Hello.

I would like to ask your opinion about the possible theoretical causes of engine failure in my 4x4 YB Sierra.

The photo below shows the damage to the piston on the 3rd cylinder.



At the time of the failure, the engine done about 315,000 km and it was rather outworn (burned 0.5L of oil for every 400km normal driving) There were no modifications. The oil was Motul 300V 15W50. I've been using Denso IK24 sparkplugs. (on the third cylinder the electrode was melted) I did not hear any knocking from the engine just before the failure.
At the moment the engine was rebuilt: skimmed head, new valves, bearings, new pistons, rings etc. (also new fuel pump and fuel filter)
Injectors (blue Weber IW-044) have also been tested, their flow rate and efficiency of solenoid valves is basically almost identical. Electrical connections for fuel pump, injector plugs are clean. The other pistons were not damaged in any way. They have a small spray of carbon deposits.
Question:

What could have caused this type of damage? What should I pay attention to not to repeat the such "entertainment"? What else to check?

Thank you in advance for any suggestions.