Ever since the first small-block V-8-powered Corvette rolled off the assembly line for the '55 model year, "Corvette heads" have been coveted by anyone building a high-performance small-block. In truth, not all Corvette cylinder heads were/are exclusive to the Corvette; many of the same castings were also installed on high-performance Camaro, Chevelle, Chevy II, and full-size Chevrolet engines. Irrespective of their original usage, Chevrolet small-block cylinder heads are most commonly identified by casting numbers, which are located in a variety of locations depending upon the particular head.
On traditional "Gen 1" small-block heads, the complete casting number is usually located beneath the valve cover, directly above a pair of intake ports. Additionally, the last three digits of the casting number can usually be found beneath one of the ports when a head is viewed from the combustion chamber side. These are the three digits commonly used when referring to a particular casting. As an example, in machine shops, bench racing parlors, and other houses of high performance, LS6/LS2 heads, which are casting number 12564243, are called simply "243" castings. Whether you're referencing iron castings from the '50s or current production LSx aluminum castings, only the last three digits are commonly used to reference a particular head. That is, unless you're ordering a cylinder head from a dealer GM Performance Parts distributor. In that case, the actual part number is required, so if you want to order an LS6 head, you have to specify part number 124564825 for the bare casting, or part number 124564824 for an assembled head, which includes valves.
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 and possibly a casting mark. The most sought-after traditional iron castings are the ones with a "double hump," which indicates a performance head with large ports. Some cast-iron performance heads also have a triangular identification mark. Late-model aluminum heads are easier to identify because the last three digits of the casting number are externally visible.
The best small-block Corvette heads include the following:492 (double hump)-The prototypical double-hump, cast-iron, small-block head is casting number 3991492. The double hump or "camel hump" casting marks can be found on a variety of small-block iron heads and designate a high-performance casting. Many heads with the double-hump mark were installed on fuel-injected Corvette engines, dating back to 1960. As a result, 492 and other double-hump castings are routinely referred to as a fuel-injection or fuelie heads, even though they were also used on the original LT-1 and the 302ci Z/28 Camaro engine. Common features include 2.02/1.60-inch valves, 64cc combustion chambers, and 160cc intake runner volume. As part number 3987376, this head featured screw-in rocker arm studs and pushrod guide plates. But as part number 3958603 (discontinued in 1991), a 492 casting had pressed-in studs and was machined for 1.94/1.50-inch intake/exhaust valves.
462 and 186 (double hump)-These castings have specifications similar to the 492 head. The 186 was the first large valve/64cc chamber head with accessory bolt holes.
461 and 461X (double hump)-These were the original double-hump castings and feature 64cc combustion chambers and a 2.02/1.60-inch valve combination, but have slightly larger intake ports; 461 castings typically have 165cc intake ports, while 461X castings have 170cc intake ports. The 461 and 461X castings achieved notoriety during the heyday of Super Stock drag racing, when porting was not allowed (and acid-etching had yet to be perfected). The 461 castings do not contain accessory mounting bosses and bolt holes, so they can't be used in many late-model vehicles unless special brackets are installed.
624 (double triangle)-Originally used on '71-'72 LT-1, '71-'74 Z/28, and '73-'79 L-82 engines (with 9:1 compression ratio), this head has 76cc combustion chambers and is in essence a high-performance smog head. It was also available as a service replacement for high-performance, low-compression engines. part number 464045 was supplied with 31/48-inch screw-in studs and is machined for 2.02/1.60-inch valves. This casting isn't a lightweight type, but weight reduction was incorporated in its design.
441-A late-model smog head installed on 350-cid engines, the 441 casting has 76cc combustion chambers and 1.94/1.50-inch valves. Although the intake ports are only 155cc's, these heads deliver amazingly good performance, especially when modified and machined for 2.02- or 2.05-inch intake valves.
441X-This version of the 441 casting was used on 350- and 400-cid small-blocks. It's virtually identical to the 441 casting, except that it has 80cc combustion chambers and 1.60-inch exhaust valves when factory installed on 400-cid engines. 441X heads installed on 350s have 1.50-inch exhaust valves.
997 and 493-These are late-model "smog" heads with 76cc combustion chambers and 1.94/1.50-inch valves. Functionally almost identical to the 441 head, except intake ports measure 160 cc's.
Aluminum Gen 1 and Gen 2 HeadsCorvette aluminum heads were released in the middle of the '86 model year and feature 58cc combustion chambers, 1.94-inch intake, 1.50-inch exhaust valves, and angled spark plugs. Although they're reasonably priced and offer excellent performance potential, the original L98 aluminum heads fall short of the revised castings, which first appeared on '88 model year Corvettes. These castings (casting number 10088113, PN 10185087) were revised to increase exhaust port flow and feature a port roof that was raised .100-inch and a floor flattened to create a "D" shape. Exhaust valves were also treated to a 65-degree backcut to improve flow capacity. These changes brought about a 15-percent increase in exhaust port flow.
LT1/LT4-From many angles, '92-'96 LT1 aluminum heads look similar to their L98 counterparts, but they're obviously not interchangeable. On the other hand, LT1 and LT4 heads are completely interchangeable, so many Corvette owners have upgraded their LT1 engines with a pair of LT4 castings (casting number 12555690). The enhanced air flow capacity of LT4 heads results from enhanced port design, (195cc intake ports) along with 2.00-inch intake and 1.55-inch exhaust valves (compared to 1.94-inch intake and 1.50-inch exhaust valves in LT1 heads).
LSX HeadsLS1/LS2/LS6-Standard LS1 cylinder heads offer impressive air flow characteristics; the flow of the LS6 heads is even better. In fact, the same castings are found on LS2 engines and a variety of other GM LSx engines including the 303hp, 5.3-liter V-8 in the '06 Impala SS. Formally known as casting number 12564243, these heads feature 65cc combustion chambers, 210cc intake ports, and 70cc exhaust ports. When the ports are CNC-machined, intake and exhaust volumes are in the range of 250 cc and 85 cc (depending on the company doing the actual port modifications).
The King Kong of small-block cylinder heads is the new LS7 casting. Featuring 2.20-inch intake, 1.61-inch exhaust valves, and CNC-machined 270cc intake/85cc exhaust ports, these heads are in a class by themselves. Unfortunately, they were designed for a 427ci engine, and the valves are too large to safely clear cylinders less than 4.065-inch in diameter. These heads also have rectangular, rather than cathedral-shaped intake ports, so standard LS1/LS6 style intake manifolds are not compatible.
According to internal combustion lore, "An engine is nothing more than an air pump." That phrase has certainly been written and said more times than anyone cares to remember, but it does capture an essential aspect of creating horsepower. In order for an air pump to be efficient, it must be equipped with cylinder heads that provide exceptional air flow capacity. For over 50 years, Corvette small-block cylinder heads have been doing just that.