Piston cooling jet

A piston cooling jet is a fixed oil nozzle that sprays lubricating oil onto the underside of the piston crown to carry away combustion heat. In trunk-piston four-strokes the jet sits in the crankcase aimed at the crown or at a cooling gallery drilled inside the piston; in large two-strokes oil enters through the crosshead and telescopic or articulated pipes feed the crown cooling space. Removing crown heat keeps the ring grooves and the top land below the temperature where the oil cokes and the rings stick. Loss of cooling oil overheats the crown, risking burn-through and ring collapse, so crown-outlet temperature is monitored per cylinder.