Chevrolet Performance Bowtie big block Chevy cast iron engine blocks have all 8620 steel main caps and are CNC-machined. They are race-prepared blocks that feature blind head and accessory mounting bolt holes, a deck configuration that enables the use of any big block Chevy cylinder head, and cylinder bores that may be safely bored to a 4.600 in. maximum diameter, while maintaining a .250 in. minimum cylinder wall thickness. The Chevrolet Performance Bowtie big block Chevy cast iron engine blocks differ from the production-machined Bowtie versions in certain areas--they start from raw castings and are not "conversions."