referring now to the figures in greater detail , fig1 shows a vertical section through a small single cylinder hermetic refrigeration compressor of the type commonly used for household refrigerators and food freezers . the compressor includes a shell 10 comprising an upper half 11 and lower half 12 which are joined together along a joint indicated at 14 to provide a hermetic enclosure for the internal compressor mechanism . it should be noted that the lower half 12 includes a depressed oil reservoir section 16 at the center , and it also has mounting feet 17 welded to the outside for mounting the compressor through rubber bushings and suitable frame members of the refrigerator or food freezer . the internal pumping mechanism includes a cylinder block 20 which is resiliently mounted within the shell 10 by suitable means such as spring 21 secured to the underside of the cylinder block 20 and engaging a projecting spring support 23 welded on the inside of the lower shell half 12 . it will be understood that the spring supports are three or four in number and are so positioned and constructed to support the pump mechanism in both the stationary and running positions . a motor stator 24 is bolted or otherwise securely fastened on the upper side of the cylinder block 20 , which also has secured to it at the center a bearing boss 26 , which may either be integral or a separate member as shown . the bearing boss 26 has a vertically extending bore 25 therein which serves to journal a vertically extending crankshaft 27 . above the bearing boss 26 a rotor 29 is press - fitted on the end of crankshaft 27 to rotate within the stator 24 , and the rotor 29 includes a thrust bearing 28 engaging the upper end of the bearing boss 26 to absorb the vertical loads and weight of the rotor 29 and crankshaft 27 . at its lower end , crankshaft 27 has an offset crank pin 31 , and at its bottom end is an angularly extending oil pickup tube 32 extending angularly from the offset crank pin 31 to have a lower end submersed in the oil reservoir 16 and located on the axis of rotation of crankshaft 27 . thus , the oil pickup tube 32 , because of the portion extending at an angle upwardly and away from the axis of rotation , serves as a centrifugal pump to force oil from the oil reservoir 16 up into the interior of the crankshaft 27 through various internal passages ( not shown ) which provide oil to the surface of the crank pin 31 and the bearings in the bearing boss 26 . it will be understood that the top end 33 of crankshaft 27 extends upward above the rotor 29 and stator 24 to fit within an inverted cup 34 secured to the inside of the shell upper half 11 . the cup 34 normally has a substantial clearance from the crankshaft end 33 , but does serve as a restraint to movement of the entire assembly on the support springs 21 during shipping . in alignment with the crank pin 31 , the cylinder block 20 has a cylinder bore 35 extending away from the crank pin 31 and opening onto an end face 36 of the cylinder block , which is closed off by means of a cylinder head 37 including a suction muffler 38 and a valve plate ( not shown ) for mounting the valve structures for providing the necessary suction and discharge valves for the compressor in the manner well known in the art . the pumping action is provided by a piston 40 slidably mounted within the cylinder bore 35 , and a connecting rod 41 has a big end 42 journaled on the crank pin 31 and the opposite end connected to a wrist pin 44 mounted in the transverse bore 46 in the piston 40 . as shown in fig2 and 3 , there is a longitudinal oil passage 48 in the connecting rod 41 extending from the big end 42 to the wrist pin 44 for conducting oil from the crank pin 31 to the wrist pin 44 , where the oil may be distributed around an annular groove 49 formed in the middle of wrist pin 44 . wrist pin 44 is nonrotatably held in the transverse bore 46 by a suitable lock pin 51 extending through the piston 40 and into an opening provided on the wrist pin . it will be understood that the above - described structure is conventional and well known in the art , and the above description serves primarily as background for the following description of the preferred embodiment of the present invention . an oil film must be present between the piston 40 and cylinder bore 35 not only to provide lubrication between these parts as the piston moves back and forth in the cylinder , but also to provide sealing action to prevent blow - by of the refrigerant gas being compressed back past the piston into the space within the shell 10 . heretofore , the oil has been supplied to the walls of cylinder bore 35 and the surface of piston 40 from several sources . the first of these is the flow of oil around the wrist pin 44 as supplied through the oil passage 48 along the sides of the transverse bore 46 to flow out the ends of the bore and to the walls of the cylinder bore 35 . it has generally been found that the amount of oil supplied to the walls of cylinder bore 35 from this source is rather small because of the generally tight fit between the wrist pin 44 and transverse bore 46 . a main source of oil to the head end of cylinder bore 35 is through the oil that is mixed with the returning refrigerant gas and passes into the suction muffler 38 , from which the oil can pass into the cylinder on its suction stroke past the suction valve ( not shown ). another source of oil for the skirt end of the piston and cylinder bore is the oil that leaks out of the joint between the big end 42 of connecting rod 41 and the crank pin 31 , and therefore flies off by centrifugal force , with some of the oil landing within the lower end of cylinder bore 35 when the piston is on the forward stroke . in addition to the above , some compressors have been supplied with oil suction tubes , such as that shown at 53 ( see fig1 ). a small capillary tube has one end immersed in the oil reservoir 16 and has the other end entering into the suction plenum to provide additional oil which enters the cylinder bore through the suction valve . the presence of oil in the cylinder head area is particularly important not only to supply oil to the cylinder bore , but also to provide lubrication to the suction and discharge valves as well as to provide a flow of cooling oil to these areas , which become heated from the compressed refrigerant gases particularly in the areas of the discharge valve and discharge plenum within the cylinder head 37 . in accordance with the present invention , advantage is taken of the fact that there is a certain amount of oil flowing from the upper end of the bearing boss . accordingly , an oil supply recess 55 is located on the cylinder block 20 directly above the cylinder bore 35 , and a certain amount of oil from the bearing boss will flow downward off the motor stator 24 into the oil recess 55 , where it normally tends to accumulate . an oil feed hole 57 is provided in the form of a small vertical bore extending through the cylinder block 20 from the oil supply recess 55 into the cylinder bore 35 adjacent its midpoint . the piston 40 has a circular side wall which is provided with an elongated annular oil groove 59 which is of only slightly reduced diameter defining a cylindrical surface having a head edge 61 adjacent the cylinder head 37 and a skirt edge 62 adjacent the crankshaft 27 . the groove 59 has a depth of only about 0 . 010 to 0 . 020 inch and the edges 61 and 62 are provided with filets and radiuses to avoid any burrs and sharp edges . the spacing between the edges 61 and 62 is preferably such as to provide a maximum groove width while also providing adequate load support area , and therefore leaves a head land 64 at the top end of piston 40 adapted to make a tight sliding fit within the cylinder bore , as well as a skirt land 66 at the bottom end of the piston . as shown at fig2 and 3 , it can be seen that the oil groove 59 and the oil feed hole 57 are so positioned that at bottom dead center of the piston , as shown in fig3 the head land 64 , depending on the length of the piston stroke , may partly , but never completely , block the oil hole 57 , so that substantially all of the area of the head land 64 is available to provide a sealing fit with the cylinder bore 35 as the piston begins it compression stroke . on the other hand , when the piston is at top dead center , as shown in fig2 it is desirable , but not necessary , that the skirt land 66 be positioned to slightly uncover a portion of the oil feed hole 57 to allow additional oil to drip into the cylinder bore 35 to ensure adequate lubrication in the area of the skirt land 66 during reciprocation of the piston . it will therefore be seen that the presence of the oil at the midportion of the cylinder bore 35 through the oil feed hole 57 provides an additional oil supply in this area to ensure better sealing and better lubrication of the piston at the head and skirt lands 64 and 66 , and , because of the reduced area on the piston in contact with the cylinder bore , friction is slightly reduced . according to another feature of the present invention , to ensure positive supply of fresh oil through the feed hole 57 , means are provided for oil within the oil groove 59 to escape from the groove 59 to ensure a constant positive flow of oil in this area and to provide better cooling . as shown in fig4 and 5 , this may be accomplished by the use of passage means in the form of bleed holes 68 , which may be one or two in number , extending inwardly through the piston from the oil groove 59 into the interior cavity 69 . the total area of the bleed hole or holes 68 is such as to provide a reduced flow rate into the interior cavity 69 below the flow rate of the oil that can be admitted through the oil feed hole 57 to ensure that the oil groove 59 is always maintained full of oil at all times , for improved cooling and sealing of the area between the piston 40 and cylinder bore 35 . as an alternative arrangement , as shown in fig6 and 7 , instead of the use of a bleed hole 68 , it is possible to provide a passage in the form of a small flat or groove 71 on the skirt land 66 to allow oil to flow from the oil groove 59 past the skirt land 66 into the lower end of cylinder bore 35 . again , the size of the flat 71 must be such that the outflow past the flat 71 can never be as great as the inflow rate through the feed hole 57 to ensure that the oil groove 59 remains full of oil at all times during operation of the compressor . although several preferred embodiments of this invention have been shown and described in detail , it is recognized that these are by way of example only , and various modifications and rearrangements may be resorted to without departing from the scope of the invention as defined in the claims .