A lock mechanism for use with lever handles includes a lock core and a latch mechanism that is rigidly connected to the lock core to prevent the lock core from rotating in the door. Through-bolts are not needed, allowing a small diameter rose to be used. Stops and a spring return mechanism are entirely located within the lock core, allowing the rose to be thin. The lock core defines the rest position for the lever handles at an angle slightly above horizontal. To avoid the necessity for producing different locks in left and right-hand configurations, the lock core 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, including a pair of locking lugs that directly engage the lock core near its inner perimeter, allow the lock to withstand abusive forces applied through the lever handle. Endplay is nearly completely eliminated to provide a quality feel through the use of collars that connect the interchangeable inner and outer sides of the lock to the lock core.