There are several different types of distributor gears out there, including iron gears, bronze gears, melonized gears, and composite gears.

So what’s the right gear for your distributor?

(Image/Wayne Scraba)

Iron Distributor Gears

The vast majority of distributor gears out there are iron. They were (and are) designed to work with a cast camshaft that features an iron distributor drive gear. This combination goes back to almost the beginning of time.

This is a standard iron distributor gear for an MSD distributor. These differ from a stock Chevy gear in that the ID is 0.500 inch. A stock Chevy Delco gear has an ID of 0.491 inches. (Image/Summit Racing)

Then roller camshafts appeared. Most were machined from steel cores. The steel cores (often billet today) were and are necessary because of spring pressures commonly required for roller cam operation. A common iron distributor gear didn’t live long in these applications, simply because the two materials (iron and steel) weren’t compatible.

The end result of using this mix is usually engine carnage resulting from lots of iron chunks in the oil.

The initial solution? Use a bronze distributor gear.

Bronze Distributor Gears

While bronze gears can still wear out, the rate of wear isn’t as severe as the iron gear. This proves great for race cars, but not so great for cars that see extended use. Basically, bronze distributor gears are sacrificial and will eventually wear out.

Several different manufacturers offer bronze-aluminum gears. This one is again from MSD (part number MSD-8471). Typically, these are race-only pieces for use on steel core camshafts. As noted in the text, a bronze gear tends to be self-sacrificial, so you have to keep an eye on it. (Image/Summit Racing)

In the Eighties, hydraulic roller cams made their appearance in production cars. Here, the manufacturers started to use cam cores manufactured from ductile iron. Iron distributor gears won’t work and neither will bronze gears (at least for long).

Melonzied Distributor Gears

The big factor fix was a melonized steel distributor gear (GM began using the process in 1985).

What exactly does “melonized” mean?

The short answer is, it’s a form of salt bath nitriding—a process that improves wear resistance, corrosion resistance and scuffing wear. It also increases fatigue strength of the material that is processed (in this case, the distributor gear).

The stock distributor gear is on the right while a melonized replacement is on the left. Melonized gears are easy to spot due to the dappled finish. (Image/Wayne Scraba)

Today, GM and Ford both use melonized gears in several of their street crate engines equipped with steel camshaft cores. Because of their wear properties, melonized gears are a good fit for cars with either ductile iron camshafts or cams manufactured from billet steel. Melonized gears are compatible with conventional iron core camshafts too.

By the way, the writer’s engine is equipped with a custom billet roller camshaft from Bullet Racing Cams, and they recommended a melonized gear for the application.

You can easily spot a melonized gear by its distinct surface finish: To the eye, it appears dimpled.

Composite Distributor Gear

A relatively new offering for distributor gears are composite examples, like this one here from Comp Cams. (Image/Summit Racing)

Another option is a composite distributor gear. This is basically a plastic polymer, often mixed with a carbon compound. A couple of manufacturers offer them, including Howard’s Cams and Comp Cams. The Summit Racing catalog has them listed for various Ford and GM applications.

FYI, composite gears are compatible with all types of camshaft cores. They are the softest gears on the market and they are also the most-costly.

Composite distributor gears usually feature a carbon mix and they’re designed to replace bronze gears used with steel camshaft cores. Composite gears are not recommended for use with high volume or high pressure oil pumps. (Image/Summit Racing)

Distributor Gear Indexing

We’re not quite done. When replacing a distributor gear, consider gear indexing: A factory GM Delco distributor gear has a dimple in it that aligns with the pointer of the rotor (some aftermarket replacement distributor gears do not have a dimple) however the gear can be installed 180 degrees off in order to fine tune the distributor housing/vacuum cannister position on the engine.

As noted in the text, you’ll find a dimple on stock GM Delco gears. This dimple helps you index the distributor body with the rotor. (Image/Wayne Scraba)

For example, if you cannot set the initial timing without the advance canister hitting an OEM spark plug wire bracket or an intake manifold runner, simply remove and reinstall the distributor drive gear 180 degrees and reinstall the distributor. Flipping the drive gear will provide approximately 14 degrees of distributor body movement in relation to the surroundings.

Basically, there are two ways to install the gear. One way will position the vacuum canister in the appropriate spot on the engine. The other will make it more difficult to work with. (Image/Wayne Scraba)

Installing the gear with the dimple aligned with the rotor pointer is important in Corvettes with ignition shielding since it has to fit a pre-determined opening. Some aftermarket camshafts either ignore the indexing layout or they don’t take into account its importance in an early Corvette application with distributor shielding.

distributor vacuum advance canister on an msd distributor for a big block chevy engine
Check out the way the distributor vacuum advance cannister is situated on the writer’s BBC. This allows easy advance or retard on the engine, without the cannister interfering with a manifold runner or other bits. Yes, I use a vacuum advance setup on my street engines, and yes, I prefer to mount the distributor clamp this way for various reasons—they can be flipped over. (Image/Wayne Scraba)

The bottom line here is, it really doesn’t affect timing. It affects the location of the distributor body and vacuum advance canister in relation to the intake manifold (or the ignition shielding in a Corvette).

For a closer look at distributor gears, check out the accompanying photos.

When installing a new gear, it’s always a good idea to install a fresh roll pin. (Image/Wayne Scraba)
A good example of a distributor drive roll pin is this one from Allstar Performance. (Image/Wayne Scraba)

Keywords
distributor, distributor gears, distributor tech
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Author: Wayne Scraba

Wayne Scraba is a diehard car guy and regular contributor to OnAllCylinders. He’s owned his own speed shop, built race cars, street rods, and custom motorcycles, and restored muscle cars. He’s authored five how-to books and written over 4,500 tech articles that have appeared in sixty different high performance automotive, motorcycle and aviation magazines worldwide.