The MGA With An Attitude

In recent years there has been a rash of bad rotors for the Lucas distributor. It has been widely rumored that too much carbon in the plastic makes it conduct spark to ground on the shaft. This cannot be true, because carbon black used as pigment and reinforcement in molded plastic rotors is non-conductive (insulating similar to the plastic). There are other reasons for rotor failure and spark leakage to the mounting shaft, mostly an issue of bad design and a misplaced rivet. Here are some examples of bad and good rotors.

The rotor above left is a good rotor. This one has the contact bar attached by ultrasonic welding (melting and pressing down) a protruding plastic finger that works like a rivet (but non-metallic). In other cases the contact arm may be molded into the plastic part. The rotor above right is a bad rotor. This one has the contact bar attached with a metal drive rivet, and the rivet is too close to the driving shaft. Pictures at right illustrate this point. The white circle represents diameter of the insulating base of the rotor. The rivet is clearly positioned inside of the circle, therefore leaving less insulation between the rivet and the shaft. Failure mode here is for spark to arc through the insulation between the rivet and the metal shaft.

The next rotor pictured to the right should be good. The rivet here is positioned farther out from center, outside of the base circle. Insulation distance between the rivet and the shaft is equal to or greater than wall thickness of the base of the rotor. I do not have any reports of failure of this type of rotor.

The red rotor shown here is from Distributor Doctor. I don't believe the color or material has any advantage over black ones, but the design is sound. It is similar to original Lucas rotors with the contact arm molded into the part, and no metal rivet. So far all notes from end users say this is a good part never known to fail.

Incidentally, the cap and rotor on my Mallory Dual Points distributor are the same color red, and have been for decades past. Mallory has a reputation for top quality products, and indeed I have never had a cap or rotor failure in 20 years and 215,000 miles. However, do not let the red color affect your common sense. If you might find a cheap rotor on eBay that happens to be red, you should not assume that the color has anything to do with quality or function.

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