The goal of the proposed paper is to present and to discuss hydro-lock and related failures on four stroke medium speed marine Diesel engines.

Hydro-lock occurs at start-up when a piston is impeded from reaching the top dead centre due to the presence of an incompressible fluid that saturates the combustion chamber.

If the energy of the system is sufficient the hydro-lock is overcome and the engine is started, in case the energy is insufficient the rotation of the crankshaft is stopped abruptly.

In either cases the combination between the amount of fluid and the energy of the system can lead to the bending of the connecting rod, with or without immediate adverse effect on the engine.

As a matter of fact an hydro-lock does not necessarily put the engine immediately out of service: depending on the severity of the hydro-lock the engine could function indefinitely or it could fail after several running hours (in general few hundred).

In any case the engine will give to the crew little hints, if any, of anomaly.

Hydro-lock is usually discovered either during a routing inspection / maintenance or at the time of the final failure, which occurs due to fatigue fracture of the connecting rod.