Ken and the crew at Indy Cylinder Head started with Mopar Performance Parts' cast iron street RB block (P4529851). Termed a water block, it maintains water flow between cylinders for the demands of street use as opposed to the siamesed bore offerings. Siamesed-bore RB blocks can be bored to 4.500-inches. As received from MPP, the "water block" maintains the stock 4.25-inch bore diameter; however, Indy Cylinder Head opens it up to a 4.375-inch bore--which is "the largest bore diameter this cylinder block can accommodate," according to Ken Lazzeri. All of the MP RB blocks have the stock 10.72-inch deck height.