An assortment of expansion/well nuts from The Hillman Group. Expansion nuts or well nuts consist of a flanged neoprene bushing with a threaded nut embedded in the non-flanged end. As the bolt is screwed into the nut the rubber on the other side of the hole expands and seals the bolt hole. Variants included in this assortment range from #6-32 to 3/8 -16 in fine & coarse thread sizes. Full assortment contains twelve (12) refills comprised of Hillman item numbers #3319 – #3330.From the ManufacturerAn 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.