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Hydrostatic lock, hydraulic lock, hydro-lock  or hydrolock occurs when liquids enter an engine cylinder. This can occur from a coolant, oil or fuel leak, but the chief cause is drawing water into the engine through the air induction system (air filter, intake manifold). Internal combustion engines must employ a compression stroke to compress the charge. Liquids are incompressible; the presence of a liquid in the engine cylinder during the compression stroke generates destructively high cylinder pressures.

Abnormally high cylinder pressures can bend and break pistons, piston pins, connecting rods, crankshafts and ruin bearings and can crack or break cylinder heads and engine blocks. Small amounts of liquids may pass through an engine cycle without causing damage.  Hydro-lock may occur while the engine is running, the work of the compression stroke being supplied by engine's rotational inertia. Or a liquid may leak into the cylinder while the engine is being stored; the work of the compression stroke will be supplied by the starter motor.