A lock mechanism for use with lever handles (36, 74) includes a lock core (10) and a latch mechanism (18) that is rigidly connected to the lock core (10). Stops and a spring return mechanism (130, 132) are entirely located within the lock core (10), allowing the rose (50) to be thin. The lock core (10) defines the rest position for the lever handles (36, 74) at an angle slightly above horizontal. The lock core (10) is made substantially symmetrical about a vertical plane, instead of a horizontal plane, and the inner and outer sides of the lock mechanism can be interchanged. A heavy-duty locking piece (86), including a pair of locking lugs (82, 84) that directly engage the lock core (10) near its inner perimeter, allow the lock to withstand abusive forces applied through the lever handle (36). Endplay is nearly completely eliminated through the use of collars (48, 70) that connect the interchangeable inner and outer sides of the lock to the lock core (10).