A threaded core includes a first half core and a second half core. The first half core includes a core body and a movable core portion. The core body has a body surface and an end face adjacent to the body surface. An outside threaded portion is defined in the end face. The core body defines a recess on the body surface. The recess communicates with the groove. The movable core portion is movably mounted in the recess. The movable core portion has a front end face. An inside threaded portion is defined in the front end face and configured for forming part of the screw thread cooperatively with the first outside threaded portion. The second half core has an identical structure with the first half core. The inside and outside threaded portions of the first and second cores cooperatively define a screw hole.