The MR-1 and MR-1A Pontiac blocks have Siamese cylinder bores. This allows for the over bore to be as great as 4.400 which yields 572 cubic inches when used with a 4.75 inch stroke crank. Max bore size on the MR-1A blocks are 4.350. Minimum bore size of the cast iron MR-1 blocks is 4.150 and the MR-1A minimum bore size is 4.250.