Stopped the ops till the cowls were lifted and engine inspected, three blown plug inserts, big cracks, engineers put it down to shock cooling.
This was fifty years ago, no where near the engine instrumentation available then as is now
So my question would be... sure, the engineers may have put it down to shock cooling... but what ACTUALLY caused it?
Back in the day, with no hard data, it was pretty normal to blame failures on mis-handling, whether it actually was or not. Many engine shops would blame any failure of one of their engines on the pilot, and "shock-cooling" was a pretty common charge.
Back when I was younger I used to race motocross bikes, and it always used to make me wonder why my two stroke motor, operating pretty much on the edge of seizing for most of the race, could endure being occasionally dunked in freezing water without ill effect... because that really IS shock-cooling...