Cylinder heads are understandably a vital ingredient in any recipe for "porcupine" horsepower pie, so whether the big-block in your Corvette is being built for street performance, drag racing, road racing, autocrossing, or simply holding down the garage floor, the heads must be properly matched to the pistons, camshaft, and intended usage if an engine is to deliver anticipated performance. Countless numbers of big-blocks have had all visions of glory destroyed by misdirected "experts," who either selected the wrong castings, or took a wrong turn while modifying them for maximum power. Such mistakes aren't uncommon because ego and poor judgment frequently win out over common sense.
In spite of the vast array of cylinder heads produced by Chevrolet and a host of aftermarket manufacturers, there are usually only a few models that are truly applicable for an engine of a particular personality. Just as passenger car heads rarely provide the ultimate power levels on a race engine, race heads usually provide less than impressive results when installed on a true street engine. It all has to do with combustion chamber volume, port size and shape, and air flow characteristics.
Big-block cylinder heads cast by GM are most commonly identified by their casting numbers, which may be found in a variety of locations depending upon the particular head. However, the complete casting number is located beneath the valve cover, usually above an intake port. Additionally, either the last three digits of the casting number or the complete number can usually be found beneath one of the intake ports when a head is viewed from the combustion chamber side.
Rather than the complete number, only the last three digits are commonly used when referring to a specific casting. As an example, in machine shops, bench racing parlors, and other houses of high performance, casting number 6272990 is called simply a "990" casting. In the real world, the first four digits are virtually meaningless, as is the actual part number (6260482), which is used only when ordering a head from a Chevrolet dealer's parts department. However, once a cylinder head is out of its original box, a part number is of no use because the only identifying marks on the head itself are the casting numbers.
Back in the day when the big-block was introduced (1965), Chevrolet offered a variety of engines, each designed to deliver a specific level of performance. As fate would have it, cylinder heads were also designed to deliver specific performance characteristics. Big-block (Mark IV) engines originally configured for low and medium performance were fitted with cylinder heads having oval-shaped intake ports; high-performance big-blocks derived increased breathing capacity from heads with rectangular intake ports. The word "Pass" cast into a big-block head identifies it as being of the oval port persuasion; "Hi-Perf" cast into a head denotes rectangular ports. Early models of both oval and rectangular port heads have a bathtub-shape closed chamber; 1971 and later castings are usually of the open-chamber design.
Another characteristic that distinguishes early from late-model cast iron heads is the spark plug hole. The '69 and earlier heads are machined for gasketed 31/44-inch reach plugs (Champion "N" type) with a 131/416-inch hex; 1970 and later heads require a tapered seat 11/42-inch-reach plug with 51/48-inch hex. The aluminum heads installed on production engines continued to be machined for 31/44-inch-reach 131/416-inch hex spark plugs.