[drolston]

I shimmed with a piece of paper for a snug fit of the rotor on the shaft. Be sure to spin the rotor with the cap on before you install the distributor, to be sure the rotor does not contact the spark plug wire connectors inside the cap. if you get any contact, it is probably because the cap is not on perfectly straight or completely seated.
In most cases you have to install the distributor and then the cap. Once you get the cap on and snapped, tap it down firmly with a leather or plastic mallet, or a hammer with a piece of wood as a cushion.
Here is what happens if the cap is not seated perfectly straight. The car might start and run but very badly. This was part of my winter of ignition discontent.