Abstract: A lock assembly includes a lock body having a hole longitudinally therethrough, a plunger arranged longitudinally in the hole, a single strong spring coiled around the plunger and arranged longitudinally in the hole inside the lock body for returning the plunger to an open position, a key for turning the plunger to a closed position, and a plate for receiving the plunger in the closed position. A hole in the head portion of the plunger and a hollowed end of the key have matching hexagonal cross-sections for engagement with each other. A first roll pin is seated in the hole in the head portion and guides the hollowed end of the key into the hole. A second roll pin is inserted crossways into another hole in a middle body portion of the plunger for retaining the plunger in the open position.