Product Description An expansion or well nut consists of a flanged neoprene bushing with a threaded nut embedded in the non flanged end. It is used in blind bolt holes to fasten a piece and seal the bolt hole. The nut is inserted into the hole flange side up as the bolt is screwed into the nut the rubber on the other side of the hole expands and seals the bolt hole. From the Manufacturer An expansion or well nut consists of a flanged neoprene bushing with a threaded nut embedded in the non flanged end. It is used in blind bolt holes to fasten a piece and seal the bolt hole. The nut is inserted into the hole flange side up as the bolt is screwed into the nut the rubber on the other side of the hole expands and seals the bolt hole.