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post #1 of 12 (permalink) Old 04-11-2005, 06:56 PM Thread Starter
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Distributor gear nightmare

I broke in the cam on a rebuilt 351 Windsor and am experiencing the worst case of distributor gear failure that I have ever encountered. The motor in question is a rebuilt 1978 block with stock crank & rods with ARP fasteners installed, hypereutectic pistons, Clevite 77 bearings and Speed Pro/Federal Mogul cam & timing set. The distributor is a remanufactured Ford Dura-Spark and drove a high volume oil pump. The motor is in a Cobra replica. By trade, I am more a machinist than a mechanic, therefore I am competent in my abilities to do precise measurements and found the following:

- Distributor gear to distributor base dimension is 4.035”
OEM tolerances as stated by Ford Racing are 4.031 – 4.038”

- Distributor base counter bore (on top of block) to machined surface where gear thrust face contacts machined surface in block (where oil pump shaft protrudes) is 4.035”. This means that when end play is removed from distributor shaft, bottom gear face is lightly contacting machined thrust surface in block

- Distributor shaft end play is .029”
OEM tolerance as stated by Ford Racing is .024” - .035”

- Cam is cast iron flat tappet hydraulic and distributor gear is cast iron (Ford racing blue color coded gear)

- Camshaft endplay is .003”

- I ran the oil pump numerous times with a drill before starting the engine and observed an abundance of oil supply on the machined surface in the block where the distributor gear face makes contact

I broke the cam in for approximately 20 minutes at 2000 rpm, and shut it off to make some adjustments. I then restarted intending to run the cam in for an additional 10 minutes, but after about 4 minutes the engine quit. I pulled the distributor and found that 2 or three teeth of the cam gear were completely sheared and in the bottom of the oil pan. The remaining teeth were shaved down to about half thickness and starting to become “rolled over” to the point that they were cracking and on the verge of breaking off. The roll pin was intact and showing no signs of damage, so I suspected that this was more a case of poor gear mesh rather than excessive load from the high volume oil pump (or, that load was constant instead of abrupt ie: jammed oil pump gears).

Weighing the cost of a new gear and labor time to install it, I instead bought another distributor. I measured the gear dimensions and positions on both distributors and shaft end play. To reduce load on the gear, I replaced the high volume pump with a stock one. I also removed the oil pan, cleaned it and refilled with new oil and inspected the inside of the motor for metal particles and also checked oil pump shaft for correct endplay. I then ran the engine for approximately 8 more minutes & pulled the distributor to check gear wear. Gear showed signs of excessive wear that indicated it would be in the same condition as the first gear, if run for as long.

I then researched this problem and found the below posted chat forum threads that suggested an incorrectly ground cam gear. In one of the responses, the same condition that I experienced was described by a person as, “having the distributor gear teeth shaved off by the sharp corners of the cam gear.

Lastly, I purchased a Cranes Cam Part # 443942 and an iron distributor gear from MSD. I installed the distributor gear to the correct specifications and installed it and the cam. Once again, I removed the oil pan, cleaned it and refilled with new oil. I broke in the new Crane cam & lifters for approximately 25 minutes and took a short drive around the block. I removed the distributor and the gear is once again worn beyond reasonable limits.

I am running out of ideas:
- Are these gears surface hardened? (nitriting process?)
- Could there be a batch of bad distributor gears with insufficient surface hardening?
- Could there be a batch of bad cam blanks with incorrectly ground gears?
- Is there something incorrectly machined in the block in terms of distance or angle (squareness) between the gears? I have a hard time believing this, because this problem should have already surfaced within the last 27 years / 3 rebuilds and would have been cause for someone to have already junked it.

Following is a thread from the club cobra forum: http://www.clubcobra.com/t61733.html

- After voicing a concerned opinion to Comp Cams about abnormal distributor gear wear with the last two cams from them I was told about a new gear made by them that is claimed to last as much as 100,000 miles with no significant wear.
It arrived today…..It's PLASTIC!!!..A composite, but plastic, nevertheless.
The cam gear is also different than the past two cams. No more razor edges... they've been factory smoothed out and are actually a bit rounded on the edges. Sure hope this setup lasts....

- The last two cams I bought from comp came with very sharp gear edges. You can almost see where the gear has shaved off the distributor gear teeth. This last cam is totally different though....You can't feel a sharp edge anywhere.

- Don Scott did a write-up on why the dist gears go out. It's the cam blanks used on aftermarket cams, and 351W's seem to be one of the worst offenders. A little work with a bench wire brush seems to clean it up as you can tell with your new CC cam. Had I known this, I'd have reworked the cam gear before I installed it.

- Someone over a year or two ago on this forum mentioned that burnishing the cam gear on a wire wheel took care of most of the problem they were having with 351W cams. He indicated that they had built 36 engines and had not had a problem after taking off the sharp edges on the cam gear.

- That was me talking about burnishing the gear. I called Comp Cams I don't know how many times to tell them about the problem and they said no way. They always blamed the way the gear was installed, using the high volume oil pump, or whatever else they could come up with. I think it's funny they came up with a new gear to take care of a problem that's so easy to fix. It's been several years since I started burnishing the cam gear and I have not had 1 failure. Don

- Rick Lake: The gear you are talking about is made by Greg Depree. 803-730-7574. As long as you are not a nascar builder, he will sell you one for $80.00. He makes them for comp cams. They are the gears used in nascar motors ford and others make. 3 thing are causing gears to wear out, no or poor oiling to the 2 gears where they meet. Too much end play on the camshaft, causing the gears to rub against each other. The distributor shaft not being square with the block and cam gear. A high pressure, high volume oil pump will help wear out the gear but not in 5k miles. You need more than oil splash to keep those gears with proper lube. There is a trick to put a .030 groove in the lower distrib housing so the oil pours onto where the gears meet.

- Following is thread from the Ford trucks forum:
- http://www.ford-trucks.com/dcforum/perf/714.html

Dave, what brand of camshaft are you running ? At the shop where I work we had a 351w with a cam from Comp Cams. It chewed its way through 6 distributer gears before Comp 'fessed up to the problem. We knew that the cam and dizzy gear need to be made of the same material but after the first two gears were chewed we tried steel and bronze gears and had the same failures. It turns out that Comp incorrectly ground the gears on the end of some of their cam blanks. Who knows how many of their small block Ford cams are out there ? I would try a new cam from another manufacturer with a new dizzy gear. I have always had good luck with Crane. DF

I am running a PAW cam. when I complained to PAW they said that all the cams are ground by just a few companies. They did cave in and send me another cam of the same type. What do you think of the High performace oil pump theory?

I know this is a long message and I thank you for your time in reading and replying. The point that I can’t stress enough is that this goes beyond the incorrect height distributor gear or wrong gear material for the cam (steel vs iron) prognosis. I have shown above that all dimensions were measured correctly and all were within OEM specifications. This is also not a lubrication issue, as I have a flood of oil on the gear thrust surface in the block when the oil pump is turned with a drill.
This is a gear mesh / wear pattern, tooth profile or materials hardness issue. If you know of any leads not mentioned above that I may follow it would be greatly appreciated.

Thank You,

Tom Clark
Owner, The Fab Ford

[ This Message was edited by: TheFabFord on 4/12/05 6:57am ]

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post #2 of 12 (permalink) Old 04-11-2005, 07:36 PM
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Distributor gear nightmare

I'd buy a new cam if I were you. Most likely the angle of the teeth is incorrect.

However, if you wanna try for one last hurrah, try a distro gear from Mallory or Crane; they offer a steel (not Iron gear) of the correct alloy and hardness so that they are compatible with all types of camshafts and offer superb wear characteristics.
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post #3 of 12 (permalink) Old 04-11-2005, 07:52 PM Thread Starter
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Distributor gear nightmare

I have bought a new cam, the Crane cam currently in the motor is replacing the Speed Pro that the short block came with.
I thought that surely the gear on the Speed Pro cam was the culprit, that is why I was sure I had solved it with the Crane cam.
Are you saying that the steel gear from crane is compatible with an iron flat tappet cam? Because my understanding is that IRON MUST BE RUN WITH IRON AND STEEL WITH STEEL. Im not ruling out the possibility that both cam blanks had incorrectly ground gears, especially in light of the fact that the Cran tech told me to send it back for replacement.

[ This Message was edited by: TheFabFord on 4/12/05 7:54am ]
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post #4 of 12 (permalink) Old 04-11-2005, 08:14 PM
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Distributor gear nightmare

According to Crane Cams: "Steel Gear; Works with cast cams, induction hardened roller cams or 8620 steel carburized roller cams.

In a tech question I submitted to mallory on a similar issue they responded "The gear thay comes on this distributor is an alloy steel gear designed to run with hydraulic flat tappet cams as well as the factory Ford hydraulic roller cams. Check with the cam manufacturer and see if this gear is compilable with their cam." This was in reference to a question I had about the gear that came on my recently purchased mallory 47 series distributor.
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post #5 of 12 (permalink) Old 04-11-2005, 10:54 PM
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Distributor gear nightmare

More information is here:


Topic 2: Excessive Distributor Gear Wear on Ford
Excessive distributor gear wear can be a problem with Ford engines. Most of the time, the problem is a result from the use of a high volume oil pump. High volume oil pumps put a lot of stress on the distributor and cam gears. A high volume oil pump is not necessary on a street driven engine that turns no more than 7000 rpms. Only extreme racing engines require the high volume pump. Should you use this type of oil pump, the following precautions will save your distributor gear from early destruction.

(1) Drill a .030" hole in the lifter/oil galley plug behind the distributor. This will allow additional lubrication on the distributor and cam gears. This will not lower oil pressure enough to hurt the engine.

(2) Ford oil pump mounting brackets have elongated holes. Due to this, the distributor shaft and oil pump shaft should be aligned so that the distributor turns freely before tightening the mounting bracket bolts. Failure to do this will cause a binding situation, thus damaging the gear.

(3) Stock Ford hex oil pump drive shafts are know to vary in length which could cause a bottoming or binding situation.

(4) Brass distributor gears can be used to avoid damaging the cam gear. Brass gears are softer and can wear out quicker than the cast gear, but will not cause damage to the cam gear. If using a brass gear, check it occasionally for wear.

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post #6 of 12 (permalink) Old 04-12-2005, 01:22 PM
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Distributor gear nightmare

I had similar problem with a 289.
Stripped the gear drive right off of the cam!
After reading many posts and manufacturers literature, I decided to ditch the high volume pump and go with a stock unit.
From what I understand is that the engines that run loose tolerances benefit from the high volume but engines with tight ones actually are prone to the types of problems you/we have had when running the high volume version.
My .02
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post #7 of 12 (permalink) Old 04-12-2005, 03:33 PM Thread Starter
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Distributor gear nightmare

I appreciate all of the replies........... finkster, thanx for input, but replacing the HV pump with a standard was the first thing that I did.

Being that I have more than adequate lubrication at the gear when priming the pump with a drill, and all of my installed specs are in the middle of tolerance, I am looking at one of two things.
-Gear metallurgy or incorrect profiling of teeth
-Block has out of tolerance machining internally that is placing the distributor gear either too close to the cam gear or out of perpendicularity.

If block dist counterbore to cam bore alignment were out that far this motor would have been scrapped 25 years ago.

I did find out from MSD that iron dist gears are not surface hardened. So, any fault of the gear would have to be due to quality of the cast iron (metalurgy) or an incorrect profiling. Being that these are made by Ford who has a huge budget to cover quality control and re-call procedures, I highly doubt that the dist gear is the culprit. Also it has happened to 3 dist gears all of different year of production.

This leaves the cam gear. I talked to a Crane tech who informed me that although they manfufacture their own steel billet blanks, they do not manufacture cast iron blanks. I imagine this to be true with most any cam grinding company due to the costs of operating a casting foundry. It only takes one foundry to grind the wrong profiled cam gears on a batch of blanks to make a lot of after market cam grinders look bad.
My point is, I think that I got two bad cams in a row. This also seems to be pointed out by the fact that, after ruling out lubrication, the Crane tech told me to send the cam back for replacement. This also happened to the person in the chat forum that I pasted in the first post of this thread, where he stated that Comp Cams fessed up to a bad batch of gears and replaced his cam.

I would like to paste pictures of the visual differences between the teeth of an older production cam and the last two that I have had but my digital camera just died (what next?). O took an Edelbrock 302 cam out of the box that I have had on my shelf for about 4 years and the teeth all have a nicely radiused bevel at the peak of the tooth. The angle where the ground front thrust face of each tooth and the machined O.D. meet is very abrupt and sharp. Instead of the two teeth making contact on the broad surface area of their ground thrust faces, the cam gears sharp leading edge contacts the dist gear first and shaves the gear. Since this leading edge doesnt just make contact in the same place evertime, but instead swipes across the tooths face, it just keeps wittling down the tooth, thinner and thinner until the tooth breaks. If this leading edge was beveled and radiused, the tooth would make contact further back on the intended smooth thrust face and this shaving effect would be eliminated.

I am going to install a new cam and try out the polymer gear from Greg Depree like stated in my first post in the pasted forums. I can imagine that cam companies are going to throw a bone at us in the form of a polymer gear. But, if this was the necessary fix, then why for the last 75 years have we not needed them. Its because they were profiling the cam gear teeth correctly until now. Comp sells these gears for $138.00, Summit sells them for about $110.00 and Greg, (who they buy them from) sells them for $75.00. Apparently they are designed for Nascar cup car motors and are very durable. BUT, if the cam blanks were machined properly in the first place then they wouldn't be needing to sell us a $75.00 gear.

Even though the cam companies suggest exchanging your cam for a new one, the dilema is that you don't know if they are just going to send you the same problem back when you get the new cam. This makes mail order a problem because of downtime and shipping costs. I am now going to have to go to the auto parts store and have them order me cam after cam until I see an acceptable gear tooth profile. This is a very expensive problem and no one is going to accept responsibility, just a bunch of Mom n Pop independent operations that can't afford the downtime suffer at the hands of some large corporations that go on with business as usual. I guess you could say that I AM PISSED.

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post #8 of 12 (permalink) Old 04-12-2005, 03:42 PM
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Distributor gear nightmare

I just put an MSD steel gear on my distributer and the info sheet specified the dimension from the base of the gear to the distributer housing with the play removed as 3.996" to 4.005"
Is the 351 dimension not the same as a 302?

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post #9 of 12 (permalink) Old 04-12-2005, 03:48 PM Thread Starter
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Distributor gear nightmare

That is when you measure gear depth with the shaft pressed in to remove end play.

When doing it Ford Racing's way they have you pull the shaft out and measure.

Fords way tells you more because when you pull the shaft out and measure, then you can compare that measurement to the depth measurement taken inside the block from the dist base counter bore to the gear thrust face where the pump shaft stick through. This will tell you if the thrust faces of the block and the gear are making contact (as they should) when shaft end play is pulled out.
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post #10 of 12 (permalink) Old 04-12-2005, 03:58 PM Thread Starter
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Distributor gear nightmare

I went back to one of the forums where I first learned of the cam gear issue and a picture of the culprit has been posted check out the picture on the second to last post by "FXBILL"
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post #11 of 12 (permalink) Old 04-12-2005, 04:08 PM
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Distributor gear nightmare

My research indicates the gear should be installed towards the SHORT end of the spec. They seem to have a bad habit of bottoming out against the block.

MSD offers a STEEL gear for hydraulic roller cams.


EFI 302 with hydraulic roller cam... part #8456
NON EFI 302 with hydraulic roller cam (0.468" shaft)... part #85833
351W with hydraulic roller cam (0.531" shaft)... part #85834

I replaced a bronze gear that ate itself in a hydraulic roller equipped 393 in less than 2000 miles with the #85834 last summer, and it is still in perfect shape after quite a few miles of run time. (he's taken it out a couple times to check) I ended up installing it a tad shorter than what they called for.

Good Luck!

66 Mustang, Toploader 4-speed
Pump Gas 306, 9.88@135.67
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post #12 of 12 (permalink) Old 04-12-2005, 05:52 PM Thread Starter
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Distributor gear nightmare

The gear thrust face is supposed to lightly contact the machined surface in the block. The cam gear teeth pull down on the distributor gear teeth which pulls the endplay out of the dist shaft. The contact of the gear surface to the block surface is what limits the shaft from continuing to travel downward, not the mechanical advance plate on the bushing inside the top of the distributor body.

This is how height of the dist gear to height of the cam gear is maintained. If the gear thrust face does not make contact with the block thrust face, then you will have your dist gear floating at any undetermined height relative to the cam and have incorrect mesh and a bad gear pattern will result.

If you go to the clubcobra link that I pasted earlier and look at the cam gear pic, you will also note that this is more a case of incorrect deburring than tooth profiling.

As I look at the wear pattern between my two gears, I can see that the contact area is correct in terms of centeredness and height, what is wrong is, the leading edge of the tooth (at rear edge of tooth, not along length of outside diameter) is contacting the distributor much the way you would push a chisel into a piece of wood to peel up a shaving. This sharp, chisel like leading edge of the gear is in need of radiusing and deburring. The rest of the tooth profile is fine as far as I can tell.

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