Distributor Gear Meshing Can Make Or Break Your Valvetrain

Many performance buffs use a “grab and go” approach when installing many aftermarket parts. New heads: bolt them on. New intake: bolt it on. New distributor: bolt it on. With so many aftermarket components available for your performance or racing engines today, a few thousandths of an inch here and there can stack up when piecing together multiple components. Combined, this “tolerance stack” can mean that every part, individually, is within spec, but when combined, there are fitment issues.

We recently rebuilt a 489 cubic-inch big-block using new aluminum heads and a sheetmetal intake. Considering those aftermarket components, it’s not a matter of just dropping that new distributor into the engine with a gasket and considering it full steam ahead. To make sure we had our distributor installed as needed, we worked with Moroso Performance Products to get the job done.

distributor gear

When dealing with accumulative tolerances of aftermarket heads, sheetmetal intake, and an aftermarket distributor, you always want to make sure the fit between these components adds up correctly.

The first of two adjustment points are between your distributor gear and the distributor housing. This specific blueprinting process has many reputable builders differing slightly in opinion.

Blueprinting The Distributor Gear

Distributor endplay is critical because the distributor gear wants to naturally climb the cam gear due to the angle of the teeth between the two. Add to that the harmonics in all directions caused by the oil pump — also driven by the distributor gear — and it is easy to imagine why these tolerances are critical.

The clearance between the distributor housing and the gear change as the engine warms. Short of describing complex calculations of a metal expansion, just know that you must not remove all distributor endplay. When the metals expand, the endplay could become an interference fit between the gear and housing as an engine reaches full operating temperature.

The gap between the distributor gear and your housing is an all-important dimension. Our new performance distributor came out of the box with a .040-inch gap, which is too much. You can measure your gap with either a dial indicator or feeler gauges.

We referenced our handy ’70s-era General Motors service manual, which states the gap between the top of the distributor gear and the housing should be .005- to .007-inch. Many enthusiasts take that tolerance as gospel for pre-1976 performance engines and have also applied it to later model distributors. But wait, there’s more.

All Distributor Tolerances Are Not The Same

Today, a distributor housing like a late-model GM HEI or performance aftermarket distributor is machined from aluminum. Many have suffered damage because the tight tolerances listed in era-specific manuals are for early distributors with a cast-iron housing.

We use Moroso distributor gear shims to adjust the gap between the housing and gear. Simply determine your gap and install the combination of shims to correct the end play.

Most professional shops will factor looser than the factory specified gap for aluminum housings (.010- to .025-inch). I have a trusted friend and professional engine builder who says that he will tighten the aluminum tolerance to as little as .006-inch if it is for a drag racing application.

Why would he tighten the tolerances? Because he said a racing engine does not reach as high an overall operating temperature over a long period during a run. Also, consider that most racing engines utilize a roller camshaft made from billet steel. When mated to a billet cam, distributors require a bronze gear in order to survive. If you slightly tighten those tolerances, you have less up and down walk with the distributor gear, which is good for the life of the sacrificial bronze material.

Peeking down your intake's distributor hole, some take for granted the additional stresses and harmonics put on the distributor with the oil pump drive. The angled-gear contact between the Chevrolet cam and the distributor gear wants to push upward on the distributor gear.

Distributor Drive Gear Material

Most distributor gears are created from one of three essential materials. For use with a cam made of cast iron, a melonized or hardened-steel gear is typically used and found on most performance distributors. A bronze gear is actually comprised of a soft alloy and is mandatory if you use a cam made of billet steel. Bronze gears do not damage the billet-steel material as an iron gear would. Relatively new to the gear game are composite/polymer gears. They are compatible with all camshafts but are comparably more expensive than other gears.

Shown are your choices between a melonized steel gear, bronze-alloy gear, and polymer distributor gear. We custom-machined an old damaged punch to the precise diameter to effectively drive out the roll pin that fastens the gear to the distributor shaft. We use a soft brass drift punch when driving in a new roll pin.

Once you confirm and adjust the distributor’s gear endplay, it is time to address a second distributor adjustment. Remember that you should work on the gear endplay first before worrying about this next process. We finished our calculations and used Moroso’s distributor gear shims to compensate for the excessive endplay.

Overall Distributor Installed Height

The second means of achieving gear sync is through the installed height of the distributor. When our engine was undergoing a test fit of the various components, we saw our distributor bottoming out on the oil pump drive. This was indicated by the distributor flange sitting well above the intake manifold face. Bottoming out is addressed by Performance Distributors in one of their tech columns available on their website.

To adjust the distributor’s height when mounted in the engine, we added nylon distributor shims by Performance Distributors. These shims are made from durable, high-impact nylon and are available in packs of three that include .030-, .060-, and .100-inch thickness. You may be tempted to stack gaskets to raise the distributor’s height. Stacked gaskets will compress and the distributor will eventually bottom out again. Distributor shims will not compress.

We first noticed our distributor was resting .210-inch above our intake during its first test fit, requiring us to use shims (left). Performance Distributors offers its Deluxe Street/Strip HEI with an adjustable collar as an alternative (right). Instead of using shims, you can move the collar up and down the distributor's shaft to achieve a proper installed height.

To achieve overall installed height, use feeler gauges to determine the gap distance between the distributor housing clamp-down collar and the intake manifold surface. When you have a gap, that means the distributor is bottoming out against the oil-pump drive. If you have no gap and the distributor shaft will move up and down in the housing, you should be okay to clamp it down. If not, once your distance is determined, add .010- to .025-inch (the desired gap with no gasket or shim) to the distance you measured. This extra distance is to elevate the distributor to its proper installed location. Don’t forget to include the thickness of the gasket in your measurement. In our case, we had a gap of .210-inch.

Adding .010- to .025-inch to our .210-inch gap gives us .220- to .235-inch. By stacking Performance Distributors’ .100-inch and .060-inch shim, plus our .080-inch-thick gasket, we now have a shim pack height of .240-inch. This will properly mesh the distributor gear with the camshaft gear. The shims and gasket might seem a little thick, but factoring in some gasket crush puts us right in our target range.

Cam And Distributor Gear Tooth Contact

Like checking tooth contact on a differential ring-and-pinion gear, you can use machinist dye or a light grease applied to the distributor gear to check fitment between the camshaft and the distributor gear. Many engine builders perform this practice for high-RPM or extreme racing applications.

distributor gear

With the shims and gasket installed, our aftermarket distributor now meshes properly with our camshaft gear.

To do this, apply your checking compound on the distributor gear. Then, install your distributor and ensure the distributor is clamped down and the cap is removed. Now, rotate the engine by hand several rotations. Use your other hand as resistance for the distributor rotation. After a few revolutions, remove the distributor and inspect the gear wear pattern. You should have a consistent pattern through the middle of the gear.

The Knee Bone Is Connected To The… Well, You Get It

Building a high-performance engine is a long process of checking and rechecking tolerances, fitment, and measurement specifications throughout every single component. The distributor might appear to be a passive component as it spins around for ignition purposes, but the mesh with the cam, plus its duty to properly drive the oil pump is critical. Do not take its importance lightly. The load on the distributor drive is heavier than you may think when rotating a high-volume and/or high-pressure oil pump.

If the gear is not properly fit, you can grind away at the distributor and cam gears, adding unwanted metal shavings to your oil. But now, thanks to this article,  you can ensure to get the mesh where it needs to be and not have to worry about that.

Article Sources

About the author

Todd Silvey

Todd has been a hardcore drag racing journalist since 1987. He is constantly on both sides of the guardwall from racing photography and editorship to drag racing cars of every shape and class.
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