This steel (not aluminum) fitting is 9/16"x 28 threads per inch on the ID, and 13/16"x 16 tpi on the OD. It fits nicely into the attachment cap on all our filter kits. It is a popular size on 40 caliber and 45 caliber firearms. It is coated in black nitrite for anti-corrosion properties. We recommend steel in this critical connection.