Piston rings
Seal combustion gas and control the oil film on cylinder walls.
In grooves around each piston.
Compression rings limit gas leakage; the oil-control ring meters oil while allowing the piston to slide.
Flexible sealing bands around a moving plunger.
Excessive oil use or blow-by may involve rings, but ventilation and valve seals can cause similar symptoms.
Usually condition-based rather than a routine replacement. Lubrication, overheating and the exact engine affect wear.
Internal-engine work is usually labor-intensive. Diagnosis and the engine code determine the repair scope.
BMW oil-consumption diagnosis considers the whole engine before concluding that piston rings are worn.
BMW petrol and diesel engines; petrol-only systems are identified individually.