12. The lower water seal on a diesel engine wet cylinder liner must allow for liner axial movement. This seal is most commonly a __________.

A neoprene O-ring is used to seal wet cylinder liners, preventing water leakage while allowing necessary axial movement due to thermal expansion and vibration. This flexible seal accommodates liner movement without rigid locking, unlike metallic seals, precision flanges, or soft copper gaskets which are unsuitable for applications requiring movement.