How to Change a Mortise Lock: A Professional Locksmith’s Guide
Published Date: Oct 6, 2025
If you’re searching ‘how to change a mortise lock’, you probably want clear steps, not guesswork. Mortise locks sit inside the door, so swapping one takes a bit more care than a standard deadbolt. As working pros, Sure Lock & Key explains the process of changing a mortise lock in plain words and clear steps. Want a smooth, damage-free result on the first try? Keep reading — your step-by-step starts here.
Getting Started: Prep for Changing a Mortise Lock
New to this hardware? Start with our quick primer — what mortise locks are — to see how they work and where they’re used. A smooth mortise lock installation comes down to prep: confirm your mortise lock measurements, gather tools, and dry-fit the parts before you cut. If you’re learning how to fit a mortise lock, these steps keep the swap clean and simple.
You’ll need to measure the backset (distance from the door edge to the keyhole center), the depth and height of the pocket for the mortise lock case, and the center-to-center distance between spindle and keyhole.
Grab these tools: a tape measure and a screwdriver. Also, keep a drill and chisel ready if the new mortise lock case isn’t identical.
When you replace the mortise lock, you’re working with the case inside the door. If the new case matches the old, it should drop in cleanly; if not, mark high spots and chisel lightly — don’t pry against the edge. Not sure about the backset or case size? Sure Lock & Key can double-check measurements and help you pick a compatible replacement before you pull the old set.
Changing a Mortise Lock: A Step-by-Step Guide
Once your new lock and tools are ready, you can start changing a mortise lock. If you hit a mortise lock stuck in the door — stop. Check for a hidden set screw on the door edge, paint bridging the faceplate, or a bent pocket lip. If everything checks out, here’s a general guide to replace a mortise lock:
- Remove the old hardware. Unscrew the edge faceplate. Remove the handles or knobs and the cylinder. Loosen the small set screw on the door edge first, then back the cylinder out to avoid stripped threads.
- Extract the lock body. Slide the old mortise lock case out. If it resists, rock it gently front to back — don’t pry against the door edge. Score any paint at the perimeter with a utility knife.
- Install the new lock. Dry-fit the new mortise lock case in the pocket. If it doesn’t seat flush, mark high spots and chisel lightly for a snug fit. Match the backset and spindle alignment to your existing trim.
- Secure the cylinder and trim. Do the mortise cylinder replacement: insert the new cylinder, align the tailpiece (or cam) with the case opening, then snug the set screw. Refit the faceplate and handles. Not sure which part you’re replacing? See our cylinder vs mortise lock guide for a quick comparison.
- Test and adjust. With the door open, turn the key several times and check the latch throw and handle return. Close the door and test again. If the latch drags, adjust the strike slightly; if the key binds, back off the set screw a quarter turn.
Short on time or managing multiple doors? Sure Lock & Key’s commercial locksmith services can replace the mortise lock, tune the fit, and keep your space moving.
Your Next Steps for a Secure Door
Now that you know how to change a mortise lock, remember to double-check your measurements and make sure the new lock is compatible with your door. If the project feels too complicated or you’re dealing with an older, trickier lock, call Sure Lock & Key. With 35+ years in the field, we show up prepared, explain options in plain English, and leave hardware working cleanly. Need fast help for a property or storefront? With our locksmith for business service, mortise set rekeys, adjustments, and replacements happen with minimal downtime.