CC'ing a cylinder head is a process that measures the volume of the combustion chambers in the cylinder head. The term CC'ing is the cubic centimeters of the pocket (combustion chamber) around the valves. the purpose of cc'ing the head is to determine the compression ratio of the engine. You cannot compute the compression ratio if you do not know the size of your combustion chambers. Also it is important in performance heads to have equal cc'ed heads to balance the engines compression to each cylinder.