... -coded thread protectors which protect the threads, keep them clean and sharp, and aid in trade size recognition. Rigid metal conduit is available ... . Thread protectors for trade sizes 1, 2, 3, 4, 5, and 6 are color-coded blue, trade sizes 1/2, 1 ... , 2-1/2, 3-1/2 are black, and trade sizes 3/4 and 1-1/4 are red. The nominal finished ...