Distributor Cap Air Gap

[Joe C.]

Does anybody have any thoughts about what the gap should be between the rotor tip and the cap electrodes?

[Louis T.]

Re: Distributor Cap Air Gap

Hi Joe,

Great question! (It never occurred to me that this could be a variable with correct-as-to-application parts.) I don't have an answer (I'm sure others will), but in the meantime, here's an interesting article about this very topic:

http://masacc.org/Documents/Distributor_Rotor_Cap_Anomalies.pdf

Regards,

Louis

[Joe C.]

Re: Distributor Cap Air Gap

Hi Joe,

Great question! (It never occurred to me that this could be a variable with correct-as-to-application parts.) I don't have an answer (I'm sure others will), but in the meantime, here's an interesting article about this very topic:

http://masacc.org/Documents/Distributor_Rotor_Cap_Anomalies.pdf

Regards,

Louis

Yes, Louis. You are on the ball!
I read that, this morning after Googling :"distributor cap rotor gap".
Very interesting. But, here's a possible rub. Scenario:

ASSUME an ideal situation where the gap is set very close, with no contact, at all 8 electrodes....................say, .002". If the coil secondary side puts out, say, 14 KV, then as the rotor approaches the electrode, and at the instant the points break, the energy should jump the gap between the rotor and electrode, WHILE THE ROTOR IS STILL APPROACHING IT. The only way that this won't happen, is if the points break at the exact instant that the rotor is dead center the electrode, and the field in the coil collapses completely and instantaneously. Dwell would therefore have to be adaptive. Is the make/break timing this precise with old fashioned points?

[Michael H.]

Re: Distributor Cap Air Gap

Yes, Louis. You are on the ball!
I read that, this morning after Googling :"distributor cap rotor gap".
Very interesting. But, here's a possible rub. Scenario:

ASSUME an ideal situation where the gap is set very close, with no contact, at all 8 electrodes....................say, .002". If the coil primary side puts out, say, 14 KV, then as the rotor approaches the electrode, and at the instant the points break, the energy should jump the gap between the rotor and electrode, WHILE THE ROTOR IS STILL APPROACHING IT. The only way that this won't happen, is if the points break at the exact instant that the rotor is dead center the electrode, and the field in the coil collapses completely and instantaneously. Dwell would therefore have to be adaptive. Is the make/break timing this precise with old fashioned points?

What say you electrical engineers?

The position of the rotor tip in relation to the distributor terminal at the instant of ignition changes as the vacuum advance unit moves the point plate to different positions. Rotor tip/terminal absolute alignment, only occurs in the center of the range of the vacuum advance.

[Clem Z.]

Re: Distributor Cap Air Gap

later GM distributors came with a rotor that had a "E" stamped on the rotor tip and this was done for emission reasons. this tip was shorter than the older rotor tips. we always used the longer tipped rotors in performance applications. when using a HEI ignition there were no long tip rotors available so i built a special jig the fit in side the cap centered on the center contact hole in the cap and bent the cap contacts in closer to the rotor tip.

[Joe C.]

Re: Distributor Cap Air Gap

The position of the rotor tip in relation to the distributor terminal at the instant of ignition changes as the vacuum advance unit moves the point plate to different positions. Rotor tip/terminal absolute alignment, only occurs in the center of the range of the vacuum advance.

I'll take your word that absolute alignment occurs at the center of the breaker plate's travel. Isn't it more probable that it occurs @ zero degrees vacuum advance, seeing as how there are different vac cans providing different amounts of total vacuum advance? According to the "center" argument, then each breaker plate and vacuum can would have to be sold as a matched pair.

Not only is the rotor/cap alignment dependent on the vacuum advance system, but it's dependent on the centrifugal advance, as well. At base timing, the rotor and cap are aligned differently relative to the breaker plate, than at full centrifugal advance.

At steady state cruise, vac advance is all in for about 16 crank degrees, and centrifugal is all in as well, for about another 26. So, with the exception of base timing, the total effect of this, would be (16 + 26)/2, or 21 degrees camshaft (rotor) rotation. So, the combined effect of full vac plus full centrifugal advance would cause a "shift" in the rotor's position by 21 degrees, at the instant the points open.

A GM distributor is about 4 inches diameter, so the rotor's complete arc covers 3.14 x 4 = 12.56" per 360 degree revolution.
Rotor tip width is about 1/4", so :

.25/12.56 = x/360 where x = rotor tip width in degrees of rotor arc

x = 7.17 degrees
So, the number of degrees in camshaft (rotor) arc which sweeps the width of the rotor tip's electrode is equivalent to 7.17 degrees.

You can see from the above that the combined effect of vacuum plus centrifugal advance equals approx. three rotor electrode widths.

How does this correlate to rotor-to-cap air gap?

If the distributor cap is indexed to the breaker plate, such that the points "fire" each time the rotor is perfectly aligned with each electrode in the center (as Michael stated) of the breaker plate's travel due to vac advance. Then, at SS cruise, there would be another 8 degrees vac advance left, plus 26 deg cent. (8 + 26)/2 = 17 degrees rotor arc. This translates to about 2 1/2 rotor electrode tip widths!
This means that the points would "break" as the rotor is approaching it's corresponding electrode, but 2 1/2 tip widths before it arrives.

[Michael H.]

Re: Distributor Cap Air Gap

I'll take your word that absolute alignment occurs at the center of the breaker plate's travel. Isn't it more probable that it occurs @ zero degrees vacuum advance, seeing as how there are different vac cans providing different amounts of total vacuum advance? Not only is the rotor/cap alignment dependent on the vacuum advance system, but it's dependent on the centrifugal advance, as well. At base timing, the rotor and cap are aligned differently relative to the breaker plate, than at full centrifugal advance.

The "centering of the rotor/terminal probably does move/change slightly from one vacuum unit to another. Not likely all are assembled accurately.

Changing the amount of cent advance doesn't have any affect on rotor tip to cap terminal alignment because when the distributor cam rotates through it's mechanical advance, it changes the position of the point cam and the rotor.

For race engines, (no vac adv) we used to align the rotor tip and cap terminal by rotating the point plate to a position that aligned before securing the plate by some method other than using the vac adv unit.

[Joe C.]

Re: Distributor Cap Air Gap

The "centering of the rotor/terminal probably does move/change slightly from one vacuum unit to another. Not likely all are assembled accurately.

Changing the amount of cent advance doesn't have any affect on rotor tip to cap terminal alignment because when the distributor cam rotates through it's mechanical advance, it changes the position of the point cam and the rotor.

For race engines, (no vac adv) we used to align the rotor tip and cap terminal by rotating the point plate to a position that aligned before securing the plate by some method other than using the vac adv unit.

Duh! Thanks for reminding me that the points cam and rotor pivot together due to the action of the advance weights!
So, 16 degrees vac advance at crank equals 8 degrees at rotor, which is about 1 rotor tip width. If the points are indexed to "break" at the (approximate) center of breaker plate travel, then the 2 electrodes would be adjacent, at any point of vacuum advance.
So, that explains why air gap would be critical.

I needed to do that mental exercise in order to help convince myself. Your explanation drove the point home.

Joe

[Michael H.]

Re: Distributor Cap Air Gap

Thanks for clearing that up!
So, 16 degrees vac advance at crank equals 8 degrees at rotor, which is about 1 rotor tip width. If the points are indexed to "break" at the (approximate) center of breaker plate travel, then the 2 electrodes would be adjacent, at any point of vacuum advance.
So, that explains why air gap would be critical.

I needed to do that mental exercise in order to help convince myself. Your explanation drove the point home.

Joe

That stuff can be confusing.

[Clem Z.]

Re: Distributor Cap Air Gap

some aftermarket distributors have a rotor that the tip can be moved so you can "index" the tip. if you want to see where the tip is at while the engine is running take a old cap drill a hole in the side of the cap in line with the wire terminal and use your timing light to see where the rotor tip is when that cylinder fires.

[Stuart F.]

Re: Distributor Cap Air Gap

Clem;

I have one of those kits though I have never used it. It uses a Ford 5" cap and a dull red/orange housing and rotor. The rotor is adjustable. I had 2 caps, one with the drilled hole to align it with and the other for actual use. The rotor uses Nylon screws and has a brass round pin tip. The rest of the mechanism (weights, springs, shaft, Etc.) is all standard GM. The housing mounts with the 2 spring loaded "L" screws to the distributor, and the cap mounts to the housing with 2 standard Ford spring clips.

Stu Fox