Abstract: The present invention relates to a closure underpart with a tubular sleeve (2) and a fixing flange (3) arranged on the outside of the tubular sleeve (2), at least one projection (4) being provided on the inside of the tubular sleeve (2), which serves to hold the closure underpart (1) on a spike inserted into the tubular sleeve (2) when fixing to a vessel wall. In order to create a corresponding closure underpart, which can be held on a spike with a much greater play it is proposed according to the invention that the at least one projection (4) has the form of a bendable web which run inclined vis-à-vis the inner wall of the tubular sleeve (2) at an angle &agr; which is smaller than 90° and greater than 0°.