Why SB Ford Distributor Gears Fail
DistgearsIt was a dark and stormy night. Deep in his lair, the Butcher performed the last of his subtle distributor installation experiments. Suddenly the truth appeared before his eyes. He now believed he held the secret of the unexplained Ford distributor gear failures. It wasn't the high volume oil pump, it wasn't the alignment of gear teeth, and it wasn't oil starvation. The mysterious gear death was most likely caused by diabolical.
Several people have emailed me, and there seems to be some confusion. I wrote this story to try and help people who have had mysterious distributor gear failures for no apparent reason. These gear failure posts come up about once a month on the cobra message boards. I personally have not had distributor gear problems in my engine.....I blew them up too fast
What I mean is, my distributor gear teeth were shaved because of engine oil starvation and not because of the reasons I hope to explain below.
I've checked my theories on this subject with three Phd engineers who own, design, or admire cobras, and we all agree. We do not believe gears are failing because of the gear metal choice! Yet on every message board, that is the favorite piece of advice that cobra owners give to one another. Folks fail to read that several steel cam gears have destroyed steel distributor gears. Therefore, I don't think it is the damn metal! I feel the gears are failing because of the following two reasons
1. the distributor gear is forced into the bottom of the block under load after clamping.
2. the oil pump shaft bottoms into the distributor shaft, causing the total shaft legnths to be too long, and then the system is under load after clamping.
These theories are presented below. Some cobra friends have emailed and suggested to me "why did you not use a steel gear?" Sorry for not making this clear... THOSE ARE STEEL GEARS
One is an MSD and one is a Crane and both are steel. But again, my gears are sheared for very different reasons than what is occuring on the message boards. Below are my thoughts and ideas on this subject...see what you think and decide on your own why the occasional sb ford distributor gear fails.
Super Update 2 - Wayne Maybury on ClubCobra!
I had to bring this topic back up.
I visited my engine builder last week and we were discussing various things when I told him about Andy discovering the problem with the length of the MSD distributor which resulted in gear problems. He laughed as we walked over to the far side of his shop. He uncovered a 351W which had been raced all summer in a dirt modified car. This is a Ford Motorsport block with all the right parts.
He told me to look at the distributor. Under the distributor flange was a .030" spacer that he had made at the beginning of the season. When he dynoed the engine, he noticed that the distributor actually turned, changing the timing and stalling the engine on the dyno. He thought that he had forgotten to tighten the hold down bolt but that wasn't the case. It was tight, but since the timing hadn't been adjusted yet, it wasn't really tight. When he played with it, he discovered that the gear was binding on the bottom so he made the spacer. He was very lucky since no damage was done. He also mentioned that the measurement given by MSD wasn't correct.
"It appears that "The Butcher" was right on. Way to go Andy."...Wayne
This week I made some MSD distributor mods. Let me show you these modifications and how they revealed what is most likely causing unexplained distributor gear failure. Last week I thought it was tooth alignment...this week I tend to think it is more than that.
Last week I was dreaming about shims and shimming. I then open up my MSD catalog and see that those damn bowtie folks have all the luck once again. They can get MSD distributors with a slip collar. This means that they can easily set the depth of the damn distributor! This means you can get perfect gear teeth alignment. I WANT ONE! Check this pic from the MSD website.
I marched down to young Jorge's machine shop, where I have a cocaine habit of $500 a month, and asked if he could make such a part? He said no prob. I told him what I wanted and the next day the part was ready. Jeorge spun the unit in the lathe and removed the ledge, and then built a collar to my specs. I was very excited and pleased.
By now I have come to expect problems with any proto-type designs. It usually takes 2-3 tries to get a part to work and this was no exception. On this first try, the screw stuck out the side just a tad. This was enough to hang the unit in the air maybe 50/1000th. I also found out my old clamp would be way up in the air.
Day two came and I had the new and improved collar. A smaller screw and it was sunk further in. Now check out this boss highboy clamp. A perfect fit. I can feel it. Aye Carrumba. At this stage, I am ready to put this damn distributor so perfectly in the hole that the teeth are going to align so well that I should get an award or something! =) Well not so fast. Trouble was a brewing.
Here is the infamous MSD pink spec card once again. They want 3.996" - 4.005" from the collar to the bottom of the teeth (not the very bottom but the teeth bottom). Want to see how far I was off and how far many people are off out of the box...look at the shimming that would of needed to be done OR if you left it 'as is', the teeth would surely wear up high. That gab is where the old ledge would of been, now trying to get to 4.000".
Here is the secret Butcher tip. A lid of a spray can makes a great distributor stand. Originally I coated the gear in oil. I will have to say that this makes a big mess. If I did it again, I would keep the cam and distributor die, and only use machinist blue die. This is much easier to read. Some folks also like yellow paint. I like the die since it dries fast when I hit it with compressed air and is easy to see on some gears.
I know have a distributor perfectly adjusted to 4.000". I put it in the hole, and tighten the clamp. I then spin the unit by hand since I am still building the engine, and the cam gear does not have a belt or chain on it. I think to myself "man O man, this is a damn bear to spin". The cam gear was really tight and I could hardly spin it by hand. I was worried that I had done something wrong. I decided to loosen the distributor clamp bolt. Now things were loose as butter. I tighten it again and the gear is very hard to spin. What in the world is going on?!
And then it hit me. At 4", my gear was bottoming out in the hole. 4" was too long and the clamp was compressing the unit. This is just a few thousandths. A regular cobra with the engine already in, would never be able to turn the cam by hand and feel it binding. The only reason I stumbled onto this was that my engine is still apart. Normally a person drops in the distributor, cranks it down and drives off. Under engine load, the gear would spin, but it has to be under much added pressure once it is compressed like that at MSD spec!
I decided to try the test anyway and see the teeth pattern. Notice here it is not bad but on the high side. I would love to set this gear at 4.005" but I can't. That would really bind. I decided to keep moving the gear up to the lowest point I could find where it is not binding. I ended up at 3.990". This is pretty damn close to 3.996, but not perfect. If I wanted to be crazy and waste more time, I could run down to Jorge and have him machine some material off the gear. I rather run it as is, and watch it closely, and do that only if I need to in the future.
I save my first two steel gears that failed because of oil starvation. I still wanted to check them out. These gears both had 500 miles on the. Both were getting heavily worn. This gear has a tooth wear pattern that is higher, but the teeth are less warn. This gear had a very nice wear pattern , but had much heavier wear than the first. My theory was that the first gear was higher on the shaft and spun freely, where the second gear was probably to proper spec, had great teeth alignment, but I bet had the compressed binding issue. Are you sitting down? Holy sheep ****. This is exactly what happened. Here is the side view and the one on the right had scrubbed itself down on the block.
The problem I believe is that the MSD gears might be a touch long. Leaving the gear at 'out of the box height' is probably better than trying to set it to 3.996 - 4.005", even though it is probably high from a tooth alignment perspective. The best solution would be to have a small amount of material removed from the bottom of the gear since there is a small pad there, and then go to spec and blue die the gear to check the wear pattern. The worst thing that can happen is having the gear be down in the hole too far, then as the distributor is clamped, the gear is rammed into the block, binds, and heavier than normal teeth wear occurs. I am not positive, but I have to think that this is the unexplained reason that maybe 1/100 cobra owner's gear fails for no apparent reason.
I'm not sure how to tell if the unit is too long other than the way described above which is not practical with completed engines. Perhaps careful measuring or perhaps some clay on the bottom of the gear. Wait! blue die on the bottom of the gear and then install unit. Then spin the engine over the few times and check bottom of gear for wear. This inner circle had blue all on it before this picture, and so clearly it was bottoming out.
UPDATE: Wes, Grover and I have been debating some of these issues in email and on ffcobra.com
Again. The gears are
spiral they have a large thrust component as they transmit power. The
bottom edge bottoms out on the block and turns it into a thrust bearing
surface to take this load. It will obvioulsy and correctly wear the blue
off. A 45 degree spiral gear will transmit 50 % of his force into rotation
and the other 50% into thrust (up an or down the shaft) Do you see this
Wes then had a super idea of using green plastigauge on the bottom of the gear during installationg to check clearance. I think this is an excellent solution that is very easy to apply. Here are Wes's comments.
I wonder if the dye may be somewhat scrubbed off from just turning the motor because of the down thrust load that I talked about above (unless the motor is turned backwards). Would merely installing the distributor and clamping it (not turning) with a bit of Plastigage stuck to the slightly greasy bottom of the distributor gear do the measuring job? Have you ever tried it?
Mr. Fixit also wrote into ClubCobra with an excellent additional reason why the distributor gear system can be installed improperly under load. Mr. Fixit has seen several engine where the oil pump shaft was either too long, or the distributor shaft pocket was to short. Upon clamping, the shafts force excess pressure to the oil pump and distributor, causing extra load, and the teeth once again wear at an accelerated rate.
there'll be phantoms, there'll be fires on the road... and the white man dancing