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Bob's '31 Cab 06-24-2010 07:09 AM

Rotor gap to distributor body terminals

I was curious as to how critical is the gap between the rotor and the distributor body terminals?

I purchased a replacement rotor and distributor body and after checking the clearance's for the gap which in Les Andrews mechanics handbook states .025", I found mine to be in the area of approx. .012" with a feeler gauge. This is a repro unit.

What effect does this have on the overall preformance, when you are setting the timing, and the preformance of the engine?
Do you typically need to file down the rotor contact to achieve this clearance?

Thanks to all in advance for your reply's.
Bob Galli

MrTube 06-24-2010 07:30 AM

Re: Rotor gap to distributor body terminals

I'm very new at this as everyone here knows. But I figured i'd mention my personal experience with this.

I recently purchased a repro distributor body and rotor also. The gap was very close and I left it. I ended up having a problem when the flat plug connectors moved the distributor body slightly and it caused the rotor to rub on one of the terminals which took off some brass.

I then decided to increase the gap simply to allow for any movement. I did this by bending the end of the rotor slightly down. This also aligned it better with the center of the contacts in the distributor body.

Please note the movement I have is in the distributor body and the distributor it self. Not the rotor/shaft.

tuneman 06-24-2010 07:40 AM

Re: Rotor gap to distributor body terminals

when I replace mine, they generaly have to wide of a gap! I bend my rotor till i get to the setting in the book .:)

Ron/IA 06-24-2010 07:51 AM

Re: Rotor gap to distributor body terminals

Bob - MrTube's experience is valuable. The fit between the distributor base and the distributor body can be sloppy, and thus (as experienced) the close .012" clearance can be a problem. Just remember that .025" is less than 1/32".

He also has a good idea in bending the sparkplug straps so they don't put any pressure on the distributor body. The thing to remember here is to have consistency. The more everything is of the same and consistant dimension, the better the ignition and engine will operate.

I am sure others will have more valuable information.

MrTube 06-24-2010 08:00 AM

Re: Rotor gap to distributor body terminals

"He also has a good idea in bending the sparkplug straps so they don't put any pressure on the distributor body. "

I just noticed I accidently deleted that when I edited the post :mad:

I had originally installed them as is without pre-bending them and they had both moved the distributor body some and even lifted it a tad.

It took some time to bend them so they fit good and tight but did not put pressure on the distributor body. But it was well worth the effort.

Don/WI 06-24-2010 08:09 AM

Re: Rotor gap to distributor body terminals

I have discovered that the gap between the rotor and the four terminals in the distributor body can vary by more than .015" ~ .025". I bend the rotor tab so it has a .025" gap at the terminal that had the greatest gap and then I filed the remaining terminals until they all had a gap of .025". It takes a little time to do this but I feel it is worth it to have my "A" running as good as possible. Don/WI

30Tudor 06-24-2010 10:00 AM

Re: Rotor gap to distributor body terminals

To make the cap fit better sometimes you have to tighten up the slot in the cap where it interfaces with the body. This can be done by filling the slot with epoxy and then filing the slot to the correct size.

MikeK 06-24-2010 10:34 AM

Re: Rotor gap to distributor body terminals

The rotor gap is critical in setting the peak voltage the coil will reach before discharging. Too small of a gap (less than 0.025") will result in more spark plug misfires during idle and at W.O.T. The bigger the gap, the higher the coil voltage before discharge to the plug.

Those gizmos sold in the 60's-70's at auto shows that fit on top of your dizzy cap to fire fouled plugs simply added an additional gap to raise the peak voltage before coil discharge.

Purdy Swoft 06-24-2010 05:20 PM

Re: Rotor gap to distributor body terminals

I agree with Mike K rotor gap is critical. Most of the reproduction distributor caps have TOO close of gap and the wider the gap the hotter the spark, to a point. If the gap is too wide you will have early coil failure. Les recommends a gap of .025, this is a safe gap as far as coil life goes but a bit conservative. I run a .035 gap with a hotter spark . I have ran one of my model A's with an 81 year old original coil and a .035 rotor gap for 10 years and NO problems. I also set the original style points with a gap of .022 and set my Champion W18 spark plugs with a gap of .035 . This all makes a big difference in the performance.

Dennis L Oberer 06-24-2010 07:41 PM

Re: Rotor gap to distributor body terminals

I understand that if you run the gap at .035 you get a hotter spark. As for slop/play/lost motion between the rams horn and the distrib base...I installed a very small bushing over the small tiny pin and filed an equal amount needed off the bakelite rams horn and made it a very tight fit.

Thus no wiggling of the rams horn once installed on the distrib base. and so the .035 stays constant if the bushings are good, if the distrib cam is equal on all 4 sides.

Backlash in the gearing gets into the actual timing of the spark however. Try for nearl y no backlash. Snyders sells an oversize distrib driven gear/shaft.

Also if you leave the flat plug "wires" on the rams horn watch the sparking at night. You need an air gap there to prevent that.


Dennis L Oberer
Green Bay WI

SteveM 06-25-2010 06:51 AM

Re: Rotor gap to distributor body terminals

Some sources recommend that if the gap is too large, building up the gap with solder and then filing it down to the proper gap.

Given that there's a lot of spark in there, wouldn't that just melt the solder?


John Butts in CT 06-25-2010 07:58 PM

Re: Rotor gap to distributor body terminals

I have nothing to add to the gap discussion (except to suggest that the gap should be uniform at all four terminals. File them if you need to, then bend the rotor contact).

I will add a caveat that there's a batch of repro caps "on the shelves" with electrodes that are bent, too long/short, not set at 90 degrees from each other, and/or covered with plastic "flash." A reputable supplier was happy to hand pick me a new cap when I brought this to their attention.

Once I received a cap that had the electrodes "square" at 90 degrees, it was simple to achieve a uniform gap.

Moral: Don't assume that a repro part is ready to bolt on.

Have a great weekend, everyone.

MrTube 06-25-2010 09:47 PM

Re: Rotor gap to distributor body terminals

The one we bought had plastic over the spark plug terminals on it as well. I had to peal\pry it off to get a good connection. I didn't measure the distance between terminals inside of it but nothing caught my attention.

John Butts in CT 06-26-2010 08:15 PM

Re: Rotor gap to distributor body terminals

We're on the same page, MrTube. The dimensions of your new cap may be fine, but just think how your engine would sound (or not sound at all) if you hadn't cleaned off that "flash."

No disrepect to our many fine parts houses (thank you all), but the distributor body on the shelf today may differ a bit from the thousands of distributor caps on the line at River Rouge, '27 - '31.

Just saying, "before you bolt it on, check it out."

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