US2787504A - Piston - Google Patents

Description

April 2 1957 F. JARDINE ET AL 2,787,504

PISTON Filed Jan. 7, 1955 s Sheets-Sheet 1 27 o Tm13 IN VEN TORS BY arr/e5 Mjvw/k April 2 ,v 1957 F. JARDINE ET AL 2,787,504

I N V EN TORS 7 /74/74 Jkrdhe 4/14 Y Java;

April 2 1957 F. JARDINE ET AL 2,787,504

PISTON Filed Jan. 7,1955 5 Sheets-Sheet 3 States PISTON Application January 7, 1955, Serial No. 480,342

6 Claims. (Cl. 309-11) This invention relates to an improved piston and more especially to one made of aluminum alloy which is adapted to serve in automotive engines having relatively short strokes. The term aluminum alloy, as used herein refers to those alloys containing at least 70% aluminum. Such common alloying elements as copper, silicon, magnesium, zinc and iron may be present in the proportions recognized in the art to obtain the properties desired.

Aluminum alloy pistons have been extensively and successfully used in internal combustion engines for many years. The combination of light weight and relatively high thermal conductivity has been an important factor in favor or" such pistons, but to take advantage of these properties it has been necessary to compensate for the higher thermal expansion of aluminum as compared to the iron and steel used in the engine block. This has been accomplished in different ways, such as by cutting slots in the skirt, by the use of low expansion ferrous metal struts or by providing openings in the skirt around the pin bosses or by combining these features with each other as well as with other means of limiting or controlling expansion. The aim in all cases has been to obtain as close a tit as possible between the piston and the cylinder compatible with conditions of load, speed and lubrication over the entire range of piston operating temperatures. Generally speaking, the various piston designs have resulted in effecting improvement in some respects but frequently at the expense of other features. The best designs thus far developed have had so-called shoulder areas at the junction of the pin bosses and the skirt which develop high contact pressures. The previous piston designs have not permitted maintenance of a uniform bearing area over the entire thrust face because they have suffered from a deflection of the skirt occasioned either from greater expansion of other portions of the piston or from a sharp temperature gradient within the skirt itself.

The problem of maintaining a close uniform clearance between the piston and the cylinder is largely one of controlling the expansion of the skirt portion of the piston. The expansion of the piston head and dependent ring belt do not offer any difficulty since the ring lands on the ring belt are normally machined to provide ample clearance, the actual contact between the piston and the cylinder being obtained through the use of piston rings located in the grooves between the ring lands. However, the skirt cannot be machined in a similar manner and the skirt does come in contact with the cylinder over a portion of its circumference. Moreover, the expansion of the shirt section is complicated by the fact that the temperature of the skirt is not uniform but diminishes with distance from the head. Various expedients have been tried to control the expansion of the skirt as indicated above, but none of them have been entirely effective in broadening the bearing area of the thrust faces and minimizing the danger of shoulder contact near the top of the skirt. i

atent G In addition to the foregoing difiiculties, there has been a demand for shorter pistons having greater diameters to permit construction of engines with shorter strokes and a lower over-all height. New problems have arisen in meetingthis demand because of the restriction of area allowed for dissipation of heat and the sharper thermal gradients as the height of the pistons has decreased. Also, there is a greater tendency for a short piston to rock in the cylinder with the resultant rounding of the corners of the piston rings and a reduction in their useful life. In addition, rocking of the piston produces an undesirable engine noise.

With these and other problems in mind, it is an object of our invention to provide a piston of the transverse slot type which has a closer clearance with the cylinder and wider bearing area than has been considered feasible heretofore. Another object is to provide a piston which is particularly adapted to serve in engines having relatively short connecting rods and short strokes. Still another object is to provide a piston having a low compression height but which is still exceptionally stable during reciprocation within the cylinder. A further object is to provide a piston, the deflection of the head of which is not unduly transmitted to the skirt with attendant induced stresses. Another object is to provide a piston having piston pin bearing areas approximately proportional to the explosion and inertia loads imposed upon the piston. A further object is to provide a piston having a lower compression height than previous automotive engine pistons yet without reduction in heat transfer to the cylinder walls and excessive wear of the piston rlngs.

These and other objects and advantages are realized in our invention as more particularly described below, one embodiment of which is shown in the accompanying drawings where Fig. 1 is a side elevation, partially in section across the thrust faces; I

Fig. 2 is a cross section through the vertical plane of the piston pin axis;

Fig. 3 is a view on line III-III of Fig. 1;

Fig. 4 is a view on line IVIV of Fig. 1;

Fig. 5 is a view on line VV of Fig. 1;

Fig. 6 is a bottom view of the piston, and

Fig. 7 is a fragmentary enlarged sectional view of the ring belt showing the narrow wear lands.

Our piston consists essentially of a head portion, a ring belt dependent therefrom, a full round thin wall type skirt below the ring belt and a pair of relatively closely spaced columns or piers depending from the head and laterally attached to the ring belt but partially separated from the skirt with aligned bores in the lower ends thereof for receiving the piston pin. The piston pin is located immediately below the ring belt and above the center line of the piston thus establishing what is known as a low compression height. By reason of the pier construction and low compression height the diam eter of the piston can be made nearly equal to its height and it is therefore possible to reduce the over-all height of the engine block thus lowering its cost and weight. The closer spacing of the pin bearings as compared to that in a conventional piston permits the use of .shorter and therefore more rigid piston pins. 'Moreover, the attachment of the columns or piers to the piston head and the short distance between the columns provides additional support for the head and greatly reduces any deflection thereof. This is of particular importance where the diameter of the piston is increased and where the explosion loads have been increased and higher compression ratios are employed. The increased rigidity of the head has'been accomplished without im-- posing undue stresses upon the skirt and developing but preferably on both thrust faces.

q a high shoulder wear areas. support from the skirt and. utilizing a relatively thin skirt wall a skirt flexibility is produced which permits the skirtitobecomeadjusted to the contour of the cyl'inder" wallsand yet: without substantial change in frictional horse power;. It may be further noted that it' is unnecessary to employ'a crown head on our piston since the construction. provides adequate rigidity with aflat head. However, it is still possible to use either a dome or recessed head if the engine manufacturer so desires.

The; ring belt. must be of sufiicient width to accounrnodate the necessary number of compression and oil rings but in any case it should not be long enough to increase the compression height of the piston to a. dimension exceeding 50% of the piston diameter. in recent piston. designs: two compression rings and one oil ring have been usedt'an'd this number can be employed on: our piston, however a greater or smaller number mayalso be; used. Although we haveshortened the ring. belt, this has not resulted; as might be expected, in a greater deflection. of the head" with increased skirt deformation and permanent changein thrust face contour as: experienced in. previous'ipistons. Also, the rigid lateral. connection. of the: columns or piers to the ring belt. and the relatively short. space between the columns serveto increase the rigidity of the: ring. belt. By reason of these-features'an. even. shorter ring belt can be employe'd than is. shown in. the accompanying figures and still retain satisfactory rigidity in. thering belt section.

The lands between the ring grooves must be properly proportioned to carry the loads imposed upon them by the rings, and to-furnish' an adequate path for transfer of heat to the rings which in turn transmit it to the cylinder walls. The top land is generally Wider than the others to provide a better path for heat flow from the head of the piston and to offer greater protection tothe-top piston ring from the high flame temperatures. Theoretically, it would. bevdesirable if theztop land could be in contact with the cylinder wall but this is. impractical, if not -impossible,.because of the. danger of scoring and seizure. However, limited contact between the -ring' belt and the cylinder canbe es'tablished: by providing slightly raised narrow bands or wear lands on the. lands which are too thinto' cause any scufling; problem, but are wide enough to permit new of heat from the piston head to the cylinder walls. construction feature will be referred to in greater'detail'belom.

The. skirt portlonof the pistonis: relatively thin to permit 'adequate flexibility. To accomplish this, the'skirt should have athickness on the'order ofa flyto A3 inch in thickness. Anincreased flexibility is also obtained through a partial separation: of thepin supports in the columns orpiers from the skirt. Still another feature contributing to flexibility. is that-of providing a; transverse slot at the top-of the'sk-irt on at leastone thrust face,

the skirt temperature on the thrust faces since theslots' act as heat'dams and consequently the thermal expansion of the skirtis reduced. The slots may 'be'located between the-skirt and the ringbelt or they maybe.- in the lowermost ring groove. Under some conditions it may even be desirable to add a vertical slot and thus form a well known T-slot. Generally speaking, the transverse 'slotshould be as long aspossible'to obtain the maximurn flexibility but in no case should-it extend any farther than the I junction of. the :pier with the :ring belt or skirt. In most cases-the slot should extend over an arc of about 100.

The increased flexibility-of the skirt minimizes areas of high bearing pressures thus reducing danger of sending orscoring. It also permits-a closer fit with a resultant reduction in noise whenstarting a coldenginesand yet the fit is satisfactory at engine operating temperatures.'

Still another advantage'of a fiexibleunrest'r'ained' skirt resides in improved oil. control. The bottom 'edge'of By separatingthepiston. pin.

Such slots reduce 4 theskirtis. in close contact withthe. cylinder all around its periphery and the bottom edge therefore acts as an oil scraper with resultant reduction in oil consumption.

Although not imperative, it is desirable to progressively reduce the thickness of the skirt wall from the region above and below the piston p-in bore to the mid point of the thrust faces and-thus form what may be called an eccentric skirt wall. It is advantageous to make: the wall from 25.10: 40% thicker above and below the pin bore than at'the. center of the'thrust faces. The object of tapering the skirt wail section in this way is to develop a uniform strength beam condition. This minimizes localization of high stresses as well as contributing to uniform contact pressures between the skirt and cylinder wall. The-taper can be produced in a variety of manners but for simplicity in manufacturing, we prefer to use an eccentric radius.

n-addition to tapering the. skirt walls, as mentioned above, itis oftenadvisable to provide a bead or rib of tapered shape either on both sides of the transverse slot or on the lower sid'e'in' order to strengthen that portion of the skirt. The rib or'beadI tapers from a maximum thickness at thcijunction of the'pier and the skirt to a minimum or zero thickness between said junction and the'ccntcr' of the thrustface-and thus offers a cantilever type of support to the skirt adjacent the slot.

The adaptation of: the. skirt to'the contour of the cylinder: can be further improved by grinding the exterior of theskirtto a slightly oval, elliptical or cam shape, the major axis of the shape. outline being in the direction of the. thrust faces, i. e. normal to. the piston pin axis. For the sake of convenience, all of these non-circular shapes are referred to-as cam ground shapes. The outline may be either symmetrical or non-symmetrical with respect to a vertical, plane: through thelpiston pin axis, as demanded by the characteristics of the. piston with respect to the degree of thermal control incorporated, flexibility of the skirt. and engine loadv conditions. The term thermal control" as used here, referstto. the ability of the piston to. retard the. expansion ofthethrust faces under varying thermal conditions. In additionto the foregoing, it is desirable tovery slightly taper the exterior of the skirt in avertical directiomfor example, the bottom of the Skirtmayibe.from:0.0005 to tlOOZinch greater in diametcr than the top of the skirt. The amount of taper will vary with the. engines and: type of service.

Through-use of the foregoing features affecting fiexibility andf' contour of-the.skirt. it is possible to compensate .for ithermal "expansionran'd :to/provideagreater bearingiarea' without creatingobjectionable regions of high contact.- pressures. Moreover, we have found that the bearing areaon the thrustfaces ofour piston is enlarged by at'lcast.25'% as'c'ompared to conventional pistons now inuse. The bearingsarea also'extends-downwardly and laterally around the skirt so that all ornearly all of the lower skirt periphery isxin contact with the cylinder thereby reducing nil consumptionas mentioned above.

Although in most cases theskirtic'an be. of a uniform length or height around the entire circumference of the pistomit may be desirable in some cases to cut away a portion under 'the pin bores'lto 1 permit clearance between the piston and counterweights onthe crankshaft when the-fpiston is atlthe'bottom. of'the stroke. Of course, the contour of the. bottomtedge 'of the skirt may be altered for other reasons. .toraccommodateit' to aparticular eugine or to reduce Weightoftherpistonbut such alteration doeslnot affect the essentialifeatures ofrour improved piston as herein :described.

Theitexplosionload' is carried directly to the piston pin from the; head through a pair of straight closely spaced aligneddepending columns or piers. This construction eliminates to a"very..great.extent the need for reinforcing ribs'under the head and wells between the head and pin hearings. lt lalso greatly simplifies the manufacture of the pistons since there are no'undercut cosmos {sections or otherv structuralidetails whichrequire the use Lof collapsible cores;

In addition to carrying the piston pin, the columns or piers are rigidly attached to the ring belt thereby stiffening it. The separation of the piers from the skirt not only increases the flexibility of the skirt but their closer spacing permits the use of shorter piston pins. The reduction in the piston pin span makes possible increased rigidity of the pin for a given pin diameter and minimizes deflection of the pinhole center line which would result in bending of the piers; r The separation of the piers from the skirt wall also isolates any bending of these members and prevents the skirt distortion which occurs in conventional pistons with integral skirt and boss construction.

A useful and novel feature of our piston is that of providing additional metal for balancing purposes at the lower end of the piers or columns or in the head, but not at the bottom of the skirt; The balancing metal can be easily removed with simple tools and without distortion of the skirt such as frequently happens in pistons of conventional design where it has been a common practice to provide balance metal in the form of pads near the open end or bottom of the skirt. Any metal remaining in the pads after balancing tends to stiffen the skirt and reduce its flexibility.

To further strengthen the piston head, it is advisable to provide a wide shallow rib between the top of the columns or piers. Such a rib does not entail any problem from a manufacturing standpoint and yet supplies any needed reinforcement at the center of the head, adds to the rigidity of the head, ring belt and pin supporting structures.

The piers or columns are laterally joined to the ring belt and to any portion of the skirt below the belt and above the piston pin bore. in our preferred practice the attachment of the piers or columns to the skirt extends downwardly to a line not lower than approximately the center line of the piston pin bores. in any case the joined section should extend downwardly to a level so that a greater bearing area is provided in the upper por- .tion of the pin here than in the lower portion. In this manner the explosion is carried on a greater bearing, area than the inertia load. Thus, the bearing areas are roughiy proportioned to the loads they are intended to bear. This results in a more efficient distribution of metal in the pin bearing without a detrimental. increase in contact pressures.

As indicated above, our improved piston permits the use of shorter piston pins than have been required in conventional pistons Where the pin boss is integral with r the skirt. However, the pins used in our pistons should be long enoughto take advantage of the increased bearing area provided in the upper portion of the pin bore. The piston pins may be secured in place in customary manner, for example, they may be pressed into or clamped in the connecting rod or they may be of the floating type and locked in position with a spring ring positioned in complementary circular grooves in the pin and here. To facilitate insertion and removal of the pins it may be desirable to enlarge the bore in the skirt section. if the piston is cast, the enlarged opening can be easily produced by appropriately shaping the core pins. On the other hand, if the piston is forged or extruded'and the bore must be drilled, an enlarged bore through the I skirt can be conveniently machined.

The arrangement of. the piers or columns with respect to the head, ring belt, skirt and pin bores has served to reduce the compression height of the piston, that is, the distance from the top of'the piston to the center line of the piston pin. In prior automotive piston design it has been conventional to make this. distance more than 50% of the piston diameter. In our. piston this distance is preferably less than 45% of. the diameter but should not exceed one-half the diameter of the piston. This decrease in height is accomplished withoutany loss in rigidity of the head or. flexibility of theskirtor stability ofthe-piston within the cylinder- Moreover, the heat canbedissipated from the head Withsufficient rapidity to prevent. overheatmg.

Tov increase the paths of heat flowfrom the-piston. head to the cylinder a narrow band, also referred to as a Wear land, can be machined on one or more lands as the ring grooves are: cut, which will produce. an interference fit with the cylinder. Such wear lands are narrow enough andvv thin enough to quickly acquire the contour of the cylinder over which they travel. The lands, for example, may be only ,4, inch in width and 0.010 inch in depth. They do not scuff or scorethe cylinder walls and yetthey provide a tight seal and a good path to dissipate heat from the piston to the cylinder. If these lands areprovided on more than one land, the piston is extremely well stabilized and rockingjs all butzeliminatedr One embodiment of the piston described above is. illustrated in the accompanying drawings, Fig. 1 showing both the external appearance and a vertical section in the plane of the piston pin axis. With reference to the features of the piston it will be seen that head 10 is provided with a dependent ring belt 12 having ring lands 14, 1 6, 18 and '20 at the periphery between which are spaced rings grooves 22, 24 and 26. In this design, grooves 22 and carry compression rings while groove 26 is adapted to receive an oil ring, suitable oil holes 27 being provided in the base of the groove, as seenin Figs. land 3.

Immediately below the ring belt. 12 is skirt 28 which is separated therefrom at the thrustv faces by transverse slots 46. The skirt 28 hasan upper end 30, the diameter of which is slightly smaller, less than 0.002 inch, than the diameter of the lower end 32. Also, if desired, the thickness of the skirt may gradually diminish from a maximum thickness in the region of the vertical plane of the center line of the pin bore to a minimum at the midpoint of the thrustv faces 54 and. 56. The skirt, it will be noticed, is of full round design, no openings being present below the transverse slots other than the holes for insertion or removal of the piston pins.

A transverse slot 46 which terminates in drilled holes 48 may be used on but one thrust face, preferably the minimum thrust face, identified as 54 in Fig. 5, but for the best results, slots should be provided on both thrust faces as illustrated. To be most effective, the transverse slots should extend across the entire thrust. face, preferably through an arc of about as seen more clearly in the section appearing in Fig; 4 in the plane of the slots, and terminate in holes 48 located in a thickened Wall section, the inception of fillet 50, also shown in the same figure. Although the terminating holes 48 are shown drilled parallel to the piston pin axis it is to be understood that they may also be drilled in a radial direction. In addition, it is to be noted that a tapered bead 49 is provided at the underside of slots 46 which serves to strengthen the top of the skirt at the slot. The slots, as mentioned above, serve to interrupt the heat flow from the head and ring belt to the thrust face portion of the skirt and in addition they permit independent expansion and contraction of the ring belt and skirt.

A pair of parallel columns or piers 34 integrally joined to head 10 and ring belt 12 carry the explosion load: to the piston pin operating in bore 36 in the pin support portion 37 in the lower part of the columns as shown in Figs. 1, 2 and 3. Below the support section 37 and bore 36 at the lower end of the columns extra metal 38 is provided for balancing purposes. The columns or piers 34 are also laterally and integrally attached'to the portion of the skirt adjacent the columns and above the center line of the piston pin bore as more clearly shown in Figs. 2 and 4. A fillet 50 is provided to blend the columns into the ring belt and skirt sections; Since the columns are laterally connected to the skir'tdown to a level above the center line of the pin bore 36, the bore traverses the zone of connection and thus provides a larger'bearing area 40 in' the upper partofthe bore than in thelower part 42. .The enlarged'upper bearing surface, of course, carries'the explosion load while the lower and smaller bearing area takes the inertia load. The 'upper and lower bearing areas are approximately proportional to the loads they carry. The columns or piers are separated from the skirtbelow the center line of the piston pin bore by a space or recess terminating in curved surface 44.

To improve the operation of the piston the pin bore 36 is preferably slightly offset from the true center line in the direction of the maximum thrust face 56. The offset is apparentfrom the sectional views in Figs. 3, 4 and 5 and the bottom view in Fig. 6. However, it is to be understoodthat the bore may be centrally located, if

desired.

The columns or piers 34 are relatively closely spaced as compared to the conventional type of piston where the pin bosses are integrally joined to the skirt and thereby reduce the span of the piston pin. To reinforce the head, a shallow rib 52 is provided between the columns, as seen in Figs. 2, 3 and 6, which is parallel with the pin axls.

The thrust faces of the piston and the bearing areas on these faces are better shown in Fig. 5 where the minimum thrust face is indicated at 54 and the maximum thrust face at 56. The bearing area on these faces generally extends over at leastthe distance between points 58 which in this case represents an arc of at least 90 and is. considerably greater than that found on conventional pistons used heretofore. The bearing area becomes greater as the bottom of the skirt is approached, and merges into a contact area which encircles or nearly encirclesthe skirt. It is not to ,be inferred, however, that the bearing areas are equal on both thrust faces since in many cases the bearing area is greater on the maximum than on the minimum face depending upon the load and contour of the skirt. However, our piston still provides a greater bearing area with minimum evidence of shouldersi than have characterized pistons of the prior art with less flexible skirts.

To secure a closer fit with thecylinder under full load conditions, narrow wear lands 60 may be provided on ring lands 14, 16, 18 and 20 as more clearly seen in the enlarged view in Fig. 7. These lands do not score or scuff the cylinder wall and yet they provide a sufficiently "close contact to improve the transmission of heat from the piston to the cylinder. 7 The'fatigue life of pistons made in accordance with our invention is verymuch greater than that of pistons made in accordance with previous designs. in a comparative test at simulated operating temperatures a piston of the type described above weighing but 1.20 pounds withstood one million cycles under a head load of 1,250 p. s. i. whereas an aluminum piston of the steel strut type of the same diameter weighing 1.14 pounds failed after 500,000 cycles at a load of only 650 p. s.i.

4 The heat distribution and temperatures attained in various parts of a piston of the type described above have been demonstrated in a bench test where the piston head was'subjected to a constant heat input at a temperature of 640 F. and the heat flowed to all parts at a constant rate with the result that constant temperatures were obtained. The test conditions simulated in large degree the conditions existing during full load operation of an engine. Under the testconditions the center of the head reached altemperature of 402 F., the pier in the region of the pin bore attained a temperature of 352 F., at the top of skirt adjacent the transverse slot the temperature was 211 F. while at the bottom of the skirt the temperature was 192 F. These values indicate the rather uniform temperature found in the skirt which can in part account ;for its uniform expansion characteristics. a I-Iayingthus; describedflour invention, we claim: a

1. A piston comprising a head portion, a ring belt dependent therefrom, a flexible skirt having two opposed thrust faces below said ring belt and connected with said ring belt above openingsprovided in the skirt for insertion of a piston pin, said skirt also being full round over at least its lower portion thereof and separated from said belt by a transverse slot at the top of at least one of the two opposed thrust faces, and a pair of aligned piers spaced inwardly of said ring belt and said skirt, said piers being integrally joined to the head at their upper ends and laterally joined to the ring belt over its entire depth but separated from said skirt below said lateral connection, said piers also having aligned piston pin bores adjacent their lower ends for receiving a piston pin, said bores being spaced from the head such that the distance from the top surface of the head to the center line of the bores is not more than one-half the diameter of the piston, said pin bores also traversing the lower end of the lateral connection between said piers and said ring belt whereby the upper portion of the bore continues beyond the piers toward the skirt but the lower portion of the pin bore terminates at the outer side of the piers, and said skirt having circular openings therein aligned with said pin bores of a diameter no smaller than that of the piston pin bores for insertion of the piston pin.

2. A piston comprising a head portion, a ring belt dependent therefrom, a flexible skirt having two opposed thrust faces below said ring belt and connected with said ring belt above openings provided in the skirt for insertion of a piston pin, said skirt also being full round over at least its lower portion thereof and separated from said belt by a transverse slot at the top of each of the two opposed thrust faces, a pair of aligned piers spaced inwardly of said ring belt and said skirt, said piers being integrally joined to the head at their upper ends and laterally joined to the ring belt over its entire depth and to the upper portion of the skirt extending no lower than the center line of the piston pin bore but separated from said skirt below said region of lateral connection, a uni formly tapered rib at least at the lower edge of the transverse slot, said rib starting at the junction of the piers and skirt and gradually diminishing to a minimum thickness toward the midpoint of the thrust face, said piers also having aligned piston pin bores adjacent their lower ends for receiving a piston pin, said bores being spaced from the head such that the distance from the top surface of the head to the center line of the bores is not more than one-half the diameter of the piston, said pin bores also traversing the lower end of the lateral connection between the piers, ring belt and skirt whereby the upper portion of the bore continues beyond the piers toward the skirt but the lower portion of the pin bore terminates at the outer side of the piers, and said skirt having circular openings therein aligned with said pin bores of a diameter no smaller than that of the piston pin bores for insertion of the piston pin, said skirt being characterized by having a bearing pattern extending over an arc of at least across the thrust faces. I

3. A piston comprising a head portion, a ring belt dependent therefrom, a flexible skirt having two opposed thrust faces below said ring belt and connected with said ring belt above openings provided in the skirt for insertion of a piston pin, said skirt also being full round over at least its lower portion thereof and separated from said belt by a transverse slot at the top of at least one of the two opposed thrust faces, the wall of said skirt tapering .from ,a maximum to a minimum thickness, the maximum thickness being in the vertical plane through the center line-of the piston pin bore which gradually diminishes to a minimum thickness at the midpoint of the thrust faces, said maximum wall thickness being from 25 to 40% greater than said minimum thickness, and a pair of aligned piers spaced inwardly of said ring belt and said skirt, said piersbeing integrally joined to the head at their upper ends and laterally joined to the ring'belt over its entire depth and tothe uppcr'portion of the skirt extending no lower than the center line of the piston pin bore but separated from said skirt below said region of lateral connection, said piers also having aligned piston pin bores adjacent their lower ends for receiving a piston pin, said bores being spaced from the head such that the distance from the top surface of the head to the center line of the bores is not more than one-half the diameter of the piston, said pin bores also traversing the lower end of the lateral connection between the piers, ring belt and skirt whereby the upper portion of the bore continues beyond the piers toward the skirt but the lower portion of the pin bore terminates at the outer side of the piers, and said skirt having circular openings therein aligned with said pin bores of a diameter no smaller than that of the piston pin bores for insertion of the piston pin, said skirt being characterized by having a bearing pattern extending over an arc of at least 90 across the thrust faces.

4. A piston comprising a head portion, a ring belt dependent therefrom, a flexible skirt having two opposed thrust faces below said ring belt and connected with said ring belt above openings provided in the skirt for insertion of a piston pin, said skirt also being full round over at least its lower portion thereof and separated from said belt by a transverse slot at the top of at least one of the two thrust faces, and a pair of aligned piers spaced inwardly of said ring belt and said skirt, said piers being integrally joined to the head at their upper ends and laterally joined to the ring belt over its entire depth and to the upper portion of the skirt extending no lower than the center line of the piston pin here, but separated from said skirt below said region of lateral connection, said piers also having aligned piston pin bores adjacent their lower end for receiving a piston pin, said bores being spaced from the head such that the distance from the top surface of the head to the center line of the bores is not more than one-half the diameter of the piston, said pin bores also traversing the lower end of the lateral connection between the piers, ring belt and skirt whereby the upper portion of the bore continues beyond the piers toward the skirt but the lower portion of the pin bore terminates at the outer side of the piers, said skirt having circular openings therein aligned with said pin bores of a diameter no smaller than that of the piston pin bores for insertion of the piston pin, and said skirt having an external non-circular outline of cam ground contour, said skirt being characterized by having a bearing pattern extending over an arc of at least 90' across the thrust faces.

5. A piston comprising a head portion, a ring belt dependent therefrom, a flexible skirt having two opposed thrust faces below said ring belt and connected with said ring belt above openings provided in the skirt for insertion of a piston pin, said skirt also being separated from said belt by a transverse slot at the top of at least one of the two opposed thrust faces, the wall of said skirt tapering from a maximum to a minimum thickness, the maximum thickness being in the vertical plane through the center line of the piston pin bore which gradually diminishes to a minimum thickness at the midpoint of the thrust faces, said maximum wall thickness being from 25 to 40% greater than said minimum thickness, a pair of aligned parallel piers spaced inwardly of said ring belt and said skirt, said piers being integrally joined to the head at their upper ends and laterally joined to the ring belt over its entire depth and to the upper portion of the skirt extending no lower than the center line of the piston pin bore but separated from said skirt below said region of lateral connection, a tapered rib at least at the lower edge of the transverse slot, said rib starting at the junction of the piers and skirt and gradually diminishing to a minimum thickness toward the midpoint of the thrust face, said piers also having aligned piston pin bores adjacent the lower ends for receiving a piston pin, said bores being spaced from the head such that the distance from the top surface of the head to the center line of the bores is not more than one-half the diameter of the piston, said pin bores also traversing the lower end of the lateral connection between the piers, the ring belt and the skirt whereby the upper portion of the bore continues beyond the piers toward the skirt but the lower portion of the pin bore terminates at the outer side of the piers, said skirt having circular openings therein aligned with said pin bores of a diameter no smaller than that of the piston pin bores for insertion of the piston pin, and said skirt having an external non-circular outline of cam ground contour, said skirt being characterized by having a bearing pattern extending over an arc of at least across the thrust faces.

6. A piston comprising a head portion, a ring belt dependent therefrom, a flexible skirt having two opposed thrust faces below said ring belt and connected with said ring belt above openings provided in the skirt for insertion of a piston pin, said skirt also being full round over at least its lower portion and separated from said belt by a transverse slot at the top of at least one of the two opposed thrust faces, the wall of said skirt tapering from a maximum to a minimum thickness, the maximum thickness being in the vertical plane through the center line of the piston pin bore which gradually diminishes to a minimum thickness at the midpoint of the thrust faces, said maximum wall thickness being from 25 to 40% greater than said minimum thickness, a pair of aligned parallel piers spaced inwardly of said ring belt and said skirt, said piers being integrally joined to the head at their upper ends and laterally joined to the ring belt over its entire depth and to the upper portion of the skirt extending no lower than the center line of the piston pin bore but separated from said skirt below said region of lateral connection, a uniformly tapered rib at least at the lower edge of the transverse slot, said rib starting at the junction of the piers and skirt and gradually diminishing to a minimum thickness toward the midpoint of the thrust face, said piers also having aligned piston pin bores adjacent the lower ends for receiving a piston pin, said bores being spaced from the head such that the distance from the top surface of the head to the center line of the bores is not more than one-half the diameter of the piston, said pin bores also traversing the lower end of the lateral connection between the piers, the ring belt and the skirt whereby the upper portion of the bore continues beyond the piers toward the skirt but the lower portion of the pin bore terminates at the outer side of the piers, said skirt having circular openings therein aligned with said pin bores of a diameter no smaller than that of the piston References Cited in the file of this patent UNITED STATES PATENTS 2,120,019 Cooper June 7, 1938 2,130,923 Jardine Sept. 20, 1938 2,506,811 Rodger May 9, 1950 FOREIGN PATENTS 885,516 France May 31, 1943 608,142 Great Britain Sept. 10, 1948

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