Thanks for the comments. We hear/ see much here about blown head gaskets but they're not often related to overheating as a cause. That is, ie. sometimes you read "car overheated [radiator/ hose went, whatever] ... should I worry about head gasket?" but more often it's "I have coolant in the oil [or vice-versa]".

So I believe your summary that the gasket moves & weakens with each warm-up cycle and eventually just fails to seal.

In this case the folks were driving ~ 60 MPH and the car suddenly started smoking. The drove to the garage nearby they use for service work and the mechanic said "blown head gasket" after some testing.

So, who knows? It just reached that point in the warm-up cycle where the compression began leaking past the gasket and voila.

I think I am going to just replace the engine. The PO's "didn't overheat" could mean that. Or that the gauge didn't show overheating - but it could have just dropped to the bottom with lack of coolant. Or the "garage nearby" could have been 10-20 miles on an oil-water mix.

Thanks again.
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240s: 2 drivers and some parts cars