in fig1 - 4 , an e - type coupler embodying the improved design of the present invention is shown generally at 10 . the coupler 10 comprises generally a coupler head 12 and a shank 14 which is adapted to be mounted in a draft gear ( not shown ) on the end of a car sill ( not shown ) of a railway vehicle ( not shown ). the knuckle side of coupler head 12 is shown at 16 and the guard arm side at 18 . a rotary lock - lift assembly 20 is mounted on coupler head 12 to release a knuckle 22 from its closed position . coupler head 12 has a front face 24 including a throat portion 26 extending toward knuckle side 16 in a curved manner toward upper pivot lug 28 and lower pivot lug 30 . located behind pivot lugs 28 and 30 are buffing shoulders 32 and 34 which form a pocket for receiving knuckle 22 . projecting from buffing shoulders 32 and 34 are upper pulling lug 36 and lower pulling lug 38 which are engaged by corresponding pulling surfaces 40 and 42 respectively of knuckle 22 . upper pivot lug 28 narrows toward knuckle 22 into pin protector 44 . likewise , lower pivot lug 30 narrows toward knuckle 22 into pin protector 46 . upper pivot lug 28 and lower pivot lug 30 include pin apertures 48 and 50 respectively having elongated non - circular cross - sections . it is these apertures 48 and 50 combined with a third aligned aperture 52 in knuckle 22 having a circular cross - section that form a single passageway 54 through which knuckle pin 56 is received . pin apertures 48 and 50 may have a major axis x -- x being in the same direction as the longitudinal axis of coupler 10 and the minor axis y -- y being in a direction transverse to the longitudinal axis of coupler 10 as shown in fig6 . the elongated pin apertures 48 and 50 provide the slack necessary for the primary pulling load to be transmitted from knuckle 22 to pulling lugs 36 and 38 as well as pin protectors 44 and 46 without exerting excessive loads on knuckle pin 56 . they also provide the slack necessary for the primary buff load to be transmitted from knuckle 22 to buffing shoulders 32 and 34 and pin protectors 44 and 46 again without exerting excessive loads on knuckle pin 56 . coupler head 12 includes outwardly protruding flange sections 58 and 60 from the top surface 62 of upper pivot lug 28 and the bottom surface 64 of lower pivot lug 30 respectively . in addition to pulling and buffing loads on coupler head 12 during use , a bending load is present which acts to pull upper pivot lug 28 away from lower pivot lug 30 . these flange sections 58 and 60 provide the necessary strength to preclude pivot lugs 28 and 30 from bending away from each other . it is downwardly protruding flange section 60 upon which the cotter pin of the prior art has become caught thus restricting knuckle pin 56 from further rotation and reducing the fatigue life of knuckle pin 56 . knuckle pin 56 comprises a longitudinal cylindrical member having a circular cross - section and a length greater than that of passageway 54 such that a bottom portion 66 of knuckle pin 56 extends downwardly from bottom surface 64 of lower pivot lug 30 when knuckle pin 56 is placed within passageway 54 . knuckle pin 56 has an enlarged cap 68 having a diameter greater than that of passageway 54 so as to abut the top surface 62 of upper pivot lug 28 and preclude knuckle pin 56 from moving downwardly during use . according to the present invention , knuckle pin 56 is locked within passageway 54 and restricted from unintentional upward removal during use by retaining means that are free to fully rotate with knuckle pin 56 . for the sake of convenience , it is preferred that the retaining means be located on or near the bottom portion 66 of knuckle pin 56 that extends downwardly from the bottom surface 64 of lower pivot lug 30 . it is important that the retaining means conform closely to the surface of cylindrical knuckle pin 56 so as not to become caught upon downwardly protruding flange section 60 . at the same time , it is also important that retaining means project from the surface of knuckle pin 56 at least enough so as not to pass upwardly through passageway 54 . the retaining means may take the form of one or more surface indentations on knuckle pin 56 combined with an arcuate gripping member which at least partially encircles knuckle pin 56 and which is seated in the surface indentations . the preferred form of the retaining means include an elastic ring 70 located within a circumferential groove 72 on the bottom portion 66 of knuckle pin 56 that extends downwardly from the bottom surface 64 of lower pivot lug 30 . elastic ring 70 projects from circumferential groove 72 a distance which makes the total diameter of the cross - section of knuckle pin 56 combined with elastic ring 70 greater than that of lower pivot pin aperture 50 of passageway 54 . thus , when knuckle pin 56 moves upwardly during use , elastic ring 70 comes into abutting contact with bottom surface 64 of lower pivot lug 30 restricting knuckle pin 56 from any further upward movement which would result in unintentional removal . unlike the cotter pin retaining device of the prior art , elastic ring 70 fits closely about knuckle pin 56 , and there is no contact between elastic ring 70 and downwardly protruding flange section 60 . accordingly , knuckle pin 56 and elastic ring 70 are free to rotate continuously during use of coupler 10 . consequently , the slack point stress on knuckle pin 56 at the longitudinal point along knuckle pin 56 between knuckle 22 and upper pin protector 44 is distributed along the circumference of knuckle pin 56 resulting in a longer fatigue life for knuckle pin 56 . due to the fact that lower pivot pin aperture 50 of lower pivot lug 30 has an elongated non - circular cross - section , it is preferred that the bottom surface 64 of lower pivot lug 30 include chamfers 74 and 76 about aperture 50 as shown in fig5 - 7 . chamfers 74 and 76 are located on the sides of aperture 50 at either end of minor axis y -- y and create a circular area of contact for elastic ring 70 when knuckle pin 56 moves upwardly during use . this is important because if elastic ring 70 were to contact the rim of the non - circular aperture 50 without chamfers 74 and 76 , the area of contact would be at the two points at either end of minor axis y -- y and elastic ring 70 would be much more easily overcome by the upward movement of knuckle pin 56 . the uniform circumferential area of contact created by chamfer 74 and 76 makes for a much greater resistance against undesired removal as well as wear of elastic ring 70 . it is preferred that elastic ring 70 be of a resilient material such as rubber so that it may be expanded around the bottom portion 66 of knuckle pin 56 and will automatically contract around circumferential groove 72 without the use of a tool to remove or apply elastic ring 70 . in the alternative , elastic ring 70 may comprise a split metallic ring ( not shown ) which is either spring biased or tightened down with a tool after it has been placed in circumferential groove 72 . while , it is also preferred that elastic ring 70 have a circular cross - section and circumferential groove 72 have a corresponding arcuate cross - section as shown in fig8 other alternatives are possible such as the rectangular shaped cross - section of elastic ring 78 and corresponding rectangular shaped circumferential groove 80 in alternative knuckle pin 82 of fig9 . the foregoing description and drawings explain and illustrate the best known mode of the invention and those skilled in the art who have the disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention which is defined in the following claims .