General Motors Distributor Gear Information


This section explains questions I have been asked over the decades about GM distributor gears

Bowl type distributors. These distributors have gears retained by a staked mild steel pin (staking is peening the end of a pin over to create a rolled over end stop of pin metal). If the staked ends are carefully removed, they will allow removal of the staked pin, gear removal. A regular .750 inch long, .187 inch diameter common hardware store hardened steel roll pin will directly  replace the original soft steel pin, without needing to be staked back into place


On early Tri-5, and other "bowl type" Chevrolet, and other GM bowl type distributors, the gears are not marked as to how they phase onto the shaft to the distributor rotor. Using the rotor tip as a reference, the gear can install either straight on, or 180 degrees rotated, both mountings are correct, with exceptions. Those gears have 13 teeth. One end of the hole in the gear will have one end of one tooth aligned with the pin hole, the other end of the pin hole will have the center of the hole aligned between two gear teeth. On engines that have two four barrel carbs, there can be an issue in setting the timing on bowl type distributors WITH OILING TUBES hitting the rear of the back carb, then, the other direction, against the firewall, and not allowing timing to be correctly set (even if the distributor is moved" one gear tooth in the engine". If this occurs, the single gear hole end of the gear is probably the way the gear is in place on the shaft aligned with the rotor tip. Simple fix that will give a 'one half gear tooth' phasing between the firewall and carb is to remove the roll pin from the gear, rotate the gear 180 degrees on the distributor shaft, and reinstall the roll pin, gear tooth phasing between two gears at rotor firing tip center, the oiling tube shouldn't hamper timing setting any further. 


Later "dimpled" gears for both window cap point, type and early HEI distributors. The dimple has two functions. First, the dimple is a balancing hole to balance the gear in its rotation. Second, the reason for the balancing and dimple, to adjust for the rotor firing bar weight on the distributor rotor. To install the gear in its correct position, the dimple should be aligned with the firing bar tip on the rotor.


"Melonized" coated gears. "Melonized" is a friction reduction coating process that is applied much the same as a chrome finish. GM manufactures engines that run the hard chrome plated hydraulic roller cams use these gears to help break in the gear tooth interface between distributor drive, and driven gears for break in. This is simply a coating, and is usually dissipated between the gear tooth connection pattern after a fairly moderate break in period, with no ill after effects.


Bronze gears. Bronze gears are used with steel solid roller camshafts, and are considered to be "sacrificial". They are designed to wear away, instead of wearing the drive gear down/off the steel camshaft. Steel cams are usually solid roller cams, sometimes run on the street, usually used on drag race engine builds. Average lifespan on the street is usually 5,000 miles, but varies.


Composite gears. Composite plastic/nylon, and other soft material gears are being used on some specialty applications, in place of stock metal, bronze and other gear materials, and are very specific in application. Please be aware, composite materials are not bronze, cast iron, nor steel, life span will vary. 


As always, one of the best things to do when researching a distributor conversion, especially if the engine is being rebuilt, and/or camshaft is being changed, is to ask both the engine builder, and camshaft manufacturer just which gear they require for their product.