US2551488A - Controlled expansion piston - Google Patents

Description

y 1951 D. D. DEMING 2,551,488

CONTROLLED EXPANSION PISTON Filed Feb. 18, 1948 5 Sheets-Sheet l l I :ZZTFEH 2:1?

DONAL 0 D. DEM/N6 May 1, 1951 D. D. DEMlNG CONTROLLED EXPANSION PISTON 5 Sheets-Sheet 3 Filed Feb. 18, 1948 Patented May 1, 1951 ATENT OFFICE CONTROLLED EXPANSION PISTON Donald D. Deming, Chester-land, Ohio, assignor to Thompson Products, Inc., Cleveland, ()hio,

a corporation of Ohio Application February 18, 1948, Serial No. 9,109

9 Claims. 1. This invention relates to-a trunk piston used in. internal combustion engines and, more particularly; to pistons constructed of light metals such as aluminum, magnesium or various alloys thereof.

It has long been recognized that pistons formed from a light metal or alloy require some form of expansion control mechanism when employed in a cylinder bore defined by material of substantially lower thermal expansion coefficient, such as cast iron. Variousconstructions have. been heretofore suggested for the purpose of controlling the expansion of a light metal piston. For eX- ample, U. S. Patent No. 2,238,087 to Bowser et a1. discloses, as one modification, the well known U-slot' piston wherein the expansion of acantilever bearing section of the skirt wall, defined by an inverted U-s1ot,. is controlled: by an expan sion band of ferrous material which is of generally U-shaped configuration and has the bight portion thereof secured to. the. central portion of the cantilever well section and the end portions thereof inturned and anchored in: the metal forming. the wrist pin. bosses. The expansion, control effected by this construction is. not all. that can be desired. inasmuch as the expansion controlling band is of. substantial Width relative to the overall length of the piston and hence practically the entire length. of the cantilever section of the skirt Wall is unitarily controlled. Furthermore, such band. would not permit expansion. movement. of the: wrist pin boss portions in the direction transverse to the wrist pin. axis.

It has also been heretofore suggested, forexample in U. S. Patent No. 2,262,132 toBerry, that the expansion of the cantilever section of a U- slot piston be controlled only at the top portion thereof which portion, of course, is subjected to a higher degree of heating than the lower portions of the piston skirt. Hence in the Berry patent it is proposed that the skirt ofthe piston be an oval configuration withthe bottom portions of the skirt approaching more nearly a circle than the top portion. However, the Berry patent employs a continuous ring or band as the expansion controlling member, and such band requires special care in locating such upon the piston mold during the casting operation so as to insure that the band Will'be cast into the top portion of the skirt in proper location. Furthermore, the continuous band necessarily mustbe provided with a coating of graphite or similar lubricant to permit relative circumferential movement of the. skirt portion of the pistonv with: respect to the band to occur during the heating cycle. Lastly,

2 the utilization of the continuous band necessarily requires that a plurality of recesses be formed in the interior wallof the piston during the casting which recesses correspond to the location or the supporting members for the band upon the core section of the piston casting apparatus.

In accordance with this invention, a, controlled expansion piston construction is provided. which will reliably control or limit the expansion of a trunk piston both in the direction of the wrist pin axis and in the direction tarnsverse thereto with an unusual degree of precision. In addition, the expansion control member employed in this invention is of substantially simpler configuration than hasheretofore been known and may be cast in proper position in the metal of the piston without requiring any special-facilities or producing any complications in thenormal cast-- ing of the piston by conventional apparatus. The. piston construction embodying this invention does not require that any special. support be provided on the core of the molding apparatus for supporting. the expansion controlling element during the casting process so that in the finished piston no portion of the skirt wall is weakened ioy the recesses which have beenv heretofore necessary to accommodate the supports for the expansion control member during the casting operations. Furthermore, it is not necessary to coat the expansion. control element with graphite or similar lubricating device inasmuch as itsconfiguration is such as to readily permit the necessary relative movement of the skirt of the piston. with respect thereto without requiring a lubricant.

Accordingly, it is an object of this invention to provide an. improved light metal piston construction, and particularly to provide an aluminuln alloy piston so constructed that the expansion thereof under the temperatures encountered during the operation of the piston in an internal combustion engine will be controlled so as to maintain optimum clearance conditions between the piston and the cooperating cylinder bore at all times.

Still another object or" this invention is to provide a controlled expansion piston wherein an expansion controlling element is cast into the metal of the skirt portion of the piston and the construction of such does not require any modification of the conventional casting technique and apparatus employed in the fabrication of ordinary pistons.

A particular object of this invention is to provide an improved expansion control element for a light. metal piston which is of U-shapecl 3 configuration, having an arcuate bight portion adapted to be secured to the cantilever section of a U-slot piston skirt and substantially straight arm portions which are respectively secured in the piers of metal which communicate between the wrist pin bosses of the piston and the head portion thereof. By such construction, unusual and highly desirable expansion control characteristics of the skirt of the piston are obtained.

The specific nature of this invention, as well as objects and advantages thereof, will become apparent to those skilled in the art from the following detailed description taken in conjunction with the annexed sheets of drawings which, by way of preferred example only, illustrate several embodiments of the invention.

On the drawings:

Figure l is a vertical sectional view of a complete piston embodying this invention;

Figure 2 is a transverse sectional view taken on the plane II--II of Figure 1;

Figure 3 is an elevational view of the core element of a piston casting mold for producing the piston construction of Figures 1 and 2, showing the expansion controlling band assembled thereto in position for the casting operation. The dotted lines in this figure indicate the external periphery of the cast piston;

Figure 4 is a plan view of Figure 3;

Figure 5 is a sectional view taken on the plane VV of Figure 3;

Figure 6 is a perspective view of a modified form of expansion control band;

Figure '1 is a transverse sectional view of a piston construction incorporating the expansion control band of Figure 6;

Figure 8 is a perspective view of still another modification of an expansion control band;

Figure 9 is a transverse sectional view of a piston construction incorporating the expansion control band of Figure 8;

Figure 10 is a side elevational View of a third modified form of expansion control;

Figure 11 is a transverse sectional view of a piston construction incorporating the expansion control band of Figure 10; and

Figure 12 is an enlarged scale partial sectional view taken on the plane XII-XII of Figure 11.

As shown on the drawings:

The piston l6 embodying this invention'is preferably formed of light metal or alloys thereof, such as aluminum or magnesium and comprises a head portion II having a depending ring flange He provided with the customary peripheral grooves I Ib for reception of the piston rings (not shown). A skirt portion I2 is provided havin opposed wrist pin bosses l3 integrally formed therein. While the skirt portion I2 is preferably cast integrally with the ring flange portion Ila of the head II, those portions of the skirt intermediate the wrist pin bosses I3 are separated from the head portion II by saw cuts I4. To provide a reinforced connection between the wrist pin bosses I3 and the head portion II, upwardly extending piers I5 are integrally formed in the skirt portion running between the wrist pin bosses I3 and the head portion I I.

While not limited thereto, the periphery of the skirt portion I2 is preferably cam ground so as to provide a slightly elliptical cross section, such ellipse having its major axis located perpendicular to the axis of the wrist pin bosses I3. Furthermore, the peripheral surface I2a of the skirt portion E2 may be ground so that the degree of ellipticalness decreases along the length 4 of the skirt portion from the top to the bottom thereof, so that the bottom portion of the skirt approaches a circle more nearly than the top portion.

With the described configuration, a hoop stretching effect will be produced in the skirt portion I2 when the piston is subjected to the high temperatures encountered in its operation in an engine block. Due to the fact that the skirt portion i2 is separated from the head II by saw cuts I4 except in the vicinity of the wrist pin bosses l3, the heat flow from the head II to the skirt I2 will necessarily be concentrated in those portions of the skirt I2 adjacent the wrist pin bosses I3. Hence the skirt I2 will tend to expand in a radial direction, substantially aligned with the axis of the wrist pin bosses.

Those portions of the peripheral surface of the skirt I2 intermediate the wrist pin bosses must withstand the side thrust against the cylinder walls which is inherent in the operation of any piston and connecting rod system. Hence to avoid piston slap, it is desirable that the dimension of these bearing surface portions, 1. e., the dimension across the piston skirt in the direction perpendicular to the wrist pin axis, be substantially equal to the cylinder bore under starting conditions of the engine. Since these bearing sections of the piston skirt will also be heated during the normal operation of the engine, it is therefore imperative that radial expansion of the bearing section of the piston skirt be definitely limited, or preferably, entirely neutralized, so as to prevent a binding of the piston in the cylinder bore under abnormal temperature conditions.

To more effectively control the expansion of the bearing sections of the piston skirt, a cantilever section I6 is formed in such skirt by the well known inverted U-slot construction. A pair of circumferentially spaced, vertical slots I8 are cut into one of the bearing sections of the piston skirt I2 intermediate the wrist pin bosses I3, and the slots I8 extend from the horizontal saw cut I4 downwardly a substantial distance in the piston skirt wall, but do not extend entirely through such wall. Hence by controlling the expansion of the cantilever section I6, the effective clearance between the bearing section of the piston skirt I2 and the bore of the engine cylinder may be effectively controlled.

In accordance with this invention, an expansion control element 20 is provided for controllin the expansion of the cantilever section [6. In the modification illustrated in Figures 1 through 5 of the drawings, the control element 20 comprises a generally U-shaped rod which may be conveniently produced by bending operations upon a length of wire. The material of the control band 20 is, of course, selected from a group of metals having substantially lower coefiicient of thermal expansion than the material of the piston. Hence an iron or steel wire may be conveniently utilized.

The control wire 20 has an arcuately shaped bight portion 20a and a pair of spaced, parallel, substantially straight arm portions 202) which, when assembled to the piston I0, are disposed in a substantially horizontal position and lie vertically intermediate the wrist pin bosses I3 and the piston head II. The bight portion 20a of control wire 20 is preferably rigidly secured to the top end of the cantilever section I E of piston skirt I2 as by having at least a portion of the metal of such cantilever cast therearound, such as indicated at I 5a. Likewise, the straight arm portions 28b of the expansion control band 20 are respectively secured to the piers it by being cast therein. The free ends Etc of each of the arm portions 291) are preferably arcuately outturned and also imbedded in the piers l5-so as to provide a secure anchoring of the ends of the band 20.

i From the foregoing description, it will be noted that the entire contact of the wrist pin boss portions or piers E5 of the piston skirt !2 with the control band Ed is along the straight arm portions 2% of such band, while the arcuate bight portion 20d of the band is secured to the cantilever section it but is substantially unsecured to the remaining portions of the piston skirt. In fact, a preferred construction locates the vertical slots 18, which defines the cantilever section it, so as to pass adjacent those portions of the expansion control band 26 where the straight arm portions 20b join with the arcuate bight portion 2%..

With the aforedescribed. construction, it will be apparent that any heating of the head I: of the piston it will tend to produce a radially outward expansion of the wrist pin boss portions of the piston ski-rt E2 in a direction substantially parallel to the axis of the wrist pin bosses. Such expansion will, through the action of the expansion control band produce an inwardly di rected force upon the top of the cantilever section it of the piston skirt I2. However, it is equally important to note that any tendencies of wrist pin boss portions of the piston skirt 42 to expand in a direction perpendicular to the axis of the wrist pin bosses will be substantially unopposed by the expansion control band 21? inasmuch as such elements are engaged with only the straight arm portions 26b of the expansion control band 2t and hence may readily effect slight displacements along the length of such straight arm portions.

This latter action is a unique concept in the expansion control of light metal pistons. Those skilled in the art will recognize that any heated mass of metal will tend to expand concurrently in all directions. The piston constructions heretofore known, however, have permitted expansion of the wrist pin boss portions of the piston skirt only in a radially outward direction; parallel to the wrist pin axis. The expansion control bands heretofore utilized have had arcuate or non-linear portions disposed in the wrist pin bosses or the metal piers connecting such bosses to he head of the piston. As a result, the tendency of the wrist pin boss portions of the piston skirt to expand in a direction perpendicular to the axis of the wrist pin bosses has been substantially prevented by the incorporation of arcuate or nonlinear sections of the expansion control band therein. Hence, an excessive outward expansion along the axis of the wrist pin bosses was produced, resulting in an over accentuation of the hoop stretching effect, hence an over control of the piston expansion. With an expansion control element arranged in accordance with this invention, only the tendencies of the wrist pin boss portions of the piston skirt to expand in a radial direction parallel to the axis of the wrist pin bosses is effective in producing a restraining force upon the cantilever section it. The wrist pin boss portions of the skirt i 2 are substantially free to expand in a direction perpendicular to the wrist pin boss axis without any substantial restraint or interference from the expansion control band 20 and hence without producing over-controlling effects upon the cantilever section it.

b As the result, the piston construction embodying this invention has been found in practice to provide very desirable expansion characteristics, and a desired operating clearance between the piston skirt and the cylinder bore in which the piston is operating may be reliably and precisely obtained and maintained.

The described expansion control arrangement is not only desirable from the standpoint of improved performance but also results in a construction which be conveniently and economically manufactured. The most commonly known process for producing light metal pistons is that wherein the pistons are cast in a mold employing a remunerable, sectional core. Generally, the sectional core is in three pieces, respectively a center piece and two side pieces which respectively define the recesses for the wrist pin bosses;

and after the casting operation the core is removed from the casting by first axially with drawing the central section and then collapsing the two outer sections inwardly and then axially withdrawing these sections.

As is best shown in Figures 3 through 5, an expansion control band 2t embodying this invention may be very conveniently cast in proper po sition in the piston casting without complicating the casting procedure or the construction or" the mold or core sections. A core section 2i is shown being comprised of a central core section 2 la and substantially identical side core sections 2 lb and 250. The side core sections are each provided with recesses 2 id which, when the casting is finished, define the wrist pin bosses it of the finished piston. At a point intermediate the recesses 2 id and the top of the core section, a semi-peripheral groove 22 is formed in the outer wall of the core sections and is of sufficient extent to acconimodate an expansion control rod 2e therein. By virtue of the U-zhaped configuration of the expansion control rod 2 .3, such rod may be readily snappedinto position in the groove 22 and will be retained therein by its inherent resilience. Hence, no special supporting arrangements are required to support the expansion control els ment 2:? on the core 21 and, as a necessary corollary, there will be no recesses in the skirt wall of the finished piston casting. The downturned ends 2550 of the expansion control band it respectively lie in grooves Zie provided in each or" the side core sections 2 itand 2 lo. Such grooves of course define the piers id of the finished piston casting.

The remainder of the casing operation is completed in conventional manner. A pair oi mold halves are brought into surrounding relationship with respect to the core section 2! and define a space therebetween indicated by the dotted lines in Figures 3 and 4 which is filled by the molten metal poured into the mold define the completed piston casting. It is therefore apparent that an expansion control element may be easily yet accurately located with respect to the core section and cast into the metal of the skirt portion of the piston without introducing any complication of the normal casting procedure.

In Figures 5 and 7, l have illustrated a modified construction of an expansion control element which embodies the principles of this invention. The expansion control element iii be conveniently stamped from sheet metal, making fabrication very economical. Like the expansion control rod 26 heretofore described, the element 3:? comprises a U-shaped member having arcuately shaped bight portion 36a and substantially straight, parallel arm portions 3%, and downturned end portions c. It will be noted that the width of the expansion control band 36 is substantially greater than the thickness dimension and that the axis of the arcuate bight portion 39a is perpendicular to the width dimension but parallel to the thickness dimension of the band.

To incorporate this band construction in the skirt Wall of a piston, the bight portion 30a. is provided with a plurality of spaced aperture 30d, and an aperture 38c is provided in each or" the downturned end portions SEC. The band 39 is then arranged on the core section of the casting apparatus in a manner similar to that heretofore described in connection with the modification of Figures 1 to 5, and the band 3*] will then be cast in proper position in the piston 31 as indicated in Figure 7. As in the previous modification, the modified piston 3! has a skirt portion 32 defining opposed wrist pin bosses 33 and a cantilever section 34. The piston metal of the cantilever section 3 fills the apertures 38d in the bight portion of the expansion control band 3] and rigidly secures this portion of the band to the cantilever section. Likewise, the piston metal in the vicinity of the wrist pin bosses 33 flows into and fills the apertures 328 in the downturned end portions 33c of the expansion control band and hence rigidly secures the free ends of the expansion control band to the Wrist pin boss portions of the skirt. Since the arm portions 38b of the expansion control band are again substantially straight, any tendency of the wrist pin boss portion of tr e piston skirt to expand in a 1 direction perpendicular to the axis of the wrist pin bosses will be substantially unimpeded by the control band Ell.

In another modification of this invention illustrated in Figures 8 and 9 the expansion control band is again of such shape as to be conveniently formed by bending sheet metal. While the width of the expansion control band 49 is substantially greater than its thickness,

the axis of the arcuate bight portion 40a is this time substantially perpendicular to the thickness of the band and parallel to the width dimension of the band. A ain the arm portions 491) of the band are substantially straight, and

the end portions 480 are arcuately turned outwardly.

When cast into a piston 4: of the same general configuration as has heretofore been described, and as illustrated in Figure 9, the arouate bight portion 49a is surrounded by metal of the cantilever section 44 while the straight arm portions 40b of the expansion control band 45 pass through the piers 23 of the wrist pin bosses l2 and the outturned ends 400 are imbedded in one of such piers. the expansion control of this modified construction is identical to that heretofore described.

Still another modified piston construction embodying this invention is illustrated in Figures 10 and 11. Here the expansion control band 58 may again be formed as a sheet metal stamping. The band 58 is of generally U-shape configuration having a generally arcuately disposed bight portion 58a, parallel straight arm portions all?) and outturned end portions 500. The high; portion 55a, however, is suitably deformed so as to provide a generally undulating configuration as viewed in a horizontal cross section. Thus circumferentially spaced reent-ran portions d are formed in the bight por- The operation of tion a. Furthermore, each of the straight arm portions 5012 are deformed into angular cross section as best indicated in Figure 12.

When cast into a piston 5| of the same general construction as the modification of Figures 1 through 5, the bight portion 50a is secured to the cantilever section 54 of such piston by virtue of the piston metal filling the recesses defined by the reentrant portions 50d. To illustrate the adaptability of the principles of this invention to other forms of piston construction, the piston structure shown in Figure 11 embodies only a single pier 52 which communicates between the wrist pin bosses 53 and the head portion of the piston (not shown). The straight arm portions 50?) of the expansion control band 50 pass through the pier 53 and the outturned ends 590 and the expansion control band are imbedded in the thickened metal provided adjacent the wrist pin bosses 53.

From the foregoing description, it will be apparent to those skilled in the art that piston constructions embodying this invention will not only yield greatly improved expansion control characteristics but may be economically manufactured with existing equipment and by the practice of time-proven methods. It will, of course, be understood that various details of construction may be modified through a wide range without departing from the principles of this invention, and it is, therefore, not the purpose to limit the patent granted hereon otherwise than necessitated by the scope of the appended claims.

I claim as my invention:

1. A trunk piston formed of metal having a relatively high coefficient of expansion and having a head and a skirt, said skirt having wrist pin bosses integrally formed therein and being integrally connected to said head solely by piers of metal adjacent said bosses, said skirt having a cantilever bearing section formed therein intermediate said piers, said cantilever section being defined by a pair of spaced slots extending downwardly through said skirt but terminating short of the bottom of said skirt, and a narrow U-shaped band of flexible material having a substantially lower coeifi'cient of expansion, said band having its bight portion cast into the top of said cantilever section between the head and the wrist pin bosses, said band having substantially parallel straight arm portions cast into the metal of said piers intermediate said in bosses and said head portions, the free ends of said arm portions being outturned and respectively cast within the metal of said piers, the points of joinder of the straight arm portions with the bight portion lying proximately adjacent the spaced slots in said skirt.

2. For use as a thermal expansion control member for the cantilever portion of a U-slot, light metal piston, a U-shaped band of material having a substantially lower thermal expansion coefilcient than the piston material, the bight portion of said band being constructed and arranged to be cast into the free end of the piston cantilever section, the axis of the bight portion of said band being perpendicular to the thickness dimension of the band and parallel to the width dimension, the arm portions of said band being substantially straight and the ends of said arm portions being arcuately outturned for securement in the wrist pin boss piers of the piston, the width of said band being restricted so as to permit said arm portions to be located intermediate the wrist pin bosses of the pistons and the head portions.

3. For use as a thermal expansion control member for the cantilever section of a U-slot, light metal piston, a U-shaped band of material having a substantially lower thermal expansion coefficient than the piston material, the bight portion of said band being of arcuate configuration, the axis of said bight portion of said band being perpendicular to the width dimension of the band and parallel to the thickness, said bight portion being constructed and arranged to be cast into the top of the piston cantilever section and having a plurality of apertures therein to receive the piston metal, and the free ends of said band being arcuately outturned for respective securement in the wrist pin boss piers of the piston.

4. In a light metal, trunk piston having a head portion, a skirt portion, wrist pin bosses in said skirt portion, integral pillars connecting said bosses and said head portions, and an inverted U-slot in said skirt portion defining a cantilever bearing section, the improvements of a horizontally disposed, U-shaped expansion control member of material of substantially lower thermal expansion coefficient, said control member having substantially straight arm portions and an arcuate bight portion and being disposed in a horizontal position intermediate the piston bosses and said head portion, said arcuate bight portion being secured to said cantilever section by casting the piston metal therearound, said straight arm portions being respectively cast into said piers, the free ends of said arm portions being outturned and imbedded in said piers, and the junctures of said bight portion with said straight arm portions being respectively located adjacent the vertical slot portions of said inverted U-slot.

5. In a light metal trunk piston having a head portion, a skirt portion, wrist pin bosses in said skirt portion, integral pillars connecting said bosses and said head portion, and an inverted U-slot in said skirt portion defining a cantilever bearing section, the improvements of a horizontally disposed U-shaped expansion control member of material of substantially lower thermal expansion coefficient, said control member having a generally arcuate bight portion, said bight :7

portion being of undulating configuration in horizontal cross section and being secured to said cantilever section by casting the piston metal in said undulations, and said arm portions having outturned ends respectively anchored in said r piers, whereby the radial expansion of said cantilever section is controlled as a function of the radial expansion of the wrist pin boss portions of said skirt.

6. In a light metal trunk piston having a head portion, a skirt portion, wrist pin bosses in said skirt portion, integral pillars connecting said bosses and said head portion, and an inverted U-slot in said skirt portion defining a cantilever bearing section, the improvements of a horizontally disposed, U-shaped expansion control member of material of substantially lower thermal expansion coefiicient, said element having substantiall straight arm portions and a generally arcuate bight portion, said bight portion being of undulating configuration in horizontal cross section and being secured to said cantilever section by casting the piston metal into said undulations, said straight arm portions being cast into said piers and having outturned ends respectively anchored in said piers, whereby the radial expansion of said cantilever section is controlled as a function of the radial expansion of the pin boss portions of said skirt.

7. In a light metal trunk piston having a head portion, a skirt portion, wrist pin bosses in said skirt portion, integral pillars connecting said bosses and said head portion, and an inverted U-slot in said skirt portion defining a cantilever bearing section, the improvements of a horizontally disposed U-shaped expansion control member of material of substantially lower thermal expansion coefilcient than that of the piston, said element having substantially straight arm portions and an arcuate bight portion, said bight portion being secured to said cantilever section by casting of the piston metal therearound, said straight arm portions being respectively cast into said piers, the free ends of said arms being outturned and imbedded in said piers, said control member being of restricted width permitting the disposition of said member in a horizontal position intermediate the pin bosses and said head portion, and the junctures of said bight portion with said straight arm portions of the control member being respectively located adjacent the vertical slot portions of said inverted U-slot, whereby radial expansion of said skirt in the direction along an axis of said pin bosses produces a radially inward force on said cantilever section and expansion of the pin boss portions of said skirt in the direction generally parallel to the straight arm portions of the control member is permitted without substantial effect on said cantilever section.

8. In a light metal trunk piston having a head portion, a skirt portion, wrist pin bosses in said skirt portion, integral pillars connecting said bosses and said head portion, and an inverted U-slot in said skirt portion defining a cantilever bearing section, the improvements of a horizontally disposed U-shaped expansion control member of material of substantially lower thermal expansion coefficient than that of the piston, said element having substantially straight arm portions and an arcuate bight portion, said element having a substantially greater length than width and greater width than thickness and the axis of said bight portion of said band being perpendicular to the thickness dimension of the band and parallel to the width dimension, the width dimension of the band being restricted so as to permit the disposition of said straight arm portions intermediate the wrist pin bosses and the head portion of the piston, said bight portion of the band being secured to said cantilever section b casting of the piston metal therearound, and said strai ht arm portions being respectively cast into said piers, the free ends of said arms being outturned and imbedded in said piers, whereby radial expansion of said skirt in the direction along the axis of said pin bosses produces a radially inward force on said cantilever section and expansion on the pin boss portions of said skirt in a generally horizontal plane substantially transverse to the pin boss axis is permitted without substantial eilect on said cantilever section.

9. In a light metal trunk piston having a head portion, a skirt portion, wrist pin bosses in said skirt portion, integral pilars connecting said bosses and said head portion, and an inverted U-slot in said skirt portion defining a cantilever bearing section, the improvements of a horizontally disposed lJ-shaped expansion control band 11 of material of substantially lower thermal expansion coefficient, said band having substantially straight arm portions and an arcuate bight portion, said band having a width dimension substantially greater than its thickness dimension and the axis of the bight portion of the band being perpendicular to the width dimension of the band and parallel to the thickness dimension, said bight portion having a plurality of spaced apertures therein and being secured to the cantilever section of the piston by casting of the piston metal into said apertures, said straight arm portions of the band being respectively cast into said piers, and the free ends of said arms being outturned and imbedded in said piers, whereby radial expansion of said skirt in the direction along the axis of said pin boss produces a radially 12 inward force on said cantilever section and expansion of the pin boss portions of said skirt in the direction generally horizontally perpendicular to the pin boss axis is permitted without substantial restriction by said expansion control band.

DONALD D. DEMING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,129,180 Long Sept. 6, 1938 2,238,087 Bowser et a1 Apr. 15, 1941 2,426,732 Gates Sept. 2, 1947

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