These aluminum cylinder heads come ready for the maximum street/strip Small Block Ford performance possible. They are not emissions legal. The 165 comes with 1.900 intake and 1.600 exhaust valves and the 185 sports 2.020 intake and 1.600 exhaust valves. Both cylinder heads feature 58cc or 61cc combustion chamber with 69cc exhaust ports. Both the 165cc and 185cc have available as an option nitrous or blower exhaust ports featuring a 75cc ultra high flowing exhaust port. With AFR's trademark 3/4" thick head deck, it's no problem handling that additional cylinder pressure. Each head is also drilled for the larger 1/2" head bolt hole for maximum head gasket retention as a standard feature of this serious race piece in street disguise. Ford Small Block - 185cc CNC Street Ported Heads - 61cc Combustion Chambers, Stud Mount Rockers - Fully Assembled