This cylinder liner O-ring seals the gap between the cylinder liner and the engine block, preventing coolant or combustion gases from leaking past the liner. It sits in a machined groove on the liner's outer diameter and is compressed when the cylinder head is torqued down. A failed O-ring allows coolant to enter the combustion chamber or oil system, typically visible as white exhaust smoke or emulsified oil. The manufacturer machines the O-ring to match the original groove geometry, so the seal compresses to the correct height without over-squeezing or rolling out of the groove. On reinstall, clean the groove thoroughly and lubricate the O-ring with engine oil to prevent pinching during liner insertion.