flatten orthogonal to the axis of the extension 89 the seal 77 is resilient in the sense that the seal 77 tends to return to its normal position unless deformed by outside forces the thickness 95 is greater at the internal circumference a than the external circumference b in order to resist collapsing of the external circumference b due to stressing of the seal 77 when it 77 is flattened the wall sleeve 79 is generally tubular and rigid the wall sleeve 79 may be formed from sheet metal or plastic or another preferrably non corrosive material pvc polyvinyl chloride pipe is a suitable material as it inexpensive light bonds well with rubber and is easy to cut the wall sleeve 79 is cut to the approximate depth of a wall 100 see fig1 plus seven eighths of an inch to take into account the thickness 95 of the seal 77 it is advisable to use a tube cutter not shown to obtain a smooth cut on the sleeve 79 it is particularly difficult to obtain a straight cut using a hacksaw on pvc pipe the wall sleeve 79 is covered at one end to the depth of the seal 77 at the inner circumference a with an adhesive 101 for example pvc cement when using pvc pipe for the sleeve 79 and rubber for the seal 77 the end.