Looks like one of the pistons has gently tapped a plug gap closed, fortunately without catastrophic failure.
OK, so a little catastrophic after all......
Now the head is off I can confirm there is a hole through one of the pistons, but the cause isn't detonation.
The neat round hole is from a valve stem punching through the piston.
Appears the original engine build left a few things to be desired although it was making decent power.
Exhaust valve to piston clearance insufficient and all 4 exhaust valves have been knocking clearance into the pistons.
Unfortunately I didn't pick this up when I stuck a camera down into the cylinders a couple years ago.
This was only ever going to end badly, the only question was when... the cylinder head has been shaved heavily to get the cylinder pressure up.
This is what happens sometimes when you buy a vehicle with a few mysteries about it, in this case, exactly what was inside the engine.
The past owner did a lot of good work on this car but never quite finished it.
Appears one of the things that wasn't so good was the selection of pistons used (I was told R12 Gordini - but they aren't.....) and setting up the valve to piston clearances.
Amazingly this tough little engine was still idling and you could probably put another set of pistons straight into the existing sleeves as there is no bore damage.
The head is another matter, aside from the damage the head has had too much material removed and so getting the Gordini spec pistons and valves to work is really a lost cause.
So disappointed that the build wasn't done properly in the first place, but at least the mystery of why it failed is pretty clear, this isn't a fuel or ignition related problem.
So the recovery plan is straight forward, source another head and a bucket load of correct pistons, sleeves and valves.
Pour some time=money into the head and bolt it back together for return of joy.
