It’s All in the Twist: Taking the Confusion Out of Distributor Gear

Choosing the proper distributor gear for different camshafts isn’t difficult, but there’s much more to the story than just picking the right gear material.

Choosing the proper distributor gear for different camshafts isn’t difficult, but there’s much more to the story than just picking the right gear material.

(Editors Note: This tech article was originally produced for our sister site Power & Performance News, but the tech included is so useful we thought it would be helpful here too. Yes, there are references to drag racing and hydraulic roller lifters, but much of the tech here is very useful for dirt track racing.)

Oftentimes, it is the simple little parts that can cause the most confusion. In the ’50s and ‘60s, for street performance and race engines there was just iron flat tappet cams and a matching iron distributor gear and life was easy. Then came steel billet mechanical roller cams, and the hard steel distributor drive gear would eat that soft iron gear and mess up the cam along the way.

So the solution for short use drag engines was a softer gear made of a copper-bronze alloy. When hydraulic roller cams began showing up on OE production engines in the late ‘80s, that’s when the confusion started.

That’s why we thought we’d run through all the different applications and squelch rumors, dispatch some myths, and generally shed a little light on what has become a bit of a confusing story. Most of this discussion will target V8 Chevrolet engines, but the generic information is accurate for all makes and models of engines.

When roller cams first appeared, they were generally machined out of a very hard 8620 or even 9310 alloy steel core. These cams require a soft, bronze alloy distributor gear. More recently, cam companies like Comp can press an iron gear on the end of the shaft to accept a stock iron distributor gear for street use. One difficulty is determining the gear material on an unknown cam. Steel cams can sometimes (but not always) be identified by the copper surface color between the lobes. These cams usually run the same alloy steel gear, but it is possible to have an iron gear grafted to the cam as an option.

When roller cams first appeared, they were generally machined out of a very hard 8620 or even 9310 alloy steel core. These cams require a soft, bronze alloy distributor gear. More recently, cam companies like Comp can press an iron gear on the end of the shaft to accept a stock iron distributor gear for street use. One difficulty is determining the gear material on an unknown cam. Steel cams can sometimes (but not always) be identified by the copper surface color between the lobes. These cams usually run the same alloy steel gear, but it is possible to have an iron gear grafted to the cam as an option.

This is a steel Comp Cams mechanical roller cam for a big-block Chevy. While the gear on the cam appears to be iron, it is in fact a steel gear that has been parkerized for hardening. One way to tell this is a steel cam is the surface between the cam lobes is very smooth. This indicates a billet steel cam. We ran this cam with a normal iron gear and experienced significant wear in a very short period of dyno time. There is no fix for this other than replacing the cam.

This is a steel Comp Cams mechanical roller cam for a big-block Chevy. While the gear on the cam appears to be iron, it is in fact a steel gear that has been parkerized for hardening. One way to tell this is a steel cam is the surface between the cam lobes is very smooth. This indicates a billet steel cam. We ran this cam with a normal iron gear and experienced significant wear in a very short period of dyno time. There is no fix for this other than replacing the cam.

Cam Core Materials
Distributor gear compatibility is really all about materials. The original cast iron camshafts employed a compatible cast iron gear that requires no further discussion. When steel roller cams became popular in drag racing in the ‘60s, they were and are still made out of very hard 5150, 8620, and other carburized steels. Since the distributor drive gears on the cam were of this same material, the simple fix was to use a softer copper-bronze alloy distributor gear. This is a much softer metal and these gears have a reputation for rather short life, often requiring replacement.

What muddied the gear compatibility waters was when the OE’s converted to hydraulic roller camshafts in the mid ‘80s. The cams are made from a slightly harder material, specifically 5150 steel for the GM engines, that required a more robust distributor gear. Both GM and Ford used a heat treating process called melonizing to harden the distributor gears. At least for the GM pieces, these gears were easy to identify with a light silver cast to the material. For many years, these gears were only available through Ford and GM.

This push to hydraulic roller cams has perhaps caused some of the confusion when it comes to matching up distributor drive gears. According to Comp Cams’ Valve Train Engineering Group Manager Billy Godbold, an easy way to tell if the hydraulic roller cam is compatible with a stock iron distributor gear is to look at the cam’s part number.

Any ductile iron hydraulic roller cam with a -8 suffix in the part number will be compatible with a stock iron distributor gear. As an example, a Comp Xtreme Energy Retro-Fit XR276HR cam (PN 11-423-8) uses an austempered ductile iron camshaft that will work just fine with an iron distributor gear. These cams are sometimes referred to as Selectively Austempered Ductile Iron (SADI) cams.

We had a situation recently where we ordered a Comp Cams mechanical roller camshaft that appeared to be fitted with an iron gear. We discovered it was actually a steel mechanical roller cam that was not fitted with an iron drive gear. The Parkerizing that covered the gear fooled us into thinking it was an iron gear pressed onto the end of the cam. We used a stock iron distributor gear on the engine and in less than an hour of running time on the dyno, both the cam and distributor gears suffered sufficient damage and the cam had to be replaced. Besides the obvious bronze gear, there was a simple alternative, but we were in the dark.

There are essentially four types of distributor gears. On the far left is the standard iron gear, in the center is a typical bronze alloy gear, and on the right is a melonized gear from GM. The fourth style, not shown is a composite gear that is available from Comp Cams and is compatible with all steel roller cams. Comp recommends a new distributor gear with any new camshaft.

There are essentially four types of distributor gears. On the far left is the standard iron gear, in the center is a typical bronze alloy gear, and on the right is a melonized gear from GM. The fourth style, not shown is a composite gear that is available from Comp Cams and is compatible with all steel roller cams. Comp recommends a new distributor gear with any new camshaft.

This is a bronze alloy gear that has experienced severe wear. Comp Cams recommends that if a steel gear cam has been run with a bronze distributor gear that a new bronze gear be used from then on. Substituting a melonized gear after a significant wear pattern has been established may result in severe gear wear to both the cam and distributor gear.

This is a bronze alloy gear that has experienced severe wear. Comp Cams recommends that if a steel gear cam has been run with a bronze distributor gear that a new bronze gear be used from then on. Substituting a melonized gear after a significant wear pattern has been established may result in severe gear wear to both the cam and distributor gear.

Distributor Gears
So now with three different types of cam core materials – cast iron flat tappet cams, ductile iron hydraulic (and some mechanical) rollers, and steel mechanical (including a few hydraulic) roller cams. As it currently stands, there are a total of four different distributor gears you can use – stock iron, bronze alloy, composite, and a melonized gear. There’s been plenty of confusion over which distributor gears are compatible with these different camshafts.

The simplest, all-encompassing, and easiest answer is melonized distributor gears will work with all three types of camshafts – even the steel roller camshafts. To make this a little less confusing, we’ve included a chart that will itemize each cam material and which distributor gear will work with that material. In the course of our research, there has been some significant confusion around this point, and there are published stories on the internet that state melonized gears can only be used with a SADI, or ductile iron cam. That is not true.

According to Godbold, the melonized heat treatment gives the gear a very slick surface, which makes it cozy with any cam gear material, period. To make this even easier, Comp now offers a line of melonized distributor gears for both small- and big-block Chevrolet applications and also for the Ford. Right now, for the GM engines this is limited to just the 0.500-inch shaft diameter distributors such as those from MSD, but that’s fine because GM still offers the factory gear for the standard GM shaft diameter of 0.491.

Distributor gears also drive the oil pump on most engines, and the extra load from a high volume pump and high viscosity oil can place undue stress on the drive gear, especially during engine warmup when the oil is cold. The worst case scenario is 20w50 oil, a high pressure/high volume oil pump, and tight bearing clearances.

Distributor gears also drive the oil pump on most engines, and the extra load from a high volume pump and high viscosity oil can place undue stress on the drive gear, especially during engine warmup when the oil is cold. The worst case scenario is 20w50 oil, a high pressure/high volume oil pump, and tight bearing clearances.

One way to extend distributor gear life on a small- or big-block Chevy is to mark the location of the distributor, remove it and cut a 0.030-inch slot in the distributor body’s lower sleeve. This will direct pressurized oil at the cam and distributor gear.

One way to extend distributor gear life on a small- or big-block Chevy is to mark the location of the distributor, remove it and cut a 0.030-inch slot in the distributor body’s lower sleeve. This will direct pressurized oil at the cam and distributor gear.

Distributor Gear Chart

The X’s in the chart indicate that the distributor gear is compatible with the camshaft core. We did not check bronze alloy gears for cast iron or ductile iron cams, although you can use them in these applications. We did not check them because either the stock iron or the melonized gears offer much better wear characteristics.

Distributor Gear Material

Cam Core Material Melonized Steel Iron Bronze Alloy Composite
Cast Iron X X X
Ductile Iron (SADI) X X
Billet Steel X X X

Tech Tips
Beyond selecting the correct gear for the camshaft in question, there’s plenty of useful information that can help improve gear life. This starts with ensuring that the gear is properly engaged with the camshaft in the engine. Often, custom machine work includes decking the block and cylinder heads during the blueprinting process. This lowers the distributor mounting pad and directly affects the interface with the gears.

MSD and Crane, among others, offer distributors with a movable collar so that the distributor gear can be custom aligned to the cam gear.

MSD and Crane, among others, offer distributors with a movable collar so that the distributor gear can be custom aligned to the cam gear.

To custom set the distributor height, first loosen the collar, remove the gasket, and place the distributor in the block, allowing it to bottom completely.

To custom set the distributor height, first loosen the collar, remove the gasket, and place the distributor in the block, allowing it to bottom completely.

This is why MSD, Crane, and others offer distributor housings with an adjustable height collar that allows the engine builder to properly set the distributor gear position on the cam. MSD makes a tool for checking and adjusting the distributor collar height, but you can also do this with a feeler gauge.

First, remove the gasket and then drop the distributor into the hole with the collar loose. Once the distributor bottoms out against the oil pump drive, raise the distributor body between 0.010 and 0.030-inch and tighten the collar. This will place the distributor in its proper location. You should also check the wear pattern by coating the distributor gear with white lithium grease and rotating the engine several revolutions. The pattern left on the gear should place the wear in the center of the gear.

Next, raise the distributor between 0.010 and 0.030-inch. We used a dial indicator with a magnetic base to measure 0.020-inch. Now lock the collar in place with the set screws.

Next, raise the distributor between 0.010 and 0.030-inch. We used a dial indicator with a magnetic base to measure 0.020-inch. Now lock the collar in place with the set screws.

Just to make the selection process for distributor gears a bit more confusing, for small- and big-block Chevys all OE distributors use a 0.491-inch shaft diameter. MSD distributors use a slightly larger 0.500-inch shaft diameter and therefore require a different gear.

Just to make the selection process for distributor gears a bit more confusing, for small- and big-block Chevys all OE distributors use a 0.491-inch shaft diameter. MSD distributors use a slightly larger 0.500-inch shaft diameter and therefore require a different gear.

The biggest issue involving excessive distributor gear wear can be traced to the combination of high pressure/high volume oil pumps and high viscosity oil.
Many enthusiasts believe that high oil pressure is a good thing, as if high pressure somehow does a better job of lubricating the engine.

If you think about the physics of the engine, you may realize that pressure is merely an indication of a restriction to flow. This means that we are evaluating oil flow by measuring the restriction to the flow. High pressure means there is a significant restriction holding back the amount of flow that the pump is attempting to provide. This essentially means the engine is exerting power to produce this high oil pressure. This is a parasitic load that is subtracted from overall engine output. Granted, we’re talking about low single digit numbers here, but the fact remains that these pumps place undue wear on the distributor drive gear.

In talking with engine builder Kenny Duttweiler, he thinks that excessive wear originates not at high rpm when the load is significant, but perhaps when the engine is first started with cold oil with sometimes very high oil pressure. If you think about this, the oil is cold, which increases its viscosity which also means it make take a few more moments before adequate lubrication is present at the distributor gear.

Remember that distributor gears are only splash oiled unless engine modifications are performed to direct pressurized oil onto the distributor gear. The combination of high pressure (which means high load on the gear) lubed with a sluggish, heavy high viscosity oil at idle where splash oiling may take several moments to lube the gear can be a high-load situation that will produce wear.

The quick solution may be to consider running a slightly lower viscosity oil. For example, if you have been running a 20w50 oil and experiencing excessive distributor gear wear, perhaps consider moving to a 10w40 or even a 0w50 (Mobil 1).

Driven Racing Oil offers several alternatives including a HR10w40 that offers excellent protection combined with a lower viscosity number when the oil is cold. We’ve created a chart that lists comparisons of the viscosities at various engine temperatures and the results are eye opening. These numbers indicate that just changing to a lower “winter, or w” viscosity can be worth dramatic differences in pumping requirements when the engine is cold. This simple change might give your distributor gear a fighting chance for survival.

Comp also offers a composite gear for GM 0.491-inch shaft distributors. The composite gear is softer and more conformable than the other gears, but is more expensive. It can be used with any type camshaft but it is also just as susceptible to wear if used with high viscosity oil and high volume oil pumps.

Comp also offers a composite gear for GM 0.491-inch shaft distributors. The composite gear is softer and more conformable than the other gears, but is more expensive. It can be used with any type camshaft but it is also just as susceptible to wear if used with high viscosity oil and high volume oil pumps.

Comp Cams offers an optional service to apply a parkerized heat treat to iron flat tappet cams to help prevent lifter scuffing during break-in. The cam will come with a darker appearance like this cam. This also hardens the distributor gear and requires the use of a melonized distributor gear.

Comp Cams offers an optional service to apply a parkerized heat treat to iron flat tappet cams to help prevent lifter scuffing during break-in. The cam will come with a darker appearance like this cam. This also hardens the distributor gear and requires the use of a melonized distributor gear.

Of course, another solution to excessive distributor drive gear wear would be to reduce the load it must handle by using a standard volume, standard pressure style oil pump as opposed to high volume/high pressure pumps. Reducing the amount of oil pumped also reduces the amount of oil pumped to the top of the engine that must return to the sump on a wet sump engine and it is this oil that contributes to windage issues that do cost horsepower.

As a point of reference, much of Dart Machinery’s efforts in designing the new LS Next block is intended to minimize the effects of excessive windage on the crank in the GM LS engine. While Dart has removed the deep skirt sides from the block, the company has also gone to serious efforts to reduce the amount of internal leakage around the lifters and the main and rod bearings. All this reduces the amount of oil that has to be pumped. Granted, the LS engine doesn’t drive the oil pump with a distributor, but the net gains from reduced windage are, nevertheless, very real.

On older engines still using distributors, higher engine speeds and greater engine output place greater loads on the lubrication system. We’ve covered several recommendations that should reduce the amount of load on the distributor gear and some of these recommendations may in fact lead to improved power. If nothing else, your distributor gear will last much longer.

Oil Viscosity Ratings

Oil viscosity is a major story in itself, so we’re going to just hit the critical points to explain why thicker oil is not always the best idea. When it comes to oil, we’re all familiar with viscosity ratings. Viscosity is a term that relates to a fluid’s resistance to flow. What many don’t realize is that water is used as the basis for all viscosity ratings, which carries a rating of zero (0).

The viscosity numbers on a bottle of oil indicate the SAE rating of the oil’s “thickness.” Multi-viscosity oils are rated with two sets of numbers such as 5w30. The first number in each set relates to the oil’s thickness at 0 degrees Celsius (32 degrees Fahrenheit) and is further delineated by the letter “w,” which stands for winter grade oil. The second number rates the oil’s viscosity  at 100 degrees C (212 degrees F).

A multi-viscosity oil is designed to flow like a winter grade oil when cold and then, using viscosity index improvers, “thicken” as its temperature rises. A single grade oil tends to be extremely viscous (thick) at low temperatures.

We’ve created a chart that uses centistokes (cSt) as a measuring device for flow at a given oil temperature. We won’t get into defining cSt except to say that the higher the number, the more power is required to pump the fluid at the rated temperature. This means that a higher cSt number will require more power to pump the oil at that specified temperature.

You can see from the chart that at 50 degrees F (10 degrees C), the higher viscosity 20w50 oil is roughly three times “thicker” than a 5w50. This does not mean it will require three times the power to pump, but there is a direct relationship between the two.

If we then compare the same viscosities at 212 degrees F, which could be considered a normal oil temperature for most engines, the cSt difference between 20w50 and 5w50 is less than 10 percent. If we push the envelope to nearly 250 degrees F, the viscosities are nearly the same. So if engine protection at high oil temperatures is a concern, you can see there is very little difference at 250 F between 5w50 and 20w50, but the difference between these viscosities at cold oil temperatures is dramatic. This illustrates why 5w50 multi-viscosity oil would be an excellent choice to reduce the cold oil pumping load on the oil pump drive and distributor gear compared to a 20w50.

Viscosity Ratings at Various Engine Temperatures

Viscosity cSt at 50 deg. F cSt at 212 deg. F cSt at 248 deg. F
5w30 300 11 7.5
5w50 410 17.5 11.75
10w40 530 15 9.75
20w50 1,150 19 12

Distributor Gear Parts List

Description Material Shaft Dia.  (inches) Part Number
Comp melonized SBC, BBC MSD 0.500” shaft Iron 0.500 410M
Comp melonized SBF 302/ 351W Steel 0.468 431M
Comp melonized SBF 302/ 351W Steel 0.531 435M
Comp SBC/BBC bronze 0.491 412
Comp SBC/BBC bronze 0.500 410
Comp SBF 289/302 bronze 0.467 431
Comp SBF 0.500 bronze 0.500 438
Comp SBC/BBC composite 0.491 12200
GM melonized SBC, BBC stock distributors Iron 0.491 19052845
MSD melonized for SBC MSD Iron 0.500 8561
MSD Ford 302 Steel 0.531 85834
MSD steel for 31C/460 w/ hyd. roller cams steel 0.466 85813
MSD SBC, BBC bronze 0.500 8471
MSD Ford 289/302 bronze 0.466 8583
MSD 351C. 400, 429,460, FE bronze 0.530 8581

 

 

Article Sources

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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