General Motors identifies its small-block Chevy cylinder heads with both casting numbers and casting marks. “Double-hump” heads were found on many of its high-performance engines in the 1960s and early 1970s. They were manufactured with larger intake and exhaust port volumes and valves for the period, and engines equipped with them had output ratings of more than 350 horsepower. While there are variations among them, some heads with different casting marks also performed well.
Tall Double-Hump Casting Marks
There are two casting marks with two humps — a tall one and a short one. Heads with the taller casting mark were produced in four casings. Casting numbers typically had seven digits, and heads were commonly recognized by the last three numbers. Heads with tall casting marks were identified as 291-, 461-, 461X- and 462 castings and did not have accessory bolt holes. Intake/exhaust port volumes were about 170cc and 64cc and valve diameters were either 1.94 and 1.5 or 2.02 and 1.6 inches for the intake/exhaust. Combustion chambers were typically 62 to 64cc and provided higher compression ratios.