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Bore refers to the diameter of each cylinder. It acts as a seal against combustion gases and lubricant infiltration into the combustion chamber. Also, its role is to transfer heat from the piston to the liner. In the case of a new cylinder block, there can be a few variances in the cylinder bore diameter. Typically, there are three standard-sized cylinder bore diameters. A bore size code is stamped into the top of the cylinder block for each respective cylinder.