A two member plastic pipe fitting adapter for installing a threaded sprinkler head. The members are coupled together to form a chamber for housing an annular gasket. The gasket chamber includes an expansion void and bearing surfaces for the gasket. The adapter includes a receptacle with an internal thread for engagement with a sprinkler head to load the gasket and form a fluid tight seal. The engagement between the sprinkler and the adapter is formed by dissimilar threads to minimize the outward radial forces on the adapter.