Any thoughts on what caused this?

I feel a spark plug electrode broke off, bounced around in there, and put a pin hole in the head allowing oil to get into the cylinder. Was the head all dinged up to? I be ya find one of bigger dings is actually a hole, maybe even created a hairline crack, allowing oil from the valves to seep in.

Oil into the combustion chamber would be:

- valve stem seals
- sucking oil in through intake
- bad rings/broken piston

Theoretically bad head gasket, but you would see water sooner than oil (water is pressurized, oil is gravity feed from valve cover area), burning water/coolant would be white smoke, oil is blue.

Spark plug end bouncing around causing chip/crack in combustion chamber dome would not cause oil to leak in.

Oil coming in through intake would most likely be sucked in from PCV or from oil filler neck into throttle body (this would be excessive blow by of combustion pressure into oil pan up into lifter valley into valve cover area out filler neck vent tube).

Done it! Someone lost a nut down the intake is my guess. Last one I left a nut down the throat made it half way from SLC to Boston in a 2.3 Ranger. Came back with what was obviously a carb nut that was beat to sh*&6t. He had a 1800 dollar bill he wanted us to pay for having the head pulled off and whatever else they did at the dealer in MA. Bad deal

All the damage to the piston I'd bet it's cracked causing all the other problems. Look how the rod is darker than the other.

John Dirks Jr said:

Whatever got in there and caused that damage was probably spit out the exhaust valve and could still be trapped somewhere in the exhaust system. If you ever take the exhaust off, maybe you could find it. Could be laying in the pipe somewhere or dancing around in the muffler. Theres always a slim chance it made it all the way out

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Years ago I had a screw back out of a Holley carb baseplate and go into a cylinder. "Luckily" it happened at idle and I could tell by the tone of the motor that something bad was happening. I immediately shut down the motor, still ended up costing me a piston replacement. Piston looked just like the one in the photo, minus the oil burning.