Abstract: A sprinkler head adapter comprising an internally threaded plastic fitting, a metal insert with both internal and external threads, and a sealing gasket is disclosed. The internally threaded passageway of the insert tapers inwardly. The diameters of the internally threaded fitting and of the externally threaded insert are substantially constant. The threads of the fitting and the corresponding external threads of the insert can be straight threads. The internal threads of the insert can be tapered threads corresponding to the threads of a metallic sprinkler head.