inches the outside opening diameter od is typically about 129 32 inches and the inside opening diameter id is typically about 13 4 inches it is this tapered hole 11 into which a wooden bung not shown is normally tamped and with all of the disadvantages previously recited the improved plug 15 of the invention comprises a partially hollow integrally molded conical cup like body 17 having a base wall 19 and a sidewall 21 base wall 19 is made sufficiently thick both to withstand the pressures and stresses imposed during normal service and also so as to permit a relatively rigid metallic round plate 23 to be embedded in base 19 as best seen in fig3 consideration with regard to the pressure and required flexibility is also taken into account in determining the thickness of sidewall 21 a metal shaft 25 is rigidly and centrally secured to plate 23 as also best seen in fig3 and at its outer end includes a threaded portion 27 a second rigid metallic plate 29 has a central aperture 31 through which the outer end of shaft 25 extends and which is retained by a threaded metal nut 33 however it is to be noted that a wing nut cam lock or any other suitable tightening means can be employed in place of the illustrated nut 33 as a means for drawing the plates 23 and 29 together so as to cause sidewall 21 to expand in the embodiment being used for purposes of illustration the body 17 was molded of a food grade buna n elastomer material and the overall depth d of body 17 was about 11 4 inches the outer diameter od p was about.