Patent Publication Number: US-7721431-B2

Title: Method of making a piston

Description:
RELATED APPLICATIONS 
   This patent application is a divisional of U.S. patent application Ser. No. 10/343,499, filed Mar. 15, 2004, currently allowed, which claims priority to PCT/GB01/03361 filed Jul. 26, 2001 which claims priority to GB0018840.9 filed Aug. 2, 2000, all of which are incorporated herein by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   NONE 
   BACKGROUND OF THE INVENTION 
   This invention relates to pistons for internal combustion engines and to the manufacture thereof. The invention is particularly concerned with the manufacture of a strong piston that is also light in weight and suitable for cost-effective mass production for small capacity, high compression engines. 
   Pistons for internal combustion engines for mass market automobiles are manufactured in large numbers and subjected to cost constraints, which in turn place limits on manufacturing processes. Such pistons are usually, but not necessarily, cast from a light metal alloy, typically aluminium based, and then subjected to a series of machining steps that culminate in a precision component. 
   For heavy duty use, for example in compression ignition engines, it is known to manufacture pistons of steel, usually forged, but such pistons have tended to have a weight penalty, notwithstanding extensive machining operations to remove extraneous metal, and have thus far been restricted to large capacity, low-revving engines found in trucks and the like. 
   In recent times there has been a need to provide such compression ignition engines for use in smaller, automobile engines, where it is necessary to run at higher speeds and with such high compression pressures. Although steel is a material having suitable properties, and has such strength that it could be used in relatively thin sections that mitigate most if not all of the weight penalty, there is difficulty in manufacturing a small one-piece piston that is capable of fulfilling such potential. In general such a piston has to be manufactured in steel by forging, with attendant limits to wall thicknesses and shapes that limit weight reduction. 
   It has been proposed to assemble or construct a steel piston from separately manufactured parts, as in U.S. Pat. No. 1,667,202 and U.S. Pat. No. 2,244,008. However the crown forms of the pistons shown therein are relatively simple in structure and without an in-crown combustion bowl often required by modern engines. Even without such added complexity, it is believed that the number of separate parts and assembly operations required are not conducive to providing a small piston capable of operating within a modern small engine in a cost-effective manner. 
   Notwithstanding that a small piston for mass production is subjected principally to constraints of cost, a larger piston for heavy duty application is subjected principally to constraints resulting from weight, so that the ability to produce a light weight piston cost effectively is not restricted in applicability. With this in mind, it is an object of the present invention to provide an engine piston of assembled form that is capable of providing strength and light weight in simple form and a method of producing such a piston that is capable of implementation more cost-effectively than hitherto. 
   According to a first aspect of the present invention an engine piston comprises an outer shell, including a crown centred on a longitudinal piston axis and a tubular side wall extending axially with respect to the periphery of the crown to an open end and, within the tubular side wall of the shell, a mounting member arranged to extend transversely to the longitudinal axis and bonded both to the crown and to the side wall spaced from the crown, and gudgeon pin boss means carried by the mounting member. 
   The term “longitudinal axis” is employed in relation to defining the piston with respect to the geometric centre of the crown, and notwithstanding that the cross section of the piston is other than circular, for example, is to a small extent elliptical or oval. 
   Preferably, the tubular side wall includes, adjacent the crown, a region of axially spaced, circumferentially extending ring grooves and the mounting member is bonded to the side wall at the end of the ring groove region remote from the crown and the periphery of the mounting member is bonded to the peripheral side wall substantially at the same axial position as at least one ring groove. 
   More preferably, the peripheral region the crown, the tubular side wall and the bonded mounting member define therebetween an annular cooling chamber. 
   Preferably, the gudgeon pin boss means is provided integrally with the mounting member, but notwithstanding this the bonding of the mounting member to the crown and to the side wall displaced from the crown create a monocoque type of structure which includes the ring groove region and provides great pressure resistance therefor without need for substantial wall thickness. 
   According to a second aspect of the present invention a method of manufacturing an engine piston comprises forming an outer shell part comprising a crown, centred on a longitudinal axis, and a tubular side wall, extending axially with respect to the periphery of the crown to an open end, forming a mounting member, carrying gudgeon pin boss means thereon, with a periphery dimensioned to fit within and interface with the tubular side wall, disposing the mounting member within the tubular side wall such that it interfaces with the crown at a crown interface and interfaces with the side wall at a wall interface and bonding the mounting member to the shell at said crown and wall interfaces. 
   Preferably, the method comprises forming the tubular side wall and at least the peripheral part of the crown, bounding a central crown region, as a integral shell body. 
   Preferably the outer shell and mounting member are provided separately, as unitary or pre-assembled bodies and are then bonded metallurgically to form the piston. 
   In this specification references to bonding metallurgically are intended to mean all known techniques employed in joining metal bodies directly to each other or by way of an intervening metal, and includes brazing and various forms of welding, such as friction welding and laser or other beam or jet welding. 
   The outer shell body may be formed by back extrusion or forging. Alternatively, the outer shell body may be formed by flow forming. 
   If, as may be considered the norm, combustion bowl means is required in the crown this may be formed integrally with the shell body, subject to shape constraints, or may be formed separately and metallurgically bonded. 
   The outer shell part and/or mounting member may be made from steel which is suitably ductile, but the method is equally applicable to ductile alloys of non-ferrous materials. 
   These and other features and advantages of this invention will become more apparent to those skilled in the art from the detailed description of a preferred embodiment. The drawings that accompany the detailed description are described below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:— 
       FIG. 1(   a ) is a sectional elevation through a first embodiment of an engine piston in accordance with the present invention, taken in the direction  1   a - 1   a  of  FIG. 1(   b ), comprising an assembly of outer shell part and mounting member bonded together, 
       FIG. 1(   b ) is a sectional elevation through the piston of  FIG. 1(   a ) at right angles to the plane of that figure in the direction  1   b - 1   b  thereof, 
       FIGS. 1(   c ) and  1 ( d ) are sectional half elevations through the outer shell of  FIGS. 1(   a ) and  1 ( b ) respectively, 
       FIG. 2(   a ) is a sectional elevation through a second embodiment of an engine piston in accordance with the present invention, taken in the direction  2   a - 2   a  of  FIG. 2(   b ), comprising an assembly of outer shell part and mounting member bonded together, 
       FIG. 2(   b ) is a sectional elevation through the piston of  FIG. 2(   a ) at right angles to the plane of that figure in the direction  2   b - 2   b  thereof, 
       FIGS. 2(   c ) and  2 ( d ) are sectional half elevations through the outer shell of  FIGS. 2(   a ) and  2 ( b ) respectively, 
       FIG. 3(   a ) is a sectional elevation through a third embodiment of an engine piston in accordance with the present invention, taken in the direction  3   a - 3   a  of  FIG. 3(   b ), comprising an assembly of outer shell part and mounting member bonded together, 
       FIG. 3(   b ) is a sectional elevation through the piston of  FIG. 3(   a ) at right angles to the plane of that figure in the direction  3   b - 3   b  thereof, 
       FIGS. 3(   c ) and  3 ( d ) are sectional half elevations through the outer shell of  FIGS. 3(   a ) and  3 ( b ) respectively, 
       FIG. 4  is a schematic sectional elevation through apparatus for forming the outer shell body by back extrusion from a slug of metal, 
       FIG. 5  is a schematic sectional elevation through apparatus for forming the outer shell body by flow forming a disc of metal centred on the crown and about the longitudinal piston axis, and 
       FIG. 6  is a sectional elevation through a part of a third embodiment of piston according to the invention in which the tubular side wall is of substantially uniform thickness and the ring grooves and shoulder for the mounting are defined by folding the metal of the wall radially as a function of distance along the piston axis. 
   

   DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
   Referring to  FIGS. 1(   a ) to  1 ( d ) a piston  10  for an internal combustion engine is formed from high carbon steel. It comprises an outer shell  12  and a mounting member  14  bonded to it metallurgically by brazing. 
   The outer shell  12  comprises a crown  16  centred on a longitudinal piston axis  18 , and a tubular side wall  20  extending axially with respect to peripheral region  22  of the crown to an open end  24 . 
   Centrally of the crown, and surrounded by the peripheral region  22 , is a central region  26  (denoted by boundary lines  28 ) in the form of a combustion bowl  30  having a bowl floor  32  displaced axially with respect to the crown peripheral region, and towards the open end of the tubular side wall, by a bowl wall  34 . The bowl wall conveniently has a radially reentrant form as indicated at  36 . 
   The crown  16 , including both the central region  26  and peripheral region  22 , is of integral formation with the tubular side wall by back extrusion onto a mandrel, as described below, to define a unitary outer shell body. 
   The tubular side wall  20  includes adjacent the crown a region  40  of axially spaced, circumferentially extending ring grooves  42  machined into the wall and between the ring groove region and the open end  24 , there is provided shoulder means  44  facing towards the open end effected by changes in internal diameter of the wall. 
   As best seen in  FIGS. 1(   c ) and  1 ( d ), the shoulder means  44  comprises a first, smaller reduction in thickness of the ring groove region at  46 , defining first shoulder  48 , and as second, larger reduction in thickness at  50  between the ring groove region and open end  24 , defining a second shoulder  52 . 
   The tubular side wall is, apart from the ring grooves, of substantially uniform thickness in the groove region between the crown and shoulder means, and also a reduced, but substantially uniform thickness between the shoulder means and the open end; the reduction in thickness is principally defined by the shoulder means but there is also a slight tapering of wall thickness from crown to open end to minimise overall weight by having less wall thickness where less strength is required. 
   The region  46  may, and not disadvantageously, lie at the same axial position as one or more of the ring grooves. 
   The mounting member  14  is cast by investment casting or the like and comprises an axially thin plate  54  dimensioned to fit within the open end of the tubular side wall such that at least some points at its periphery  56 , and preferably all of its periphery, interface with the wall shoulder region  46  at wall interface  57  and byway of which interface it is bonded to the outer shell. In this embodiment it is bonded to the tubular wall about substantially the whole of its periphery and to this end, the plate has increased axial thickness at its periphery, defining a flange  58  extending axially to one side of the plate towards the crown. 
   The mounting member plate  54  also carries gudgeon boss means  60  formed integrally therewith at the side facing towards the open end  24  and axially between the flange  58  and the open end. The gudgeon pin boss means includes a bore  62  for the passage of a conventional gudgeon pin (not shown) transversely to the longitudinal axis  18 . The mounting member plate  54  further includes a connecting rod space, in the form of aperture  64  extending through the mounting member along the longitudinal piston axis, said aperture defining from the gudgeon pin boss means two gudgeon pin bosses  66  and  68  spaced apart along the bore and, with the plate in position exposing the central region of the crown to the open end of the tubular side wall. 
   The central region of the crown, in particular the junction between the combustion bowl floor  32  and wall  34 , has at least one axial extension to the bowl wall, conveniently as a circumferentially complete flange  70  which provides a uniform surface extending transversely to the piston axis and against which the mounting member plate  54  can bear at crown interface  71  to define its axial position within the tubular wall. In this embodiment, the axial position of the flange  70  is such that the peripheral flange  58  of the plate is clear of the first shoulder  48 , that is, the mounting member is positioned to one axial datum only. 
   The mounting member is bonded metallurgically to the side wall at interface  57  and to the crown at interface  71  by brazing, by applying a brazing material to the interfaces between the mounting member and outer shell as they are assembled together, heating them to a temperature sufficient to melt the brazing material, followed by any heat treatment, cooling and/or quenching regime desirable to impart desired physical properties to the brazed components. Insofar as the mounting member and outer shell are fully heated, the individual components may be subjected to stress relieving prior to assembly and heating together. 
   The flange  70  which surrounds the combustion bowl floor also surrounds the connecting rod aperture  64  such that the crown, the tubular side wall and the bonded mounting member define therebetween an annular cooling chamber  80  which is substantially closed in the axial direction by the crown and by the mounting member. Channel means, indicated generally at  82 , permits passage of cooling fluid to and from the annular chamber. The channel means comprises a fluid admission aperture  84  extending through the mounting member in a substantially axial direction and disposed such that for at least part of the piston stroke a jet of fluid is directed through the aperture and into the chamber. A fluid drainage aperture  86  extends through the mounting member displaced about the longitudinal axis from the admission aperture. 
   In keeping with producing a light weight piston, the tubular side wall is, at the open end  24  cut away about the longitudinal piston axis in line with the ends of the gudgeon pin bore to an axial level between the ends of the gudgeon pin bore and the ring groove region. It will be appreciated that the side wall may be cut more severely than illustrated, to the level of the shoulder means or other demarcation of the end of the ring groove region, such that there exists, to each side of the pin boss means two circumferentially discrete skirt portions essentially isolated from each other. 
   It will be appreciated that having formed the shell  12  and mounting member  14 , there is essentially only a single assembly operation in respect of positioning the mounting member within the outer shell and bonding it thereto, more particularly to the side wall and crown combustion bowl, but that even with the use of relatively thin-sectioned outer shell and mounting member components, the resulting structure has considerable strength and resistance to deformation of the side wall in the ring groove region. In the same manner that a so-called monocoque structure gives strength and stiffness to a vehicle body, the structure here is analogous and may be considered as a monocoque type of structure. The strength is attributable to the structural shape as well as the materials and the piston can thus be formed by relatively low cost high or medium carbon steel, that is, a low alloy steel. 
   Indeed the construction is suitable for non-ferrous alloys provided they are capable of being shaped into such outer shell and mounting member and bonded together. Furthermore, it will be seen that the outer shell and mounting member may be made of different metals provided they can be successfully bonded or are compatible in terms of strength and thermal expansion and ability to form a metallurgical bond between them. The aforementioned brazing may be employed with similar or dissimilar metals. It may be possible to effect bonding between the mounting member and outer shell by a non-metallurgical bond. It may also be possible to provide one or both of the outer shell and mounting member components of non-metallic material subject to the above criteria for bonding. Where the outer shell is constructed with a discrete combustion bowl or other central region, that part may also be non-metallic. 
   As mentioned above the outer shell  12  is formed as an integral body by back extrusion. Referring to  FIG. 4 , this shows schematically a back extrusion apparatus  400  including a mandrel  402  having an outer surface  404  conforming to the internal shape and dimensions of the tubular side wall  20 , including a step  406  corresponding to shoulder means  404  and a shallow lengthwise taper that effects minimal wall thickness according to strength requirements, as well as recess  408  corresponding to combustion bowl  30 . A cylindrical sleeve  410  surrounds the mandrel, separated by gap  418  and relative movement between them exerts pressure on a metal slug  420  which deforms and flows into the gap and into conformity with the mandrel to define the shell body. The aforementioned small taper of mandrel surface  404  that creates the above discussed internal side wall taper also facilitates removal of the extruded shell. Such taper may be kept to a minimum, insofar as this is consistent with strength but may be eliminated altogether without affecting removal from the mandrel. 
   It will be appreciated that such extrusion, in distinction from forging, casting and like operations, is a precision operation that permits the formation of relatively thin walls of relatively uniform thickness which in conjunction with the strength and stiffness afforded by the monocoque type of structure permits formation of a lightweight piston from a dense material such as steel. However it is quite possible to provide the outer body shell by forging or other metal deforming processes. 
   As described hereinbefore, the combustion bowl wall has an axial extension in the form of flange  70 . It will be appreciated that the mounting member could interface directly with the floor of the combustion bowl or such axial extension could be formed on the mounting member and extend to the bowl floor. It will also be understood that the combustion bowl may be omitted altogether, that is, have a substantially flat or domed crown, and such flange extend from the mounting member plate to the bottom of the crown surface, such arrangement still providing the support between crown and gudgeon pin boss means and annular cooling chamber. 
   Although it is convenient for the central region  26  of the crown to be integral with the peripheral region  22  it need not be, and can be manufactured separately and welded into the peripheral region to effect the unitary outer shell. Such separate formation of a combustion bowl may be appropriate to avoid having to machine radial re-entrant features in situ, but it is, of course, not necessary for a combustion bowl to have such re-entrant features and it may have a side wall that is suitable for forming completely by the extrusion process that defines the shell. 
   Although the piston  10  is essentially constructed from two components brought together in a single bonding operation, it is anticipated that there will still be machining operations required to the external surfaces of the side wall and crown, such as definition of combustion bowl re-entrant features, making valve pockets or recesses in the face of the crown, forming of ring grooves, cutting of the wall end and applying a final surface finish that also defines outside dimensions to within fine tolerances. Some of these may be performed before or after assembly and bonding of the outer shell and mounting member and some may be achieved during the extrusion that forms the outer shell, for example, valve pockets  90  in the crown face and ovality of cross section defined by the extrusion mandrel. The degree of ovality required of a piston is usually a function of its overall diameter; it is anticipated that the degree of ovality required on small diameter pistons may be achieved by the final machining of the outer surface, whereas for larger diameter pistons, such ovality may be better achieved by forming the outer shell with such cross section on a suitably shaped mandrel. Also, the formation of valve pockets or other shallow crown face features with the shell eliminates at least one relatively costly machining operation. 
   Formation of the outer shell by back extrusion about a mandrel permits the tubular wall internal surface to be defined to a suitable degree of accuracy without further machining. Of particular importance in connection with forming a lightweight piston is that in addition to being able to cut away any non-essential parts of the tubular side wall, is to have all wall sections as thin as possible for the functions required thereof. To this end it is possible to extrude the side wall with only slight variation in thickness from end to end (other than at the shoulder means) and with the ring groove region also less thick than might be thought acceptable, because of the support from the transverse mounting member. 
   It is a feature of the embodiment described that upon assembly prior to bonding, the interface  57  between the mounting member and the side wall extends axially, and thus positions the mounting member radially, whereas the crown interface  71  extends radially and positions the mounting member axially. The bond at interface  71 , which is enclosed between the mounting member and crown, must be effected by the aforementioned brazing or some other technique which does not rely upon access to it. One alternative is friction welding, but that may be considered unsuitable for the axially extending wall interface  57 , and although the latter is accessible from the open end, and susceptible to bonding by a different technique, it may be preferred not to have different bonding systems in use together. 
   This and other alternative structural possibilities are addressed in a second exemplary embodiment of piston in accordance with the invention is shown at  110  in  FIGS. 2(   a ) to  2 ( d ). Many of the parts are similar to those of piston  10  and the description will concentrate on the differences, For ease of reference, corresponding parts have reference numbers increased by 100. The piston  110  is a bonded assembly of outer shell  112  and mounting member  114 , piston crown including a combustion bowl  130  therein. The outer shell  112  is generally similar to shell  12  insofar as it has tubular side wall  120  that includes a ring groove region  140  and shoulder means  144  and, integral therewith, a crown peripheral region  122 . The peripheral region includes, displaced from the side wall, an axially extending combustion bowl wall  134 . A central region  126  of the crown, defined by boundary lines  128  about longitudinal axis  18 , comprises the floor  132  of the combustion bowl which is of discrete formation from the peripheral region and bonded thereto at interface  133  which extends around the periphery of this central region and substantially parallel to the longitudinal axis  18 . 
   The combustion bowl floor  132  is carried by the mounting member  114 , being formed integrally therewith and overlying a connecting rod space  164 , corresponding to the connecting rod aperture  64  of member  14  of piston  10 , the interface  133  that the bowl floor makes with the bowl wall comprises a crown interface. 
   The mounting member  114  comprises a plate  154  carrying on the surface opposite to the combustion bowl gudgeon pin boss means  160 , including transverse bore  162  and bosses  166  and  168  spaced apart by connecting rod space  164 . The outer periphery of the member at  156  is defined to be a tight fit within the tubular side wall, in particular interfacing with the wall region  146  at wall interface  157  which extends in a substantially longitudinal direction. 
   The base of the combustion bowl wall, adjacent the floor, has axial extension  170  which abuts the upper surface of the mounting member plate in order to locate it axially with respect to the crown periphery. Radial location is effected by the crown interface  133  and wall interface  157 , although the wall interface  157 , insofar as it is defined at shoulder means, may provide the axial location. 
   It will be appreciated that the mounting member is bonded to the tubular side wall at interface  157  and to the crown at interface  133 , but significantly, in addition to the wall interface  157  being in line with the open end of the side wall the crown interface  133  is in line with the open end of the combustion bowl. 
   As each interface is accessible in the axial direction, it is possible to weld each by laser beam, particle beam, plasma jet or the like by rotating the piston assembly or the welding apparatus about the longitudinal piston axis. To facilitate such a welded bond, the surfaces bounding each interface are formed to provide a small divergence in the direction from which such welding is effected. 
   It will be understood that insofar as each interface is welded by a remote energy source in line therewith, the line of one or both interfaces may be inclined with respect to the longitudinal axis such that one or both of the interfaces are not only visible from without the piston but may have a taper that effects both radial and axial location between the mounting member and the outer shell. It is, of course, possible to effect bonding between the outer shell and mounting member at the interfaces by brazing as described above. 
   A cooling chamber  180  is defined between the outer shell and mounting member plate  154  and the plate  154  has fluid admission channel  184  and drainage channel  186  therethrough. This arrangement is shown to differ from that of piston  10  in that the chamber  180  is of greater axial extent in line with the gudgeon pin bore  162 , that is, overlying the bosses  166  and  168  as shown at  167  and  169 . Insofar as the admission and drainage channels are at an operationally higher level, these extended regions form reservoirs for cooling fluid. 
   In a modification to the above, the fluid drainage channel may comprise one or more channels  188  extending substantially radially from the cooling chamber to the connecting rod space  164 . Such arrangement of cooling chamber reservoirs and drainage channels may be applied to the piston  10 . 
   It will be appreciated that the central region  126  may be defined as being other than what is substantially the whole of the combustion bowl floor. It may, for example be a smaller region of the floor or it may be larger and incorporate the bowl wall  134 , the boundary between central and peripheral regions being at the upper crown surface, as shown by boundary lines  128 ′, and the mounting member/crown interface  133  coincident therewith, said central and peripheral regions defining together an essentially flat topped or domed crown. 
   In a further modification, not specifically shown, the central region of the crown may be formed with the peripheral region of the crown as part of the outer shell in the manner of piston  10  and the mounting member may have an upstanding closure to a connecting rod space in the manner of piston  110 , whereby the closure provides not only upstanding flange means as described above for defining the cooling chamber but also overlies, and is capable of spreading load from, the central region of the crown. 
   Sectional views of a third exemplary embodiment of piston  210  in accordance with the present invention are shown in  FIGS. 3(   a ) to  3 ( d ), parts corresponding to those of  FIGS. 1(   a ) to  1 ( d ) having reference numbers increased by 200. The piston  210  comprises outer shell  212  and mounting member  214  bonded to each other. 
   The outer shell  212  comprises a unitary body consisting of crown peripheral region  222 , crown central region  226  in the form of a combustion bowl  230  and tubular side wall  220 . The tubular side wall consists of a thicker ring groove region  240  adjacent the crown and a thinner skirt region, open ended at  224 , separated from the ring groove region by simple shoulder means  244 . The combustion bowl  230  comprises a bowl floor  232  displaced axially from the peripheral region by bowl wall  234  and the wall, at the junction with the floor, has a number of axial extensions  270 .sub. 1 ,  270 .sub. 2  with gaps between them and possibly of slightly different axial lengths. 
   Mounting member  214  comprises a relatively thin mounting plate  254 , the periphery of which is dimensioned to fit within the thinner part of the side wall adjacent the shoulder means  244 ; the periphery  256  of the plate defines an interface  257  with the tubular wall and has no axial flange or like projection to increase the axial length of the interface. 
   The lower face of the plate, facing the open end  224 , caries integral gudgeon pin boss means  260  having transverse gudgeon pin bore  262  therethrough and through the plate, along piston axis  18 , is a connecting rod aperture  264  which also effects formation of separated gudgeon pin bosses  266  and  268 . 
   The upper face of the plate, indicated at  255  and facing the crown, is substantially flat and abuts the bowl wall extensions  270 , and  2702  defining thereat interface  271  extending transversely with respect to the piston axis  18 . Insofar as the interfaces  257  and  271  correspond in position and orientation to the interfaces  57  and  71  of piston  10 , the outer shell and mounting member in the disposition shown are bonded to each other by brazing as described above, defining the strong monocoque type of structure including a closed annular cooling chamber  280  between the crown, ring groove region of the side wall and the upper surface  255  of the mounting member plate  254 . Fluid channel means  284  permits admission of cooling fluid into the chamber and channels  288  permit drainage by way of the gaps between the bowl wall axial extensions  270 , and  2702  to the connecting rod aperture. 
   As an alternative to the equally-applicable cutting away of the open end of the side wall to the extent shown for pistons  10  and  110 , the gudgeon pin bosses  266  and  268  may extend into abutment with the side wall and the latter include through apertures  290  and  292  in alignment with the gudgeon pin bore  262 . Furthermore, insofar as each boss defines an interface  294 ,  296  respectively with the side wall, it may be bonded thereto adjacent the apertures. 
   This arrangement of longer, apertured side wall and, optionally, gudgeon pin bosses extending thereto may be applied to the piston  10 , and to piston  110  provided that the mounting member is bonded by brazing or the like that does not require direct access. 
   The mounting means  214  is of substantially uniform cross section in the direction of the gudgeon pin bore  262 , that is, as viewed in  FIG. 3(   a ), except of course where the connecting rod aperture  264  is cut. Instead of the mounting member being cast, it is formed by cutting from an extruded stock and then shaped to fit within the tubular side wall and the axially extending apertures  264  and  284  cut therein. 
   It will be appreciated that a cast mounting member, having a more complex surface as seen in the above described embodiments may be employed in piston  210  or such an extruded mounting member may be employed with the pistons  10  and  110 . 
   Another difference illustrated in this embodiment is a tubular side wall which is, to each side of the shoulder means  244 , of uniform thickness, that is, without the normally slightly tapering characteristic of extrusion or forging. The outer shell  212 , although it may be formed by extrusion or forging, is produced by so-called flow forming in which, as  FIG. 5  illustrates a disc-like slug of metal  502  is caused to rotate with a profiled mandrel  504  and during rotation the peripheral regions of the disc are displaced axially to lie along and conform with the mandrel that defines the respective wall thicknesses and shoulder means. As with the back extrusion described above, the tubular side wall may be formed thereon and removed without an internal wall taper, where this can save weight. 
   Such flow forming of the outer shell may be used in respect of piston  10  and  110 . Although it may be preferred to define the tubular side wall with a thicker region into which ring grooves are subsequently machined, it will be appreciated that such flow forming permits, with the use of a radially contractible mandrel, formation of a tubular side wall  220 ′ of substantially uniform thickness from end to end but varying in radius as a function of axial position to define the ring grooves  242 ′ and shoulder means  244 ′, as illustrated in  FIG. 6 . 
   It will also be appreciated that whereas the above embodiments have described the mounting member as bonded about the whole of its periphery to the side wall at the shoulder means, it may be bonded only at a plurality of discrete points and the periphery of the mounting member may be other than conforming in shape to the tubular side wall, extending to contact the wall only at points of bonding. 
   Although it is convenient for assembly to define a bonding interface between the periphery of the mounting member to the side wall at shoulder means which demarcates between the ring groove region and the more lightly loaded skirt, it is not essential and it may be bonded to the side wall other than at such shoulder if disposing the interface elsewhere improves stress patterns. 
   It is re-iterated that the various embodiments of piston according to the invention are not limited in size and the structures and methods of manufacture are capable of being scaled to a wide variety of dimensions. 
   The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.