Chevy Double Hump Heads Specsby Moss Strohem
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. These heads were found on Chevy high-horsepower 302, 327 and 350 CID engines, including the fuel-injected 327s, from which they got the nickname "fuelie heads."
Short Double-Hump Casting Marks
Heads with short double-hump casting marks were used in later high-performance SBCs. Most of these heads had accessory bolt holes and were identified with casting numbers ending in 186 or 492. They were used from '68 through '72 on 302 CI Z-28 engines as well as '70 through '72 in LT-1 350 CI Corvette/Z-28 engines. In 1971, combustion chambers were increased to 72cc to lower compression ratios, but other specs were generally the same as heads with tall casting marks.
Double-hump heads were popular performance upgrades for engines that were not equipped with them and are in high demand today in racing applications where factory heads are required. These castings are also becoming more difficult to find for collector car enthusiasts who need factory-correct components with matching ID numbers. For performance applications, there are other GM castings that are similar in specs. The "041" heads compare favorably but the '96 to '01 GM L-31 5.7L Vortec heads are in much greater supply, still available from GM and flow better than even the best double-hump heads.
The "lure" of the double-hump heads has been their performance potential and nostalgia. Due to the decreased availability of these factory GM performance castings, aftermarket manufacturers began supplying replacement heads for the SBC in the 1980s. The cost of reconditioning OEM heads can be high, and the performance potential will typically still fall short of most aftermarket replacements. Today, many aftermarket choices are available that are cost-effective and better-performing substitutes.
Moss Strohem has a background in business and finance, and an avid interest in youth sports, health, nutrition and physiology. He writes both technical information and market commentary as a private consultant and has researched and authored business plans.