An apparatus is provided in the form of a neck for a plastic bottle or other plastic container. The neck includes an external thread with a constant major diameter that can engage with a bottle cap with an internal thread. The neck has an internal surface with an irregular shape where the internal diameter proximate to the top sealing surface is greater than the internal diameter proximate to the side wall of the plastic bottle. The internal surface of the neck engages with a fitment inserted into the neck of the plastic container to prevent movement of the fitment.