Distributor Cap-Rotor Clearances

[Drbrown]

The "complete" distributor gasket sets I see advertised for crab distributors include a narrow circular fiber gasket that fits between the Distributor cap and the metal body of the distributor.

(1) When I get a crab unit back from a reputable rebuilder should that gasket be included ? .... it normally isn't.

(2) If the distributor cap and rotor are matched to meet clearance spec's should that gasket be included ? .... granted the gasket is not very thick but it might affect the machining of the match-fit.

Related Question: How does someone check the interior clearance between the rotor tip and brass post inside the distributor cap ?

[51 MERC-CT]

Originally Posted by Drbrown

The "complete" distributor gasket sets I see advertised for crab distributors include a narrow circular fiber gasket that fits between the Distributor cap and the metal body of the distributor.

(1) When I get a crab unit back from a reputable rebuilder should that gasket be included ? .... it normally isn't.

(2) If the distributor cap and rotor are matched to meet clearance spec's should that gasket be included ? .... granted the gasket is not very thick but it might affect the machining of the match-fit.

Related Question: How does someone check the interior clearance between the rotor tip and brass post inside the distributor cap ?

The gasket is most likely a moisture barrier and it's absence will not effect air gap.
To determine air gap, measure the distance between two 180 deg. opposing distributor cap posts. (then divide by 2 ).
By measuring the rotor, determine the distance from the tip to the center line of the rotor.
Subtracting the rotor measurement from the cap measurement will give you the air gap. (Simple)

[flatheadmurre]

Better to put some masking tape on the rotor and turn it to see when it hits.
If the cap is a bit off center it can all measure just fine and be a wide gap on one side and hit on the other.
When you put enough tape on the rotor so it hits measure the thicknes of your "shim".
Next is you can put some chalk on and find out which cylinder it hits first in the cap.

[G.M.]

My eye ball guess would be .060 to .080 G.M.

[BUBBAS IGNITION]

Once upon a time (ever hear a story like that) we were concerned with rotor to cap air gap. Sun Electric and Allen Testproducts made tune up equipment and we tested on a running engine the rotor gap using a scope and large pointed probe to insert into the distributor cap. Very seldom if ever did the process show a major problem as most rotor gaps only took a few hundred volts to jump that gap. The spark plug gap was the largest gap in the circuit and that was the largest concern.
When you have 5-8,000 volts on every cylinder when firing a couple hundred isnt very much..
The cap gasket is like 51 merc mentioned above is more for moisture control than air gap.
In my experience the only issue we have had is the rotor actually hitting the cap terminal and we check each one before shipping. If there happens to be a slight clearance issue we sand a little off the rotor.
For the most part we have used the NAPA Echlin cap and rotor and found them to be very good. Part number FA64 and FA65.....

[51 MERC-CT]

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Measured the air gap on a slightly used 8BA distributor that came out of my 51 Merc. with the result that the air gap is .038 (±.005)

[Drbrown]

Thanks to each for your comments. "Learning" never stops around here ....
love it !