DE2622595A1 - IMPROVED PISTON AND METHOD OF MANUFACTURING TOWEL - Google Patents

Description

Patent attorneys W3IXKF.N & LAUFER

DR. HUGO WILCKEN DIPL.-ING. THOMAS WILCKEN DIPL-CHEM-DR-WOLFGANCLAUFEr

LOBECK LOBECK MUNICH

Associated Engineering Limited
60 Kenilworth Road

Leamington Spa, Warwickshire (UK)

Improved Piston and Method of Making the Same

The invention relates to novel pistons that are used, for example, in reciprocating internal combustion engines, Compressors or pumps can be used. The invention relates in particular to light metal pistons, which are provided with inserts which, during normal operation of the piston, are intended to resist the transversal or inclined thrusts on the piston skirt, in a direction that is generally runs perpendicular to the axis of the piston pin bosses. In this case, light metal is understood to mean aluminum alloys or

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LÜBECK 1 · Breite Strasse 52-54 ■ Telephone (0451) 75888 * (MUNICH SO · Prnwregei.tenstrae · 74 · Telephone (089) 477366

Postal check: Hamburg 138119-204 Banks: Commerzbank Lübeck (BLZ 23040022) account no. 390187 Deutsche Bank MOndien (BLZ 70070010) Account No. 65/05432

Magnesium alloys.

British Patent No. 1,286,248 describes a piston consisting of a piston head, a periphery or a circumference continuously running skirt, piston pin bosses united with the skirt, surfaces or front surfaces, which are essentially diametrically opposite regions of the shaft are formed and are arranged on an axis which is substantially in the right Angled to the axis that joins the piston pin bosses, with the shank in the range of either or both The front surface is separated from the piston head by means of a transverse slot or slots, and is made up of bracing devices, which include at least one arcuate portion disposed within and in contact with the shaft next to the transverse slot or the transverse slots, this bracing device also has parts that (on one or both sides) within parts of the piston in the region of the corresponding Gudgeon pin bosses are firmly clamped, and straight struts connecting each curved part to the clamped parts, whereby the strut device can transmit loads from one or both front surfaces of the piston skirt through the curved part or parts and the straight strut parts on the piston in the area of the piston pin bosses.

The straight strut members of the one described in the above patent Pistons are arranged in spaces or at distances from the inner surface of the piston skirt. Although such pistons work in a satisfactory manner, difficulties arise in the production and this especially when an automated pro-

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production is sought. The core that defines the inner contours of the piston when it is poured into the mold closes five parts or segments, each of which is removed from the piston mold and the cast piston relatively quickly after casting must if it is to be avoided that the piston has disadvantageous properties, such as blow molding or porosity. However, in order to cast the piston described above, the use of seven or nine core parts or segments was necessary, around the gaps or spaces between the straight strut parts and the inner surface of the piston skirt to be filled out. A core consisting of seven or nine parts has the disadvantage that it is complex and that the additional Core parts are thin and therefore easy to damage.

In the above embodiment, the bracing device is still present designed as a one-piece insert, i.e. each pair of the straight strut parts is pushed by a conventional, on the thrust responsive curved part united. This one-piece construction also poses manufacturing problems, especially when automated production is desired due to deformation of the inserts.

The invention is therefore based on the object described above To overcome disadvantages of conventional piston designs.

According to the invention, this object was achieved in one embodiment by creating a light metal piston that is marked net is through a piston head, a peripherally continuously extending skirt, piston pin bosses, which are united with the skirt and have a common axis, thrust front surfaces, which are through im

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formed substantially diametrically opposite areas of the shaft and are arranged on a second axis which is substantially perpendicular to the common axis, and by a pair of bracing devices arranged substantially symmetrically on opposite sides of the second axis are, each strut device or strut device has an anchoring part, which in the piston in the area the piston pin boss connected to this is embedded; as a pair of rectilinear .strut parts that extend from the anchoring— part on opposite sides of the common axis and extend at an acute angle to this, and a pair of thrust-responsive Parts connected to the opposite ends of the straight strut members and located inside and in contact with the shaft in the The area of the opposing sliding or sliding surfaces are arranged, being the rectilinear. Strut parts during operation of the piston transverse thrust forces from one or both thrust surfaces and from the thrust-reacting parts transferred directly to the area of the piston pin bosses, and wherein at least the straight strut parts of the strut device have a larger dimension in one Have direction generally parallel to the piston longitudinal axis than in a direction generally transverse to this axis, and wherein the piston with substantially chord-like (chordwise) Parts are formed on each side of each piston pin boss and extend in the direction of the thrust surfaces or sliding surfaces, and with bosses formed integrally with the belt-like parts and extending inwardly therefrom, wherein the straight strut members are in contact with the inwardly extending hubs but spaced from the main body the belt-like parts of the piston, so that the straight strut parts are brought into compression or squeezed together.

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Since in this type of construction a pair of struts or struts are present, the difficulties of making one-piece bracing devices are eliminated. Furthermore, since the gaps between the straight strut parts and the metal of the piston are eliminated, the number can of the core parts (coreparts) are reduced and thereby also those with one consisting of seven or nine parts Core related difficulties eliminated.

The thrust-reacting parts of the straight strut parts that assigned to each sliding surface, extend almost up to the axis of the Achub, although they are arranged separately, peripherally on the opposite sides of the second axis, i.e. the thrust axis of the piston. In addition, the thrust-reacting parts, which can also be rectilinear, a relatively short length in the peripheral direction of the shaft, so that the ends of the with these connected rectilinear strut parts also in the vicinity of the Thrust axis are arranged. In this way, the straight strut parts transfer the loads directly between the sliding surface or any sliding surface next to the thrust axis and the piston pin bosses.

The invention also relates to a method for producing the piston described in more detail above, which is characterized in that a pair of preformed bracing devices and mold core parts, which define the inner contours of the piston, are used within a piston mold or a piston casting mold that Bracing devices within the mold generally have radially outwardly opening recesses in the associated core parts fixed or locally determined by stake or holding

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devices are introduced, the space or space between the adjacent opposing surfaces of the straight strut parts and the shape not hindered or blocked by the core devices, in cooperation with the rectilinear bracing parts, stretched inward from the mold through areas of the recesses that make the inward extending piston bosses and open into those surfaces of the core parts that the inner surfaces of the im form essential belt-like parts, the molten light metal is poured to form a piston casting in the mold around the bracing device, the stake devices and the mold core parts and to fill the space or space, man allowing the casting core to cool so that the bracing device is in compression or compressed and the piston skirt is brought into a peripheral tension or circumferential tension, the casting is severed from the mold, stake devices and core parts, and the tensioned stem after it closes has cooled its final contour is machined.

The ends of the thrust-reacting parts grip or shoulders. Approaches which are preferably used in the shaft to such an extent that the thrust-reacting parts or at least the ends thereof are received in the wall thickness of the shaft and do not extend or continue inwardly from the inner contour of the shaft. This technical feature facilitates and further improves the production, since the core parts which form the inner contour of the shank r at least in the area d ends of the push-responsive components can be separated in the longitudinal direction of the piston cast, without first core parts are retracted radially inwardly need to free the ends

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to make, or after withdrawing or pushing back the core parts at a significantly reduced distance.

Since the height of the bracing devices parallel to the longitudinal axis of the piston is greater than their width and that in particular in the The area of its thrust-reacting parts and the straight-line bracing parts, is exposed to the flow of molten metal limitation exerted by these relatively thin bracing devices is reduced and, in addition, the bracing devices can cool relatively quickly.

In the following description, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings explained. ^

Figure 1 is a side elevational view of the piston looking from the axis of the piston pin bosses.

FIG. 2 is a longitudinal section along the line H-II of FIG. 2. FIG. 3 is a longitudinal section along the line IH-III of FIG. 2.

FIG. 4 is a cross section taken along line IV-IV of FIG. 1 in the direction of the lower or open end of the piston skirt.

Fig. 5 is a cross section taken along the staggered line V-V of FIG

. Figure 2, with the upper half of the section viewed in a direction towards the lower or open end of the piston skirt and the lower retainer viewed in one direction towards the upper or piston head end of the piston skirt.

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FIG. 6 is a partial vertical section along the line VI-VI of FIG. 5.

Figure 7 is a bottom plan view of that shown in Figures 1-6 Piston.

Figs. 8 and 9 are a side view and a plan view of a the bracing devices or inserts.

Fig. 10 serves to illustrate the method for producing the Piston according to the invention in partially disassembled form.

In the figures, a piston for an internal combustion engine is shown, which has a piston body made of a light metal alloy such as an aluminum alloy or magnesium alloy has been integrally cast. The piston has a piston head 1, a piston sidewall having a peripherally continuous piston ring support portion 2 and a peripherally continuous shank 3, and a pair of diametrically opposed piston pin bosses 4, which lie on a common axis 5, which are arranged diametrically or exactly opposite in the embodiment shown is. The piston pin bosses 4 are integrally connected to the piston head 1 by means of thick dependent stiffeners 6, which are arranged according to inside of the support part 2 and the shaft 3 of the piston sides wall extend. The shaft has sliding or pushing surfaces or front surfaces 7, which are formed by the outer surfaces of the diametrically opposite regions of the shaft, arranged between the piston pin boss on a second diametrical or sliding axis 8 which is perpendicular to the piston pin boss axis 5.

The bracing device, which consists of a pair of separate bracing

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inserts or strut inserts 9 is within the piston body near the end of the shaft 3 next to the piston head 1 used and consists of a material that has a substantially lower coefficient of thermal expansion than that Light metal alloy of the piston body. Every bracing insert 9 has an essentially semicircular anchoring part 10 which is embedded within an associated piston pin boss 4 or (on one or both sides) is firmly clamped to the strut insert 9 to the piston body on the light alloy in Area of the piston pin boss 4 to be anchored. The strut insert 9 can alternatively also be embedded or inserted in an adjacent part of the stiffeners 6.

It can be seen from Fig. 3 that the anchoring part 10 has a curved, i.e. substantially semicircular shape, see above that the piston pin bore 11 is cleared. The anchoring part 10 can be drilled, drilled, castellated or with other suitable changes in cross-section be provided to the mechanical fastening between the anchoring part 10 and the light alloy of the piston body to enhance. Each strut insert 9 also has a pair of straight strut parts 12 (see in particular FIG. 5), the are integrally formed with the opposite ends of the curved anchoring part 10. The strut parts 12 set on the opposite sides of the longitudinal surface, which contains the axis 5 of the piston pin bosses, in the direction of the sliding surfaces 7 at an acute angle to the axes 5 and The outer ends of the strut inserts 9 are at an angle arranged to the rectilinear strut parts 12 to form straight thrust or pressure responsive parts 13, the inside of the piston skirt around the areas of the sliding surfaces 7, wherein

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those of one strut attachment 9 on the opposite Side of the axis 8 is arranged to the other strut insert. The ends of the thrust-reacting parts work together with the shoulders 14, which are inserted in the piston skirt directly next to the axis 8 are, and the parts 13 are complete at least at their ends recorded within the wall thickness or wall thickness of the piston skirt 3, as can be seen from FIG.

Essentially belt-like shaft parts 15 extend between the Sliding surfaces 7 and 7 contain the radial outer ends of the piston pin bosses. These shaft parts are at least at the same level on the outside the straight strut parts 12 excepted (see Fig. 1, 4 and 5) to create the so-called "window". The straight ones Strut members 12 are arranged in contact with the hubs 16 (see FIGS. 3 and 6) which are integrally formed with the shaft parts 15 and extend inward from them. The shaft parts 15 in this way generally have webs extending like tendons, each with a tapering or conical convergence Opening 17 are provided through an associated hub 16 is surrounded.

A comparison of FIG. 5 with FIGS. 3 and 6 shows that that the height of at least one of the exposed zones of each insert, i.e. at least the height of the rectilinear parts 12 and the thrust reacting parts 13 thereof in the longitudinal direction of the piston is greater than its thickness in the direction of the transverse surface of the piston. This facilitates the inserts, which are made of sheet metal. It is however, it can be seen that the insert and in particular the rectilinear parts 12 and the thrust responsive parts 13 are not far from the

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Shank extend downwards and, for example, have a height which is less than half the diameter of the piston pin bore 11.

Transverse slots 18 separate the shaft 3 from the piston head 1 and in particular from the piston ring carrier section 2 in the areas of the sliding surfaces 7. The slots can be in the lowest piston ring groove 19 be open. Alternatively, there can also be only one slot which is assigned to one or the other sliding surface. To this The strut inserts 9 are arranged next to the transverse slots 18.

Each strut insert 9 can be made of steel, cast iron or other suitable materi al and can be punched or Pressing from sheet metal, casting, sintering or other suitable processes can be produced.

The shape of the strut inserts can be seen clearly from FIGS. 8 and 9 and in particular the central anchoring part 10, the two straight strut parts 12 and the straight shear-responsive parts 13.

In a preferred embodiment of the piston according to the invention For example, the piston may have a diameter of approximately 87 mm with the free end of each thrust reacting member 13 of of the axis 8 as shown in Fig. 5, at an angle of less

ο ο

is offset as 10 and preferably approximately 5, while the meeting point or junction of each part 13 with the associated straight strut part 12 from the axis .8 is offset between 20 "and 30, and preferably approximately 25

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The included angle between each straight-line bracing

o ο

part 12 and the axis 8 is between 20 and 30 and preferably

ο
approximately 25 , while the included angle between each part 12 and the associated part 13 is between 20 and 30 and preferably approximately 25.

According to the present invention, the piston described above can be used in the following manner are produced: First, two strut inserts 9 are precisely set on a pre-assembled core consisting of several parts and attached, the core having two piston pin boss forming parts 21 and 22, two sliding surface forming parts 23 and 24 and one central part 25 owns. It should be noted that the core parts which form the piston pin bores are omitted for the sake of clarity have been. The core parts 21, 23, 24 and 22, 23, 24 are formed with radially outwardly opening slots 26 and 27 to to accommodate the straight strut members 12 and 13, and are also recessed, as shown at 28, to form the hubs 16. The multi-part core 21, 22, 23, 24 and 25 is then in a not shown Piston shape inserted and assembled with this, namely together with the two strut inserts 9. The shape, which has a conventional cylindrical shape to form the outside of the piston casting is tapered with pre-pressures or ejector pegs 29 are provided, which extend from the shape inward extend and cooperate with the straight strut parts 12 of the inserts 9 and abut with these to the To hold inserts 9 in precise position on the segment core, with the one ejector peg 29 in the center of each of the recessed Parts 28 of the core is arranged, which forms the hub 16 in the piston, and wherein the ejector pegs 29 in the tapered openings 17 result in each stem portion 15 of the piston after casting.

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The light metal alloy for the piston body is then used in the Mold cast around and against the inserts 9 and the core to form the piston blank. When the casting after solidification As the alloy cools, the inserts 9 get into the correct compression state along their length and the sliding surfaces 7 of the Shank 3 are brought between the piston pin bosses 4 in a peripheral tension (circumferential tension) due to a substantial Difference between the thermal expansion coefficients of the Light alloy and the metal of the inserts 9. These compression and tension forces exist at room temperature.

The recesses 8 in the core parts around the ejector pegs 29 form the inwardly extending hubs 16 and serve to that the molten metal is more satisfactory during casting Way around the pusher pegs 29 flows.

Since, as already mentioned above, the straight strut parts and the thrust-responsive parts 13 of the inserts 9 in the lateral or lateral direction are relatively thin, the flow of the molten alloy around the inserts is facilitated. The inside directed movement and the extraction of the corresponding slotted core parts 23 and 24 with respect to the parts 13 - after extraction of the central core part 25 - can be carried out without the strut parts 13 having any appreciable angle of tension be provided. After pulling out the parts 23, 24 and 24, the core parts 21 and 22 can be brought into contact with one another and be pulled out. Since the. Strut parts, as already mentioned above, are relatively thin in the radial direction, they can be relatively cool quickly and therefore the core parts can be removed relatively soon after casting. This is of particular concern when that

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Casting process is carried out automatically. This eliminates the presence of gas inclusions and a porosity of the cast Piston blank prevented.

About the removal of the piston casting from the mold and also To facilitate deployment of the core and brace insert assembly prior to casting, the mold can be split vertically and / or parts thereof in the areas of the piston pin bosses can be radial be pulled out.

The piston casting is then machined and given, for example its final contour by turning or grinding.

The presence of the transverse slots 18 reduces the stiffness of the shaft 13 on the opposite sides of the axis 5. However first increase the strut inserts 9 the rigidity of the shaft 3 on both sides of the axis 5, since the inserts of load-bearing Are kind because the strut parts 12 are straight and are able to absorb the loads directly from the sliding surfaces 7 next to the Axis 8 to be transferred to the piston pin bosses 4. Therefore, the combined rigidity of the light alloy shaft and the inserts 9 are specified on both sliding surfaces of the shaft depending on the requirements of the machine in which the Piston is to be used and operated.

The thermal expansion of the piston skirt 3 in a direction parallel to the axis 8 would be noticeable solely through the transverse slots 18 can be reduced compared to that of a piston, because the shaft is completely firmly connected to the piston head, as the slots mechanically and thermally isolate the piston head from the skirt. the

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thermal expansion of the shaft is reduced even more by the strut inserts 9 than by the slots alone, because these bracing inserts have a lower coefficient of thermal expansion own than the light alloy. The compression forces in the straight strut parts 12 and the tension forces in the shaft, which exist at room temperature, are at operating temperature of the machine significantly reduced due to the relative expansion of the shaft in relation to the strut inserts and at the same time, the presence of these forces at room temperature will result in actual thermal expansion parallel to the thrust axis 8, which is reduced at operating temperature.

The piston pin bosses incline during normal operation of the piston 4, which are relatively solid and rigid with the piston head 1 through the fixed reinforcement 6 due to the thermal Expansion of the piston head to a radial outward movement or to a movement 6 along the axis 5 if the flask heats up. The anchoring portion 10 of the strut inserts will similarly move apart. Because the straight forward Strut parts 12 are arranged at an acute angle to the axis 5, this separating movement spreads the inner ends of the rectilinear Bracing parts 12 apart, i.e. the ends closest to the anchoring part 10, whereby their outer ends, i.e. their other ends and the thrust-reacting parts 13 tend to to move inward. In this way, the compression forces and the tension forces in the brace inserts and in the Shaft partially relaxed or relieved and furthermore the thermal expansion of the sliding surfaces and the non-sliding surfaces is reduced.

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It could be that the control effect of the strut inserts 9 could be reduced by the shaft parts or Steqe 15, since these The webs mechanically couple the piston pin bosses 4 to the sliding surfaces 7 because these webs are generally chordal in shape extend to the circumference of the piston skirt and because of the contact area the webs with the straight strut parts 12 is reduced due to the presence of the hubs 16 around the tapering ones Openings 17, however, the reduction in this control effect is small be. The control effect can be adjusted by changing it of the acute angle of the straight strut parts 12 with respect to the axes 5 and 8 and / or by changing the cross-section, the Shape or position of the strut inserts 9 within certain limits, or by increasing or reducing the size of the ejectors - pegs 29 and therefore the openings 17 and the hubs 16.

The construction of the piston according to the invention can of course can also be used with other special piston configurations. For example, one or both of the transverse slots 18 can be modified or be omitted, in this case one or both sliding surfaces firmly connected to the piston head or the piston ring carrier section.

The curved anchoring part 10 or an equivalent part can extend below or above the piston pin bore 11.

Claims;

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Claims (17)

P a I e η ί a η s ρ r ü c h e 1) Light metal piston, consisting of a Kolbeikopf (1 ;, a peripherally continuously extending shaft (3), piston pin bosses (A 1, associated with the shaft and a common axis (5) besitv-er, -Schub- front surfaces or Sliding surfaces (7), which are formed by essentially diametrically opposed areas of the sheep + es and are arranged on a second axis (8) which essentially runs at right angles to the common axis, and a pair of bracing devices (9 ), which are arranged essentially symmetrically on the opposite sides of the second axis, each strut device having an anchoring part (10) which is embedded in the piston in the region of the piston pin bosses connected to it, a pair of straight strut parts (12) extending from the Anchoring part on opposite sides of the common axis and extending at an acute angle to it, and a pair of thrust responsive T parts (13), which are connected to the opposite ends of the straight strut parts and are arranged inside and in contact with the shaft in the region of the opposite thrust or sliding surfaces, the straight strut parts during operation of the piston transverse thrust forces from one or both thrust surfaces and transmitted from the thrust-responsive parts directly to the area of the piston pin bosses, characterized in that at least the straight strut parts of the strut device have a greater dimension in a direction generally parallel to the piston longitudinal axis than in a direction generally transverse to this axis and wherein the Pistons with essentially chord-like or belt-like parts on each side of each piston pin hub. - 18 - 609850/0699
BADORLGiNAL is formed and in the direction of the thrust or sliding surfaces extend and with hubs (16) integral with the belt-like Parts are formed and extend from these inwardly, wherein the straight strut members are in contact with the inwardly extending hubs, but spaced from the main body the essentially belt-like or chord-like parts of the piston, so that the straight strut parts are brought into compression or be squeezed. 2. Piston according to claim 1, characterized in marked c h net, that the end of each thrust-responsive member is to such an extent on a shoulder (14) inserted in the shaft engages that the thrust-responsive part or at least the end thereof is received within the wall thickness of the shaft. 3. Piston according to claims 1 and 2, characterized in that that the height of the belt-like or chord-like parts in the direction of the longitudinal axis of the piston is substantially greater than the height of at least the straight strut parts and the shear-reacting parts of the struts. 4. Piston according to claims 1,2 and 3, characterized in that each inwardly extending hub one Has opening (17) which extends through the associated belt-like or chord-like part to the straight strut parts. 5. Piston according to Claims 1 to 4, characterized in that that the height of at least each straight strut portion of each strut toward the longitudinal axis of the piston is less than half the diameter of the piston pin receiving - 19 - 609.8 50/0699 Bores (11) in the piston pin bosses. 6. Piston according to claims 1 to 5, characterized in that each thrust-reacting part is rectilinear. 7. Piston according to claims 1 to 6, characterized in that the free end of each thrust reacting Part is arranged in the vicinity of the second axis. 8. Piston according to claim 7, characterized in that the angular separation of each end from the second axis about its longitudinal axis of the piston is less than 10 and preferably approximately 5. 9. Piston according to claims 1 to 8, characterized in that that the angular separation of the connection of each straight strut part with the associated thrust-reacting part ο ο from the second axis in the range from 20 to 30 and preferably O
is approximately 25. 10. Piston according to Claims 1 to 9, characterized in that that the included angle between each straight strut part and the second axis is in the range of 20 to 30 and is preferably of the order of 25, 11. Piston according to claims. 1 to 10, characterized in that each thrust-reacting part is straight and with the associated rectilinear strut part forms an included angle within a range of 20 to 30 and preferably in of the order of 25 forms. - 20 - 0 9850/06 9-9.00 12. Piston according to claims 1 to 11, characterized in that the straight strut parts against the inwardly extending hubs, but not connected to them or secured directly to them in any other way. 13. Piston according to claims 1 to 12, characterized in that each right-angled strut part in the area its connection point is embedded with the associated anchoring part in the piston in the area of an associated piston pin hub. 14. Piston according to claims 1 to 13, characterized in that the shaft of the piston zone in the region of a or both sliding surfaces is separated or separated by means of the transverse slot or transverse slots (18). 15. A method for manufacturing a piston according to claims 1 to 14, characterized in that a pair of preformed Strut devices (9) and mold core parts (21-25) which define the inner contours of the piston within a piston shape or a Piston mold are used, the bracing devices within the mold, generally radially outwardly opening recesses (26-27) in the associated core parts, fixed or fixed in place by inserting stakes or holding devices, the space or space between the adjacent opposite ones Surfaces of the straight strut parts and the shapes not obstructed or blocked by the core devices in cooperation with the straight strut parts inward of the mold is stretched through areas (28) of the recesses that the inward extending piston bosses (16) define and extend into those surfaces of the core parts that form the inner surfaces of the belt-like or tendon-like parts (15), the molten light metal - 21 - 60985Q / Ö699 is poured to a piston casting in the mold in the bracing device, to form the peg devices and the mold core parts and to fill the space or intermediate space, one the casting core lets cool, so that the bracing device is brought into compression or squeezed together and the piston shaft (7) in peripheral stress is applied to the casting from the mold, stake devices and core parts is separated, and the tensioned shaft, after it has cooled to its final contour, is machined masdninell. 16. The method according to claim 15, characterized in that that the core, which contains the core parts that define the bores (11) in the piston pin bosses (4), is divided into areas which are arranged generally parallel to the longitudinal axis of the piston casting, in a maximum of five mold core parts, the inner Define contours of the piston skirt, with the core being separated from the casting by a relative displacement between the Core parts is caused. 17. The method according to claims 15 and 16, characterized in that the anchoring parts (10) and the rectilinear Strut parts of the strut device in radially outwardly opening recesses (27, 26) in the first core parts (21 and 22) are arranged, and at least the end regions of the thrust-reacting parts (13) against the lengthwise continuous surface parts of the second core parts (23 and 24) are arranged, wherein at least after the casting has cooled, one of the second core parts is separated from the piston casting by at least one of the second core parts is shifted in relation to the first core parts in the longitudinal direction and with none at all or with only a very small initial one - 22 - 609850/0699 little initial inward movement of at least a second core part in the direction of the longitudinal axis of the piston casting. 609850/0699 • te Blank page