secured within a flat metal holder 28 to be placed in the ends of an s slip 16 so that when a drive slip is bent over it will make a tight connection the invention description is best understood by a study of the disclosure as shown in the drawing here the s slip joint 16 is used to couple two air duct sections 12 and 14 in a manner well known in the art a sheet of metal as shown in fig5 is folded to form three folds and two receiving grooves in addition to the drawing disclosure the use of slip joints is well known in the prior art as shown in the patent to zack ser no 1935690 and the one to burghart ser no 3 246918 it is interesting to note that in addition to teaching how to couple duct sections both of these patents addressed the issue of gas leakage at the coupled area the solution taught by these references is to insert packing material at the fold walls to arrest the leakage of air that enters the grooves formed by the s folds in the instant invention the problem is resolved by placing apertures in one of the fold walls of the slip joint these apertures as shown at 18 in fig5 may be of any convenient diameter and spacing across the.