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:
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.
Owner, The Fab Ford
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