  716 . Under this condition, the slider  714  causes the balls  713  to be restrained at a position on an inner circumference surface  7141  of the slider  714 , where the balls  713  are projected inwardly in the radial direction. When the slider  714  is displaced toward the other end portion against the force of the coil spring  715 , a tapered surface  7142  at an end of the slider  714  is adapted to displace the balls  713  outwardly in the radial direction. Incidentally, a tip projection  7143  of the slider  714  serves to retain the balls  713 . 
     The inner cylindrical bottom portion of the plug fitting portion  71  is provided with a push rod  717  for displacing the movable valve body member  13  to open the normally closed valve housed in the plug  1  when the dedicated jig  7  is attached to the plug  1 . The push rod  717  is adapted to extend a given length toward an end portion from the central portion of a bottom portion  718  of the plug fitting portion  71  where a plurality of axial oil drain passages is bored. The extended length is determined so as to displace the movable valve body member  13  for an appropriate distance when the dedicated jig  7  is attached to the plug  1 . Reference numeral  719  designates a seal member for preventing oil leakage when the dedicated jig  7  is attached to the plug  1 . The seal member  719  is incorporated into the inner cylindrical bottom portion of the plug fitting portion  71  to be brought into elastic contact with an edge surface of the valve seat forming member  12  of the plug when the dedicated jig  7  is attached to the plug  1 . 
     Incidentally, one end of the hose is fixed to a coupler  9  which is fitted to the coupler connecting portion  72 , while the other end of the hose  8  is connected to a negative-pressure suction device not shown. 
       FIG. 1  shows the oil drain dedicated jig  7  configured as described above, attached to the plug  1 , wherein the protective cap  4  of the plug  1  according to this embodiment fixedly attached to the oil pan  102  has been removed. Fitting the dedicated jig  7  to the plug causes the push rod  717  to press and displace the movable valve body member  13  of the plug  1 , the valve body  132  to displace apart from the valve seat  121  to open the normally closed valve, and the seal member  719  to engage the edge surface of the valve seat forming member  12  to effect liquid-tight sealing. Accordingly, a negative pressure provided by the negative-pressure suction device (not shown) through the hose  8  will allow the oil in the oil pan  102  to be forcedly sucked and thus drained. 
     Furthermore, since there is no component projecting from one end of the plug  1  into the oil pan  102 , the normally closed valve can be opened and closed smoothly without being restrained by the mechanism or structure of the oil pan at this time. 
     As described above, the present invention provides a plug having a normally closed valve built therein which needs not to be removed, which allows the operator to be hardly smeared with oil, and which provides a quick draining. It is also made possible for the first time that the plug having a normally closed valve built therein and with the same specification can be universally applied to any automobile without being affected by the type of automobile and a slight offset in attachment position of mechanisms in the oil pan. 
     Furthermore, the oil drain plug according to the present invention makes it possible to use a space for opening the normally closed valve and for ensuring therein the displacement of the movable valve body member or the screw axis portion as a portion for accommodating the spring biasing the valve body. Thus, this makes it possible to provide a plug shorter in length than a prior-art one described in the foregoing. This in turn allows the plug to protrude less downwardly from the bottom of the oil pan, providing reduced possibility of causing the plug to be damaged due to the rubbing between the bottom of the automobile and the ground. 
     Still furthermore, the oil drain plug according to the present invention provides a simplified configuration and facilitates assembly thereof to result in an improved mass-productivity, thus providing an oil drain plug at low cost. 
     Still furthermore, where the oil drain plug according to the present invention is attached to the bottom surface of the oil pan, the edge portion of the plug inside the oil pan can be placed on the same plane as the bottom surface of the oil pan, whereby the oil can be completely or substantially completely drained.