as shown in fig1 , a tapered expansion plug embodying the invention comprises a generally frustoconical rubber disk 10 sandwiched between a first washer 12 and a second washer 14 . although the second washer is illustrated as larger than the first washer , the washers could be the same size . the rubber is sufficiently flexible that it can be expanded substantially by axially compressing the disk . the washers have center holes , that of the first washer being designated by reference number 16 . the rubber disk has a slightly smaller diameter through hole 18 extending along its longitudinal axis . a bolt 20 extends through the holes of the disk and the washers . the bolt has an enlarged head 22 , a tapered portion 24 adjacent the head , and a threaded shank 26 . a wing nut 30 , having complementary threads , is tightened onto the threaded end of the shank to compress the disk . the wing nut could be made of various including nylon or suitable metals . alternatively , the wing nut could be replaced by any type of nut . the tapered portion 24 has a mean diameter about equal to that of the hole 18 in the washer adjacent the smaller side of the disk , so that it wedges in the hole as the nut is tightened . this wedging transfers torque from the bolt to the smaller washer , which bears against the rubber disk with increasing force as the nut is tightened , thus keeping the bolt from turning in the hole . as best seen in fig2 , the plug is tapered , from its larger end 32 to its smaller end 34 . the preferred taper angle α is between 3 ° and 17 °. angles in this range have been found to provide the best combination of ease of insertion and retention security . a peripheral flange 40 is integrally formed around the larger end 32 of the plug . the flange has a preferred thickness “ t ” ( fig4 ) in the axial direction in the range of 0 . 030 to 0 . 100 inch , most preferably about 0 . 065 inch . the flange &# 39 ; s width “ w ” in the radial direction , from its inner diameter , where it meets the tapered face of the plug , to its outside diameter , is preferably about 0 . 060 - 0 . 205 inch . the optimum dimensions , which are given for one plug , may vary according to the intended application , and may depend somewhat on the size of the plug , on a proportional basis . different applications may require different plug sizes , and different flange dimensions . the flange washer 14 should have an outer diameter not larger than that of the flange , yet large enough to prevent the plug from being pushed or sucked into the opening . i prefer that the flange washer 14 have an outer diameter equal to or less than that of the flange &# 39 ; s outer diameter , but greater than the flange &# 39 ; s inner diameter . in a variation of the invention , not shown in the drawings , the wing nuts are dyed different colors to identify the plugs by size . alternatively , any component of the plug could be the one color - coded . in another variation of the invention , shown in fig3 , the plug size is stamped on the bolt head . the mechanic can thus quickly distinguish between plugs of nearly equal size . plugs are sized according to diameter as shown in the table below . no . d ( inch ) used for 000 0 . 46 ⅜ ″ p 00 0 . 54 ½ ″ p 0 0 . 63 ½ ″ p 1 0 . 70 ½ ″ p 2 0 . 75 ¾ ″ t 3 0 . 89 ¾ ″ t 4 0 . 96 ¾ ″ t ; 1 ″ p 5 1 . 02 1 ″ t 5½ 1 . 06 1 ″ p 6 1 . 20 1 ″ f 6½ 1 . 27 1¼ ″ p 7 1 . 39 1¼ ″ p 7½ 1 . 46 1½ ″ p 8 1 . 54 1½ ″ p 8½ 1 . 62 1½ ″ p 9 1 . 69 1½ ″ p ; 1¼ ″ f 9½ 1 . 73 1½ ″ f 10 1 . 89 1½ ″ f 10½ 2 . 01 2 ″ p 11 2 . 13 2 ″ p 11½ 2 . 35 2½ ″ p 12 2 . 42 2 ″ f 13 2 . 58 2½ ″ p 13½ 2 . 82 3 ″ p 13¾ 3 . 02 3 ″ sch . 40 p 14 3 . 40 3 ″ p 15 3 . 86 4 ″ p 16 4 . 64 4 ″ p p = pipe t = tube f = fitting d = diameter of stopper ¾ of its height from smaller end in use , the pool mechanic selects a plug whose larger end ( not including the flange ) is as large end or slightly larger than the return line opening 50 ( fig4 ). he then pushes the plug into the opening until he feels abrupt resistance , as the flange makes 360 ° contact with the opening , so that the axis of the plug lies on the axis of the opening . now , knowing the plug is properly seated , the mechanic turns the wing nut clockwise to increase the diameter of the rubber disk sufficiently to keep the plug from coming out of the opening , and to form a tight seal notwithstanding any helical threads formed inside the opening , or other irregularities which might otherwise result in leakage . another advantage of the peripheral flange is that it makes face - to - face contact with the face plate around the return line opening , closing the path to any fluid that might leak along threads formed in the return line . it should be understood that the foregoing is a preferred embodiment , but that the invention may take other forms . for example , while the preferred shape of the plug is , as shown in the drawings , round ( that is , circular in cross - section ), the principles of the invention could be applied to a tapered plug of a shape other than round , for example elliptical or polygonal , to plug corresponding holes . since the invention is subject to modifications and variations , it is intended that the foregoing description and the accompanying drawings shall be interpreted as only illustrative of the invention defined by the following claims .