A carrier assembly for injection molding a parison with internal threads. The carrier assembly may be used with an injection stretch blow molding machine that includes a core rod. The carrier assembly comprises a carrier insert surrounding a portion of the core rod. The carrier insert includes a thread-forming portion presenting an interior radial surface and an exterior radial surface. The interior radial surface is configured to be positioned adjacent to the core rod, and the exterior radial surface includes a threaded protrusion configured to extend away from the core rod. The carrier assembly also includes a pinion insert surrounding at least a portion of the threaded protrusion of the thread-forming portion of the carrier insert. The pinion insert is spaced apart from the thread-forming portion of the carrier insert so as to present a thread-forming cavity between the pinion insert and the carrier insert.