Piston cooling jet
Sprays oil under the piston crown to carry heat away.
In the crankcase, aimed up at the underside of each piston.
A calibrated nozzle, often with a pressure valve, opens at a set oil pressure and directs a jet at the crown.
A shower that cools a hot plate from below.
A blocked or misaligned jet can let a piston overheat; symptoms usually appear only as engine damage.
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.
Turbocharged BMW engines such as the B58 rely on piston cooling because boost raises crown temperature.
BMW petrol and diesel engines; petrol-only systems are identified individually.