Melted Head and Spark Plug? Options

[Cpl620]

As the title says. Im kind of at a loss for words on this as I have never had this happen on any car i have worked on, or owned. I just picked up my 71 with an injected 1.7 and did a few normal tune up things. New Plugs, oil, filter, ect... and i was headed up north to pick up the rest of the stuff like a cap, rotor and wires. The car was running great after each of the changes and I even had a chance to get on it a bit the day prior. So about 30 miles in on these spark plugs cruising down the highway and i start to smell oil burning, then 5-10 seconds later a good size "pop" and white smoke. pulled over and let the car cool down. I noticed a lot of smoke coming from around the passenger side valve cover. My plug leads were scorched on the same side as well, to the point of melting the rubber.

Upon returning from the tow I immediately removed the plugs on that side to find my problem. A hole melted in my head, running up along the spark plug. I have been told these heads are infamous for having hair line cracks. this would theoretically lead to a lean condition, resulting in detonation. However, one problem. The plugs color on the previous plugs, and these all look good. No signs of lean mixture.

Also, the seat of the plug, which is steel is melted. This would be fine, but the threads, which are a thinner metal are not melted...
If this did happen from the inside due to detonation, why are the threads (which form the pictures were obviously exposed to the same exhaust gas as the plug seat), or the tip it self in perfect condition?

I have had other theories such as the plug it self was not seated right, but the crush ring is compressed. If there was a leak of that sorts through the plug it self, it would not be running that good to begin with. someone also mentioned a default in the plugs insulation it self, resulting in an arc in that one spot of the head.

has anyone else ever dealt with a situation similar to this? I would like to get some brain storming and answers, as I am now down an engine. Thanks.

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