looking at the score lines its clearly visable a few of them are higher than the top ring mark, so imo its been caused by the water washing all the dirt down between bore and piston.

how deep are they? the question is, are the rings damaged....the rings have defiantly caught the dirt, as you can see various height marks on the scores


if that was my engine i personally would, drop sump, remove pistons, de glaze bore, re ring pistons, replace big end shells