Sometimes the deck is not actually parallel with the main line of the block (think of an imaginary line through the crankshaft). If this is the case, then you first start with “decking” the block to the minimum tolerance. In other words, you shave the high sides of the deck of the block (to the lowest point—minimum tolerance). After the block was cleaned and decked, the cylinder walls were bored in order to get rid of any scarring or pitting and any taper in the cylinder bore. Boring the cylinders is done basing a measurement off the main line of the block, keeping the cylinder cut absolutely perpendicular to the main line as they are bored out.