Provided are methods of replacing drilling holes in castings by pre-tensioning sand cores. These methods are used in long cylindrical shaped sand cores in cold core box processes. The urethane sand core is placed in compression to increase the core strength during handling, placing inside of the mold. The pre-tensioned urethane sand core is set in a casting mold so as to form a cavity in the casting mold. The molten metal is poured to form the cavity inside of the casting. The pre-tensioned urethane sand core maintains its initial shape during casting.