[carmine]

If the carbon tip in the middle inside of distributor cap is worn out thats an indication that it was arching.
The reason for the arching is because there is a gap between the carbon tip on distributor and the spring tab on the top of rotor.
The spring tab on the rotor should be bent upward so it applies about 1/8 inch of upward pre-load on to the distributor cap carbon tip.

By manufacturing a spring tab that just barely if at all touches the carbon electrode will allow your car to run good when new but the arching will prematurely wear out the carbon electrode.

You find your self changing these items quite often.

The cap and rotor on my car has 7000 miles on it and carbon electrode still has no wear on it.

Many shops know about this but fail to correct by pre bending rotor tab upward to ensure good contact with distributor cap carbon electrode.

Win win for distributor cap and rotor sales along with more frequent visits to your mechanic for those that rely on one.