What's the gap between the distributor and the rotor?

Well, a few days ago, I had thought that my tune-up issues were over. I had been battling what I had thought was a too-thick distributor cap gasket on my '57 Powermaster. Unfortunately, after re-installing the original gasket (0.063" (rubber) vs. 0.077 (cork)) and hitting the start button, I was again faced with a broken rotor. Having no other rotors around (and being my usual cheapskate self), I broke out the epoxy and a day later had a working rotor. This time, prior to going for a start, I took one of my old distributor caps and sawed out a section with a coping saw. This modified cap was installed and allowed me to see what was really going on inside. It turned out that the rotor itself was too long, and not too high up as I had originally thought. I proved this by swapping out the rotor from my '63 4000; as I had also tuned it up at the same time. The Powermaster rotor barely squeaked by. In fact it managed to shave off a tiny bit of epoxy in the process. The 4000 rotor had at least a 0.020" gap. At last I had found the real reason why I was failing so many rotors. I promptly opened up the gap by taking a file to the end of the rotor. So, the real question is this: what should the inner (circular) dimension be within the distributor cap? I can easily mike this up and see for myself, but what is the correct spec? The other just-as-important question is this: what should the gap be between the tip of the rotor and the inner face of the four contacts within the distributor cap? I am quite inclined to fabricate some sort of fixture that would allow me to check - and possibly correct any rotor that I purchase from now on.
 
It would seem unlikely that the rotor is the cause of the problem of breakage (but it would be best to buy only high quality parts like Blue-Streak or Echlin - check with NAPA). As showcrop said, it might be a bent shaft or a worn shaft bushing. (Do you have any type of service manual?) Also, could there be any dirt or corrosion that prevents getting the cap on correctly? -Will
 
Hard numbers to find. Access to 50 year old Ford engineering drawings? Have no idea if this is helpful to you or not, but 8N side distributor cap inside post to inside post on my single example is 2.566 inches and rotor to post gap is 0.013 inch. How that might compare to your Powermaster, I do not know.
I think the solution is not re-engineering or doing your own quality control, but rather buying the best available parts from good reputable suppliers, hopefully Made in America instead of China.
 
JMOR - Thanks for that dimension, at least it's a start. I have no idea what that gap is supposed to be, but if it's 0.013" for the 8N, then I would think that there's a good chance it would be the same for my Powermaster and 4000.
 
Will/Showcrop - I know that it's not a bent shaft or worn bushing, since the original rotor was installed two years ago and remained intact. The answer is that these parts are indeed cheap and that I very definitely need to use USA-manufactured parts from now on. There's no dirt or corrosion, as I am always careful to clean everything before reassembly. In this case, the rotor was just too long and it hit the internal contacts within the cap.
 
(quoted from post at 09:33:11 10/20/14) Will/Showcrop - I know that it's not a bent shaft or worn bushing, since the original rotor was installed two years ago and remained intact. The answer is that these parts are indeed cheap and that I very definitely need to use USA-manufactured parts from now on. There's no dirt or corrosion, as I am always careful to clean everything before reassembly. In this case, the rotor was just too long and it hit the internal contacts within the cap.

I can't imagine why this has never come up here before, unless you just happen to be one of the first to get one from a new manufacturer.
 
(quoted from post at 09:33:11 10/20/14) Will/Showcrop - I know that it's not a bent shaft or worn bushing, since the original rotor was installed two years ago and remained intact. The answer is that these parts are indeed cheap and that I very definitely need to use USA-manufactured parts from now on. There's no dirt or corrosion, as I am always careful to clean everything before reassembly. In this case, the rotor was just too long and it hit the internal contacts within the cap.

Happens quite a bit.
and if you get the cap and rotor from different suppliers...more.
I don't bother changing rotor-cap combos anymore. If they are still good, no cracks, fit tightly..... clean, and back in they go. less headaches.
SOP with import parts, is to put the cap on without clipping and turn the engine slowly by hand watching for cap movement.
Even if it seems ok, pull it in a day or two to clean out the yellow dust that will be in there with 'new' parts......

Sounds like you have the dust shield covered ok.
On Fords you can never take them for granted. Some are just dust shields, some are cap spacers too. (amazingly dumb, 2 places instead of 1, that can shift)

on the gap, make sure there is one and keep it less than the plug gap and you'll be good.
 
Exact same thing has happened to me before. Rotor did not match the dis cap. Hit the ign.. Snap broken rotor. I was thinking that the rotor was packaged or labeled wrong. Same as you I put the old one back on, no problems.
 

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