[Cliff R]

Decided to pull the distributor from my 455 today and give it a good cleaning/inspection. The decision was forced by an oil leak under it from the gasket failing. It's been in service form many years and never been touched other than driving a BOP gear on it for my first roller cammed 455 and a bronze gear on it for the 2nd one.

We decided to do a durability test with the bronze gear rather than just using it to "break-in" the cam drive gear then switching to a polymer gear.

The cam drive gear area was bead blasted with ultra fine glass beads after having all the sharp edges dressed with a jewelers file.

The engine was placed in service back in 2009 and has a LOT of hard use, including quite a few street miles and many hundreds of drag strip runs. I also drive it to our local track for Friday night "test and tune" sessions.

The gear is in near perfect shape, with nothing more than a smooth area worn on the drive side where it contacts the roller cam drive gear. The timing has never budged once since it's been placed in service and no "glitter" in the oil at oil changes or visible under the valve covers when they have been off to lash the valves.

I also decided to test the timing curve after it was finished to make sure it was still working correctly. Once placed back in the engine and fired up base timing was set at 10 degrees BTDC with the engine at 700rpms, no vacuum advance hooked up. The timing was rock solid in and out of gear.

I then revved the engine slowly and noted the advance it various rpm. At 900rpms advance was up to 12 degrees, and moved slowly and steady to 22 degrees at 2000rpm's. Advance continued smoothly to full advance at 3200rpm's. The parts of all factory, center cam, weights and springs. We did add a positive stop for the advance so it is limited to 10 degrees (20 at the crank) and added a vacuum advance set for 15 degrees additional timing at light throttle cruising (ported vacuum).

This may help clear up some of the information on distributor gears to run with aftermarket steel roller camshafts. From what we've seen here the polymer gears are fine for long term use IF take steps to make sure the drive gear is smooth and lacking tooling marks and sharp edges.

It also appears that a high quality bronze gear will also make the grade using the same drive gear preparation methods........Cliff

[grivera]

Thanks Cliff- by any chance do you know what the brand of the bronze gear is?

[JLMounce]

Good information. Thanks for posting this.

I've been running the BOP polymer gear now for two seasons. All I did to prep my cam gear was to make sure there were no obvious flashing or other flaws that would bite into the gear. Cleaned it and stabbed it. This is after two years of service (probably 4000-5000 miles)

You can see that it's in pretty good shape. There's some very minor scrubbing happening on some of the teeth that would likely be better off with more thorough prep.

[Cliff R]

I'm pretty sure it is from Comp Cams, part number 451 for Pontiac distributors with the .491" stock shaft diameter.......Cliff

[Steve C.]

A fwiw, I agree with Cliff that a high quality bronze gear can make the grade under the right circumstances. Not always the horror stories often told. Before the BOP composite distributor gear I ran the MSD bronze gear with no issues in numerous engine combinations. I personally believe that the advice from MSD is key....

Articles from MSD will point out the effort that goes into a ring-and-pinion installation. Yet many don't think twice about the distributor gear/cam gear relationship unless performance starts to suffer. How the dist gear and cam gear mesh is just as important as the quality of the metals involved. MSD states the excessive clearance between the gear teeth shouldn't be fixed by shoving the distributor farther into the hole; this screws up the proper wear pattern.


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[tom s]

FYI,when I was doing a new engine breakin on Joe Shermans dyno we always ran the stock dist gear on the billet cams.After the dyno we would put on a bronze or a poly gear.He never dressed the cam before he put it in.NEVER had a issue with the poly gears.The bronze gears after a couple years with a lot of street driving would show some wear but nothing catastrophic.Tom

[chiphead]

Cliff, et al.... Would you guys dress all HR cam gears with a small file, even if they didn't appear to have sharp edges? The Lunati HR I've got, has a gear that looks pretty smooth.

The instructions with my MSD bronze gear say to block the filter relief. Is this really required with a bronze gear??

How do you judge the correct amount of backlash between the gears?

[Schurkey]

Originally Posted by Steve C.

MSD states the excessive clearance between the gear teeth shouldn't be fixed by shoving the distributor farther into the hole; this screws up the proper wear pattern..

How would shoving the distributor farther into the hole change the clearance? The gear isn't tapered. Given the helical curve of the gear teeth, you'd change timing by putting the housing in deeper--unless you turn the housing to compensate for the curved gear teeth. The gear clearance remains the same.

It's not like a differential, where the pinion gear is tapered, so moving it in or out changes backlash.

[61-63]

Cliff thank you this was informative.

[Cliff R]

The shim used above the gear against the housing will determine the actual depth of gear in the engine and where on the gear the contact patch will be established. This happens because the gear has helical cut teeth and is driven upward when the cam starts turning. So a thinner shim raises the gear, thicker one lowers it.

Mine has a heavy shim on the distributor so if you look very closely at the gear the contact patch is slightly centered more toward the bottom of the gear, maybe about .030" or so offset from center, but it is difficult to see in the pic. All contact is still on the gear so maximum contact patch combined with minimum shaft end play. The shaft end play on my distributor is .022" and that's pretty much where it started out at WAY back when first placed in service.

I've never taken the time to check "backlash" as there really isn't any way to move the driven gear closer to the drive gear that I know of, so you pretty much get what you get there.

So basically you have control of two things. Shaft end play with the distributor and to some extent depth of the gear based on that measurement.

That doesn't hold true for all GM distributors as Olds engines have the gear turning in a manner that it gets pushed down under load and against a piece in the block designed for that purpose. You also have to keep in mind that heavy decking and milling will effect distributor gear to cam gear relationships with any engine that uses a hole/mounting pad on the intake manifold for the distributor. For our Pontiac engines the hole is in the block, so we don't have to deal with anything beyond distributor shaft end play and gear depth below the block surface......Cliff

[Cliff R]

"Cliff, et al.... Would you guys dress all HR cam gears with a small file, even if they didn't appear to have sharp edges?"

Yes, no harm done smoothing the leading edges of the drive gear with a fine file, but the real work is done in the bead blaster when using ultra fine material as it pretty much polishes the gears and removes any and all tool marks from them. The material I use looks and feels like baking soda. It really doesn't remove any material but does a very nice job of providing a nicely polished surface finish.

I'd add here and for sure I'm NOT recommending this, but several very well known engine builders use stock factory iron gears on these roller camshafts. They really aren't advertising it, but I've had two engines in here to date from other sources that used roller cams and they shimmed and installed stock Pontiac gears on the distributors used in them.

Neither one showed any signs of being unhappy, no measureable wear on either the cam drive gear nor the factory iron distributor gear. This still didn't give me enough of a "warm and fuzzy" to put them back, so they were replaced with gears designed to run on roller camshafts.....FWIW......Cliff

[dataway]

I remember a story about some Harley factory mechanic for the XR750 race bikes that would run in the gear boxes on the bench with very fine "sand" in them to mate all the parts quickly and reduce friction. Of course he would disassemble and clean before putting them into use.

[Lee]

I recently talked to an old friend, whose motor I built back in 2007. He has over 50,000 miles on it now, HR cam from Bullet and a bronze gear I bought from RPM Race Parts (part # S3012). The rear oil plug was drilled to about 0.060". The motor is still running great for him. I did tell him that, with that many miles, I'd like for him to either bring the car to me or have somebody pull the distributor and check the gear!

I also am friends with the current owner of Warren Ksiez's old T/A. It has had the same bronze gear on its solid roller cam since around 2000 or so, but probably has less than 20,000 miles on it. Warren first tried running a stock distributor gear, but it was knife-edged within a few hundred miles. I think I still have that gear, in a box of my "show & tell" stuff.

[Cliff R]

I will not mention the "big name" shops the engines I got into were from, but twice now I've found stock Pontiac distributor gears being used on roller camshafts. One was on a solid roller, the other on a hydraulic roller, both from Comp and neither one had an iron gear pressed on them. Couldn't tell you exactly how many miles each of those ill fated engines had on them, but at least the stock cast iron distributor gears survived the ordeal and looked in perfect shape.......Cliff

[71 T/A]

Once, I used a Comp Cams bronze distributor gear. In a few months, the gear was already getting chewed up. Replaced it with the BOP polymer gear and haven't had any issues with two different stroked, Hroller engines.

[Scott65]

This is timely for me, as I have to decide on a gear for the cam sync I'm making. I've been running the BOP composite gear on my distributor, but it won't fit the stock shaft. Not relishing the idea of another $124 gear... Thanks Cliff, Lee, and others for posting your results with the bronze gears.

[Steve C.]

BOP gear shaft sizes include .491-inch for stock applications and .500-inch for aftermarket MSD.


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[Scott65]

Originally Posted by Steve C.

BOP gear shaft sizes include .491-inch for stock applications and .500-inch for aftermarket MSD.


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Yes, but at more than double the cost of the bronze gears. So it's nice to know that people are having good results with the bronze gears (and attention to detail)

[Skip Fix]

So why do "melonized" distributor gears work on Chevy factory roller cams? What process is that?

[77 TRASHCAN]

Originally Posted by Skip Fix

So why do "melonized" distributor gears work on Chevy factory roller cams? What process is that?

From what I've read...The factory Chevy cams are NOT billet, they are some type of iron??? Anyone?

It is a different deal...just not remembering the details...