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?
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.
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.
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).
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
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.
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.
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.
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.
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.
Hello there 🙂
My question is I am running a MSD pro- billet part # PN 85551 distributor with a bronze main gear witch is damage, I don’t know what make of CAM I have but it is pretty radical my 69 Chevy nova has a 350 motor transformed into a 406 stroker what gear do you recommend to replace my damage gear on my MSD? Thank you
I would just replace it with another high quality bronze gear.
I just figured out my issue with my turbocharged 406 SBC. its a.030 bore. Same MSD part number distributor as yours. This is what I typed to my racing buddy this morning. I think the spacer will solve yours and my problem. The web site is worth looking at. Have fun. Ken Burnside
I think we are on to something here. The odd part is that I was running the stock GM distributor in the picture next to the failed MSD gear when I was running it on the street. No damage to it. I will do some measuring today. When a hot rod machine shop line bore a block they shave a small amount off the main bearing caps then re-machine the holes to make sure the centerline of the crankshaft is parallel to the deck and perpendicular to the bores. The oil pump bolts onto the rear main bearing cap. So the oil pump moved closer to the distributor by that amount also. I have not seen this issue before, and assumed the MSD distributor would be machined to mimic the GM distributor dimensions. Assumptions. That makes it my mistake in the end.
I think this is really the problem all along. As the gear wore out more the timing got worse and the Fitech computer kept chasing bad O2 readings with timing and the fuel map. So maybe the fuel cell was unnecessary. I am sure the Fitech corrections just ran off the spreadsheet. Summit is sending a new gear. May be here in time if it gets here tuesday. Coming from arlington tx. I’ll shim the base of the distributor, and reset the computer to factory specs again. I will make a metal shim from whatever I have laying around the shop.
Nothing to it. 🙂
The camshaft I’m using is the summit brand ht 383 roller cam, it specifically states to use a melonized distributor gear. I ordered a Howards cams gear that was supposed to be melonized but running the number it states it as everwear, any thoughts on if it would work as it should?