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Old 02-26-2024, 12:10 PM   #1
Paul Howes
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Default rotor cap

the gap between the rotor cap and the distributor cap is about .036 instead of .025. How can I adjust the gap to make it smaller?
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Old 02-26-2024, 12:57 PM   #2
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Default Re: rotor cap

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Originally Posted by Paul Howes View Post
the gap between the rotor cap and the distributor cap is about .036 instead of .025. How can I adjust the gap to make it smaller?
Bend rotor
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Old 02-26-2024, 05:34 PM   #3
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Default Re: rotor cap

Ive found the current repro distributor bodies have slightly short contacts. This makes it hard or impossible to get a .025" gap no matter how you bend the rotor.
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Old 02-26-2024, 06:19 PM   #4
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Default Re: rotor cap

I've soldered a piece of brass to the end of the rotor, then adjusted the gap to 0.025". It didn't make much (if any) difference.
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Old 02-26-2024, 06:58 PM   #5
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Default Re: rotor cap

Normally you can bend the tab up. It helps to have the rotor out of the distributor when you do this.

My experience with the rotor gap has been different than Synchro. This may be because my ignition was marginal but the smaller the gap the better my car ran. If you want to set the gap smaller than 0.025, just make sure that it does not hit any of the pins on the inside of the distributor body. Sometimes the distributor body can be loose on the distributor and move to close up the gap in one are and open it up in another area.

More: The higher the pressure the harder it is for the spark to jump the gap. So in a high compression engine more voltage is needed for the spark to jump the spark plug gap. What does this have to do with the rotor gap? Maybe nothing, but I contend that a smaller gap at the rotor will provide more voltage at the plug.
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Old 02-26-2024, 08:06 PM   #6
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Default Re: rotor cap

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Be careful if you try and bend the rotor tab. I broke the re-pro rotor doing so once. I think the re-pros are plastic, maybe different than an old rotor, maybe Bakelite(?). The solder-on tab makes sense.
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Old 02-26-2024, 08:14 PM   #7
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Default Re: rotor cap

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Be careful if you try and bend the rotor tab. I broke the re-pro rotor doing so once. I think the re-pros are plastic, maybe different than an old rotor, maybe Bakelite(?). The solder-on tab makes sense.
Ditto on the breaking. I believe the original rotors used brass contact, repros are steel.
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Old 02-26-2024, 08:40 PM   #8
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Default Re: rotor cap

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Originally Posted by Paul Howes View Post
the gap between the rotor cap and the distributor cap is about .036 instead of .025. How can I adjust the gap to make it smaller?
Are you having issues ? .036 shouldn’t be a big issue, in fact it may give you a hotter spark , some here will disagree BUT do an Internet search for spark intensifiers , that will back me up
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Old 02-26-2024, 11:41 PM   #9
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Default Re: rotor cap

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Are you having issues ? .036 shouldn’t be a big issue, in fact it may give you a hotter spark , some here will disagree BUT do an Internet search for spark intensifiers , that will back me up
I agree.

.036 should not be a problem at all. Will give you a hotter spark.

We would make a spark intensifier by making an adaptor with a built-in gap if we had an issue with an old engine that wanted to oil foul a plug. Worked great.

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Old 02-28-2024, 01:31 AM   #10
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Default Re: rotor cap

I’ve never been able to bend the rotor without breaking it. Nor have I ever achieved the recommended 0.25. I gave up, and in 60+ years of driving Model A’s I’ve never noticed any trouble with this issue.
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Old 02-28-2024, 07:24 AM   #11
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Default Re: rotor cap

I believe it can be bent with a good bench vice and needle nose pliers...but I agree with Big Hammer, post #8.

If I recall correctly, I've read a post somewhere that there is a book on Model A's with a title "Secrets of Speed" which suggests .010 on the gap.

On our car, the distributor bodies are a loose fit. Again, if I recall correctly, 3 or 4 pieces of thin electrical tape placed vertically and distributed evenly around the body can serve to tighten the fit????? I haven't tried it.

I'm thinkin' that a too loose gap will add heat to the coil????
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Old 02-28-2024, 12:14 PM   #12
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Default Re: rotor cap

This slide may help you.
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File Type: jpg Rotor - Adjusting Gap 151kb.jpg (50.7 KB, 85 views)
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Old 02-28-2024, 01:13 PM   #13
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Default Re: rotor cap

Bob Bidonde - Why is the Brass part of the Rotor Cap in you illustration rounded? Is this a New Mod or........?
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Old 02-28-2024, 09:10 PM   #14
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Default Re: rotor cap

Where did the .025 gap specification come from??
The only specifications from the era thatI have found is .005” in a radio installation instructions article— that said to reduce it to create less interference, nothing in Ford literature though.
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Old 02-29-2024, 10:30 AM   #15
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Default Re: rotor cap

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Where did the .025 gap specification come from??
The infamous "Red Book", page 1-101, Rotor to body terminal gap .025". Also page 1-107
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Last edited by katy; 02-29-2024 at 10:31 AM. Reason: Addition
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Old 02-29-2024, 11:09 AM   #16
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Default Re: rotor cap

Cape Coder,
The brass distributor on the rotor is rounded with a curvature compatible with the inside diameter of the distributor body.
Kurt In New Jersey,
I think the 0.025" gap is an opinion of Model A experts. I however, use 0.015" because the smaller the rotor gap is, the lower the impedances to the spark plugs are, so the sparks have more intensity.
Attached Images
File Type: jpg Rotor Gap 164kb.jpg (54.4 KB, 35 views)
File Type: jpg Rotor Leading Edge Curvature.jpg (59.1 KB, 25 views)
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Old 02-29-2024, 01:25 PM   #17
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Default Re: rotor cap

Bob, you get an A+++ for all your helpful illustrations. THANK YOU!
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Old 02-29-2024, 07:13 PM   #18
Kurt in NJ
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Default Re: rotor cap

A larger rotor gap works to fire fouled plugs better, a bandaid for improper mixture same as all the playing with sparkplug heat ranges to cure the black carbon from too rich of a mixture.
It takes more spark voltage to jump all the gaps, at higher speeds the coil voltage is reduced ,
So other than the “red book”( I don’t have one) where is the .025 rotor gap specification listed, Ford blueprint? The later production new Ford parts in my car had a .010 gap average on my car.
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