Abstract: A method of rebuilding used cores comprising the steps of; cutting a used core substantially down the center at a midway cut thereby creating a left core and a right core. Further installing an extension unit in between the left core and right core, thereby forming a lengthened rebuilt core. Further, cutting the damaged ends off the rebuilt core to a selected finished length. The rebuilt core includes a left core obtained from a donor core; a right core obtained from a donor core; an extension unit sandwiched between the left core and the right core for rigidly joining together the left and right cores by connecting to one end of each of the cores.