A screw cap for sealing containers with threaded necks includes an outer body, having a cylindrical outer wall closed at one end by a circular lid and an interior neck seal adjacent to the circular lid. A female thread extends around an inner surface of the cylindrical outer wall. The female thread has a main portion having substantially uniform thickness, and an attachment portion having a thickness which is less than the thickness of the main portion. The female thread is attached to the inner surface of the cylindrical outer wall in the region of the attachment portion, and the attachment portion is configured to permit deflection of the female thread.