Silverado_9c3, Chevy and GMC truck dealers install "rebuilt" engines for their customers. The old engine that comes out is called a "core". The dealer must either return the "core" or pay a "core charge", usually $100.
They are glad to sell you the "core" for the "core charge". It costs them nothing and you now have something to rebuild.
Many people believe that a "core" that has been through 1000's of heating and cooling cycles is completely dimensionally stabilized and superior to a "green" casting.
