Thanks Rick. I am using the oil filled high vibration coil, #8222 and, yes, it is mounted to the head in the traditional way. When I called MSD I first spoke with a technician and explained where and how it is mounted (with an MSD bracket designed to bolt to the head). I specifically asked him about the heat in that location and he said the coil can handle that heat. That said, heat being the reason it failed as you described above still makes more sense to me. Regardless, I now have a spare MSD AL6, distributor cap, rotor and coil (as soon as their replacement arrives) to carry around in case one of these fails. Considering getting a spare magnetic pick-up as well. May be overkill, but it is so easy to replace any of these things roadside, I'd rather do that than call a tow truck.

As a preventive measure, I'm thinking of moving the coil out away from the head with some thick phenolic spacers. It is in an area of good air flow so getting it away from the head even another 1/2 inch would probably keep it cooler.

Kevin

Not sure it's the heat
 

Kevin2 Kevin IMO it's not the heat of the motor but the vibration of it. I have run ignition systems with a ballast resistor and cooked them, exploded them, melted them before I knew better. Vibration will crack wiring over time. I am talking about 10-12 gauge output wire from ALT. This wire handles from 60-95 amps, gets hot and the plastic coating doesn't melt. internal coil windings have a coating on them. I have a coil on a 6 volt tractor from 61 that still works fine. It's a oil filled round coil. It is not mounted on the motor. it's on the radiator support. I run an "E" coil on my car. These are the same coils they have been running for the last 15 years on 95% of cars and trucks. I don't know of any new cars or trucks running old style oil filled coils. I think there is a max output limit on them before burning up. I have an old accel #1 race coil and about 55,000 volts was the limit of this coil. It was oil filled and sealed. You could change the oil and replace the windings back in the 70's.
I am going to stay with common sense. Combo of vibrations and heat will burn out a coil. The other thing is spark plug gap?? Old days plugs where are .32"-.38". Now we are in the .60" - .80" air gap. That's a larger jump when you look at .80". I run mine at .45" with the bridge cut back and indexed to the hole. Last side note, was tired and got stupid one night and had the ignition on and turned the distributor and zapped myself over the hood of the car. The taste was like batterys and lasted for couple of hours. I was WIDE awake and never made that mistake again. Come to find out that these "e" coils can kill you or stop your heart. I though only amps could kill and not volts.%/ I have the same kit with my car of MSD parts. 6al (2) and complete distributor stup to drop in with timing marks on the housing. Mine is locked out with the FI system. I have gone through 1 cap (center fell out) and 1 rotor ( burnt a hole through the middle from bad cap) You are on the right path. Rick L.

Thanks Rick, makes sense even though they refer to this coil as "High Vibration". As for spark plug gap, I'm running Autolite AR3935's with a gap of .032 per the engine builder.