The first thing to remember when searching for a big-block distributor is that there are two different lengths: one for standard-deck (short) engines and one for tall-deck engines. They are not interchangeable so you must have the correct one for your engine. Most production and high-performance Chevy engines were based on short-deck blocks except for the 572 crate engine so most distributors you find are short-deck units. As previously noted tall-deck engines were only used in trucks. The talldeck version is approximately 1.25 inches longer than the standard distributor.
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If you’re not performing a detailed restoration it’s unlikely that you want to use a standard GM ignition on your project. If you do require it, finding the correct distributor housing can be difficult. Pre-1974 models used a pointstype distributor with separate coil; later versions were equipped with HEI distributors that incorporated an integral coil within the distributor cap.
As electronic controls were introduced the HEI was upgraded with a sealed connector to the vehicle’s ECM unit. These distributors are easily distinguishable because they don’t have a vacuum advance mechanism. Noncomputer-controlled units use standard centrifugal advance weights controlled by engine speed and a vacuum advance diaphragm to alter the spark advance curve for optimum performance. Computerized versions regulate this electronically.
Vacuum advance units on HEIs are individually calibrated according to the vehicle and specific application. They often have to be replaced on used distributors, and distributor shops are well equipped to do this. Specialty shops can rebuild almost any distributor including early points models and HEI units. They generally need a thorough cleaning along with replacement bushings so the shaft runs true. New advance springs and weights are also required and the distributor must be re-curved to suit your performance application.
Some restorers convert points distributors to internal electronic systems that preserve the original look while eliminating the need to adjust and service the points. Most big-block distributors use cast-aluminum housings although before 1967, distributor housings were made of cast iron. These distributors had a stamped tin identification tag wrapped around the upper portion of the distributor housing. If the tag is missing, there is really no way to tell the exact application.
For a proper restoration you need to begin with a distributor housing that has the correct identification number stamped on it. This can be difficult to find, but the best place to start is an Internet search for vintage distributor restoration services. It directs you to restorers and specialty ignition shops that can help you or guide you in the right direction. Restorer networking has improved considerably in recent years and one of the best sources is a car club devoted to your particular vehicle. If you let it be known within the club, word spreads and people often come forth with the parts you need. (Consult the chart on page 115 and 116 for distributor identification numbers.)
Because big-block and small-block distributors are interchangeable (except for tall-deck engines) your choice of distributors is large, particularly if you don’t require a numbers-matching housing. Some applications have special mods unique to the application. Corvette distributors through 1974, for example, incorporated a mechanical tach drive in the distributor housing. If you find a suitable candidate for your project, it is unlikely that it has the proper calibration for your application. Any ignition shop can check all of this and make certain your HEI is properly calibrated.
They also check for worn bushings, and install a new distributor gear. Be aware that if you are going to run a steel roller cam the gear should be replaced with a special melonized gear designed to mate properly with the cam gear. The shop will check the operation of the vacuum advance canister and replace it if necessary. They do tend to develop leaks over time.
A popular swap is to upgrade pre– 1974 engines with a more-powerful HEI unit to get rid of the points and gain a superior ignition system. The HEI requires a switched 12-volt power supply directly to the distributor. This should be a direct 10-gauge wire. Do not use a resistance wire or a ballast resistor in this HEI circuit. These are only required on points-type distributors to reduce the voltage to the points. The HEI is also more powerful and can fire plugs with a larger spark gap. Be sure to gap your plugs to .045 inch. Note that all single 4-barrel low-rise and high-rise intake manifolds accept points-type or HEI distributors, but fitment issues often occur with tunnel ram intakes, superchargers, and some EFI setups.
In many cases distributor selection must be predicated on the type of manifold or induction system you are using. Manufacturers such as MSD offer a variety of low-profile and crab-cap distributors specifically designed for racing applications and unique fitment problems. In severe cases you can also use a special offset distributor housing or even a special aftermarket front-drive distributor to relocate the distributor away from induction components. Aftermarket front drive distributors are designed to fit Mark IV engines and they can be adapted to fit Gen V and VI blocks, but some machining is often necessary. Additionally if you use manifold spacers to run a short-deck intake manifold on a tall-deck block, you still need a tall-deck distributor or a compatible aftermarket unit with an adjustable slip collar.
In addition to General Motors, aftermarket suppliers also provide HEI and points-type replacement distributors. They offer stock–type replacement distributors and they usually offer performance distributors too. These might include high-performance HEI units such as the MSD Super HEI or Performance Distributors’ racing HEI that are designed to fire up to 9,000 rpm in racing applications. Stock HEIs are very strong in the normal street driving range, but lose energy above 5,500 rpm.
The aftermarket is awash in high-quality distributors and ignition systems for big-block engines. All of them offer strong, reliable performance. As with factory distributors fitment is not generally an issue except for the difference between tall- and short-deck units or cap size to accommodate tight space as with superchargers and some injection systems. When using any aftermarket distributor with later-model steel roller cams a compatible distributor gear is required. Look for units that feature a Melonite QPQ coating (typically referred to as being melonized). This coating reduces friction and initial wear and is typically accompanied by a hardened surface (RC 55-60). Most aftermarket manufacturers offer melonized gears on their distributors.
A ductile iron distributor gear is standard fare for all factory flattappet camshafts, but since most aftermarket flat-tappet cams are made from higher-quality cores, most distributor manufacturers provide gears with slightly higher surface hardening to ensure good durability. Bronze gears have typically been specifi ed for steel roller cams because the cam cores are actually softer than fl at-tappet cores.
In this case performance distributor manufacturers covered their bets by incorporating higher-quality nickel/bronze alloy gears. Nonetheless, bronze gears often wear more quickly until they have been changed at least once. Iron gears are surface hardened and typically wear very well. Factory roller cams still use an iron gear, but the replacement distributors come with melonized gears to improve wear. Composite gears are a relatively recent development and they have proved to be successful in racing applications.
Written by John Baechtel and Posted with Permission of CarTechBooks