Distributor Parts Mis Match, Vacuum Advance

John Grady writes:

Some things you should know when buying new parts for your F distributor. There are two different sizes of rotors and distributor caps. If you mix parts, you will have problems. The wrong combination of rotor and cap can cause either mechanical interference or electrical interference or both. You need to keep your parts straight.

If you look at these pictures you will see the rotors are different in height and in the length of the contact arm.

If you install a tall rotor in a cap expecting a short rotor, it will load the distributor bearings, causing a failure. If you install a short rotor in a cap expecting a long rotor, the gap between the rotor and cap contacts is too big, likely causing cross firing.

The caps are different although both fit the distributor body. While both caps are 1.5" deep to contact, they machined back the contacts to allow for the longer rotor arm.

From an electrical perspective, I see why they made the changes. The leakage or "creep distance" on the new design (tan cap and tall rotor) is much better. There is less jump distance between the rotor and cap. The overall insulation effect is probably good for 50% more volts and less breakdown.

A few words about the vacuum advance.  

On the vacuum advance arm, there was one egg shaped hole. I very carefully squeezed the hole back to round in loaded direction for now and now the play is gone.

On the vacuum advance, I wondered about the H shaped cutout in can where arm comes out. That cutout is apparently there for factory assembly. When installed on distributor, the center of the H is a stop limit for the arm with no vacuum. On this can, the H cutout was enlarged due to wear and the arm had fallen into one leg of H. In proper operation, the arm must end up resting in the center of H.