The first thing I do when servicing or repairing any lock is look at the door and the jamb, checking the gap. I then look at the floor to see if there is any evidence of dragging. I check to see if the door operates and the door closer, if equipped, is able to close the door and latch the lock.
Once I am satisfied the door and jamb are in operating condition, I lightly lubricate the mortise cylinder keyway to make sure a key operates smoothly. Then I carefully operate the lock with the door open and closed. If the problem occurs only when the door is closed, check the door or strike alignment. If the problem discussed can be repeated, the solution can be determined.
Note: Photograph the disassembly procedure using a digital camera to have photos as reference in case the lock body explodes or spring loaded parts fly during disassembly.
Once you have a general idea of the problem, then remove the mortise lock body from the door. Behind the front, two setscrews secure the mortise cylinders or the inside thumbturn. Loosening the setscrews permits the mortise cylinder and thumbturn to be unscrewed. The interior knob or lever either has a setscrew or a threaded sleeve that must be loosened in order to remove the interior and exterior knobs or levers. If the mortise lock is through bolted, there usually is a spring cage on the interior side that must be removed in order to remove the exterior lever and spring cage. Older mortise locks can be equipped with a handleset. The handleset is usually removed by unscrewing the mounting screws on the interior side.
Once the levers, handleset or knobs and the mortise cylinder(s) and thumbturn (if equipped) are removed, unscrew the two edge mounting screws and slide the lock body out of the door.
With the internet full of lock manufacturers’ catalogs, installation instructions and parts manuals, always research the lock you are servicing before servicing. Manufacturers’ information can make the service straightforward and easy. Some mortise locks require specialized tools in order to disassemble and reassemble.
Once the lock body is on a smooth surface, carefully removed the cover, making sure no spring loaded parts will fly. I use a piece of spring steel to slide across the inside of the body once I have raised the cover about ¼”. This way, I will almost always find a problem before removing the cover.
Once I am satisfied, I will unscrew all of the cover screws, identifying any that are longer or shorter and the holes they were removed from.
With the cover removed, I photograph the body at different angles and positions to have a complete record of how the lock was assembled. This way if I forget, I can refresh my memory from the photos. A small laptop or netbook is sufficient to view the images.
After the mortise lock has been repaired and tested, I use lithium grease on the parts that slide against the case and a light spray of a dry lubricant. Install the cover and install the lock into the door. Be sure to test the operation of the lock with the door open and closed.