Yesterday morning my 68 would not start; upon opening the hood I was surprised to find the coil terminal on the distributor cap broken off. After counting myself lucky that the car ran so fine the day before with coil wire resting 2 inches away the center of the cap, I was baffled by the condition of the terminal. It had become crystallized as if subjected to extreme heat, and crumbled apart. Also, the rubber boot on the coil wire was partially eroded. After replacing the cap, I was back on the road.

I have a pertronix points eliminator and high output coil installed. The cap is standard parts store fare and has been installed for about 8 months. Is this condition common with high-voltage coils?

have you tried a NAPA cap and rotor?

If this phenomenon is due to the high voltage coil, then I need to seek out a tougher distributor cap. It's one thing to install aftermarket upgrades on a hobby car, but on a daily driver like my 68 Malibu, the constant use may create consequences when a guy deviates from factory stock.

genuine GM parts are the way to go when it comes to ignition parts like cap and rotor and wires

Do you have a picture?

From the way you describe, it sounds like there might have been a problem with the cap itself.
I doubt your trouble was due to higher voltage...afterall even a stock coil would put out 25-30KV? ...you may be running 45-50KV? At that level, it hardly makes any difference to the plastic.

The caps use a "thermosetting" plastic resin...meaning it requires heat and pressure to get the plastic to "set" or cure. Once the part has set, heating it again has little effect (it won't melt down in the engine compartment) But, if this initial heat/pressure cycle doesn't meet the proper parameters, the plastic will be weaker than normal. If you're like me...there was one bad part on the shelf and you got it.

I got an interesting email from a distributor shop owner from California - he stated that this happens when the boot of the coil wire comes loose from the dist. and there is a gap that the spark has jump between. (This is also hard on the coil/module apparently). Indeed after the wire became totally detached it started to erode a hole in the top of the cap. I didn't detect the situation until it was a damp morning and the car would'nt start. Even after re-inserting the coil boot over the remainder of the distributor post the car would not start; it mis-fired probably due to carbon tracking.

I'll have to replace my coil wire also with something that has a tighter fit.