These are used to protect threads from impact during storage or shipping as well as seal off new assemblies to prevent dust, dirt, moisture, insects, or other contaminants from entering the hose or pipe. They can be used as a cap, or turned over and used as a plug. To assure a proper fit, the lower range on the "cap" (listed below) should be 0.020" smaller than the part diameter; when used as a plug, the lower range on the "plug" should be 0.020" smaller than the opening. See our thread dimension chart at the beginning of the catalogue to identify thread diameters. Standard colour is blue. We also offer a plastic kit containing 543 pieces of an assortment of NPT, JIC & ORB threaded caps & plugs (Part #N60013-8).