How to Repair a Mortise Lock?

There are several ways to repair a mortise lock. Some of these techniques involve replacing components, such as the latch spring, spindle cam, combined arm/transit, or faceplate. You can also use a plastic handle of a screwdriver as a mallet. 

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Replace the latch spring 

If your mortise lock does not open, the first thing you should do is to replace the latch spring. You can find replacement parts at Home Depot, Lowes, and Amazon. After replacing the spring, the latch must be re-seated. You should also remove the interior knob and lever if necessary. 

Measure the length of the latch spring. If it is too short, a locksmith can provide you with a better measure. Otherwise, you can measure the spring’s pretension force by looking at other locks. You can also use a search engine on McMaster-Carr to find a replacement spring. After replacing the spring, you must install the lock mechanism, which can be a tricky task. Make sure to hold the spring firmly and re-tension it after installation. 

Replace the spindle cam 

When repairing a mortise lock, you must first remove the faceplate of the lock. Then, loosen the set screws that secure the knob to the lockset. The screws should align perfectly with the key slot of the lock. If you have a surface-mounted rim lock, you need to unscrew the cover and loosen the two cylinder setscrews that anchor the door knob to the lockset. 

Ensure that the replacement lock faceplate is flush with the edge of the door. If it is not, use a hammer and chisel to slightly deepen the mortise. 

Replace the combined arm/transit 

To replace the combined arm/transit of a Mortise lock, you must first remove the lock face from the door. A small screw driver should do the job. Once the lock face is removed, you can remove the lock mechanism. There may be springs attached to the lock mechanism. Once the lock mechanism is out of the door, you should unscrew the set screw that holds the knob in place. 

If the lock is above or below the door latch, you can insert a thumbturn through the locking hole. Remove the mortise lock assembly and test it to ensure it works properly. 

Replace the faceplate 

Replacing the faceplate of a mortise-style lock is relatively easy. First, remove the existing hardware. This usually involves removing screws. After that, insert the new lock body into the old mortise. Secure it with another screw. Remove the old faceplate by prying it up from the door edge. 

Depending on the lock type, replacing the faceplate of a mortise-style lock may require drilling new holes. It is important to check whether the new faceplate will cover the existing screw holes. Alternatively, you may wish to purchase a mortise lock that comes with interchangeable faceplates, as these are available in more varieties than just three brands.