Distributor Gear Mesh Can Make or Break Your Valvetrain


Many performance enthusiasts use a “grab and go” approach when installing many aftermarket parts. New heads: bolt them on. New contribution: screw it. New distributor: screw it. With so many aftermarket components available for your performance or race engines today, a few thousandths of an inch here and there can stack up when assembling multiple components. Combined, this The “tolerance stack” can mean that each part, individually, is within specification, but when combined there are fit issues.

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

When it comes to cumulative tolerances of aftermarket heads, sheet metal 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 is between your distributor gear and the distributor housing. This process of developing specific plans has many reputable builders who differ slightly in opinion.

Dispenser Equipment Planning

Distributor end play is critical because the distributor gear naturally wants to ride 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’s easy to imagine why these tolerances are critical.

The clearance between the distributor housing and the gear shift when the engine warms up. Short of describing complex calculations of a metal expansion, just know that you should not remove all end clearances from the distributor. As metals expand, end play can become a tight fit between the gear and the housing when an engine reaches its maximum operating temperature.

The gap between the distributor gear and your housing is a very important dimension. Our new performance distributor came out of the box with a gap of 0.040 inches, which is too much. You can measure your gap with a dial indicator or feeler gauges.

We referenced our General Motors Handy Service Manual from the 70s, which says the gap between the top of the distributor gear and the housing should be 0.005 to 0.007 inches. Many enthusiasts take this tolerance as gospel for pre-1976 performance engines and have applied it to later model distributors as well. But wait, there’s more.

Not all dispenser tolerances are the same

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

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

Most professional shops will consider a looser factor than the factory specified gap for aluminum cases (0.010 to 0.025 inches). I have a trusted friend and pro engine builder who says he will tighten the aluminum tolerance to as little as 0.006 inches if it is a drag racing application.

Why would he tighten tolerances? Because he said a race engine doesn’t reach such a high overall operating temperature over a long period of time during a race. Also consider that most racing engines use a machined steel roller camshaft. When mated to a billet cam, distributors need bronze gear to survive. If you tighten these tolerances slightly, you have less up and down walking with the distributor gear, which is good for the life of the sacrificial bronze material.

Peeking into the distributor hole of your intake, some take for granted the additional stresses and harmonics placed on the distributor with the drive of the oil pump. The gear angle contact between the Chevrolet cam and the distributor gear wants to push up on the distributor gear.

Distributor drive sprocket material

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

You can choose from 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 secures the gear to the distributor shaft. We use a soft brass pin punch when driving in a new roll pin.

Once you have confirmed and adjusted the distributor gear end play, it is time for a second distributor adjustment. Remember that you need to work on the final gear clearance first before you worry about this next process. We completed our calculations and used Moroso’s distributor gear shims to compensate for excessive play.

Dispenser Total Installed Height

The second way to achieve gear synchronization is through the installation height of the distributor. When our engine was going through a fit test of the various components, we saw our distributor bottoming out in the oil pump drive. This was indicated by the distributor flange well above the face of the intake manifold. The bottom is covered by performance distributors in one of their technical columns available on their website.

To adjust the height of the distributor when mounted in the engine, we added nylon distributor shims from Performance Distributors. These shims are made from durable, impact-resistant nylon and come in packs of three that include 0.030, 0.060, and 0.100 inch thickness. You might be tempted to stack joints to increase the height of the dispenser. The stacked gaskets will compress and the distributor will eventually come down. Distributor shims do not compress.

We first noticed that our distributor sat 0.210 inches above our intake on its first fit test, requiring us to use shims (left). Performance Distributors offer their 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 shaft to achieve proper installation height.

To obtain the overall installation height, use feeler gauges to determine the distance between the distributor housing clamp and the surface of the intake manifold. When you have a gap, it means the distributor is bottoming out against the oil pump drive. If you don’t have space and the distributor shaft moves up and down in the housing, you should be able to tighten it. Otherwise, once you have determined your distance, add 0.010 to 0.025 inches (the desired gap without gaskets or shims) to the distance you measured. This extra distance is used to elevate the dispenser to its proper installation location. Don’t forget to include the gasket thickness in your measurement. In our case, we had a discrepancy of 0.210 inches.

Adding 0.010 to 0.025 inch to our 0.210 inch gap gives us 0.220 to 0.235 inch. By stacking the 0.100-inch and 0.060-inch shims from Performance Distributors, along with our 0.080-inch thick gasket, we now have a shim set height of 0.240 inches. This will properly mesh the distributor gear with the camshaft gear. The shims and seal may feel a little thick, but factoring in some seal crush puts us right in our target range.

Cam and Distributor Gear Tooth Contact

Similar to checking tooth contact on a ring and pinion differential gear, you can use a machinist’s dye or light grease applied to the distributor gear to check the fitment between the camshaft and the distributor pinion. Many engine builders apply 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 control compound to the distributor gear. Next, install your distributor and make sure the distributor is tightened and the cap is removed. Now rotate the motor several revolutions by hand. Use your other hand as resistance for the rotation of the dispenser. After a few turns, remove the distributor and inspect the gears for wear. You should have a consistent pattern in the middle of the gear.

The knee bone is connected to the… well, you got it

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

If the gear is not properly adjusted, you can grind the distributor and cam gears, adding unwanted metal shavings to your oil. But now, thanks to this article, you can make sure you have the mesh where it needs to be and not have to worry about it.


Comments are closed.