A mortise lock sits inside a pocket cut into the edge of a door, so replacing one is more involved than matching the finish of a handle. Two locks can look similar and still differ in case size, backset, spindle position, cylinder preparation or latch direction.
Quick answer: To choose a mortise lock, identify the lock function, measure the existing case and backset, check door thickness and handle or cylinder spacing, and confirm the latch handing and strike plate. Match the replacement to the door preparation and intended use. If the door is fire-rated or part of an access-control system, use a qualified installer and the approved hardware specification.
What is a mortise lock?
A mortise lock has a lock case fitted into a shaped recess, or mortise, in the door edge. The case contains components such as the latch, deadbolt and operating mechanism. Handles, escutcheons or roses, a cylinder and a strike plate may be separate parts of the complete set.
This differs from a tubular latch or cylindrical lock where the main mechanism is installed through a round bore. Because mortise lock cases and door preparations vary, the model number or exact measurements are valuable when replacing one. A brand name and handle finish alone are not enough.
Measure the lock before you shop
Backset
Backset is the distance from the front edge or faceplate of the lock to the centre of the handle spindle or cylinder, depending on the lock design and the manufacturer’s measurement convention. Confirm the reference points on the product drawing. ASSA ABLOY publishes mortise lock models with particular backsets, such as 60 mm or 70 mm in different ranges; these are model specifications, not universal sizes.
Case size and faceplate
Measure the lock case height and depth, the faceplate length and width, and the distance from the spindle or cylinder centre to the relevant lock edges. Photograph the lock and its markings before removing it. A case that is too deep may not fit the door stile, while a shorter faceplate can leave the old mortise visible.
Centres and door thickness
Where a handle and cylinder use separate openings, record the centre-to-centre distance and compare it to the lock drawing. Measure the actual door thickness, including any relevant cladding or finish if the maker instructs you to. Manufacturers specify compatible thickness ranges, and some sets need different spindles or fixing screws for thicker doors.
Handing and latch direction
Note whether the door opens inward or outward and which side the hinges are on when viewed from the defined approach side. Some latch bolts can be reversed; others require a specific handing. Do not assume a latch is reversible because another model in the same brand’s range is. Check the installation sheet.
Choose the lock function for the room
- Passage: The handle operates the latch, with no privacy locking function. This can suit a hallway or room where a keyed lock is not needed.
- Privacy: A thumbturn or similar control provides privacy locking, typically with an emergency release arrangement. Confirm how the release works and whether it is suitable for the room.
- Keyed entry: A key cylinder operates the locking function from one or both sides according to the selected model. Check the cylinder type, keying needs and escape requirements.
- Deadlocking or security function: The lock may combine a latch and deadbolt or use another locking arrangement. Confirm what each handle, key or thumbturn does.
- Multipoint or narrow-stile applications: These use more specialized cases and door preparations. Match the exact system and dimensions rather than substituting a standard interior lock.
Names for these functions vary among brands and markets. Read the function diagram and test the intended operation before installation.
Consider the door and its use
Interior bedroom, bathroom and passage doors have different privacy needs from an entrance door. Exterior doors also need suitable cylinder protection, strike reinforcement, weather exposure resistance and security performance for the door and frame assembly. A lock alone cannot make a weak door secure.
For a bathroom or bedroom, choose a privacy function that can be released safely from outside in an emergency. For a main entry, consider how occupants will exit during an emergency and comply with local building and fire rules. A locksmith or door professional can recommend an appropriate function for the setting.
Pay attention to door material and stile width. A deep case needs enough material around it, and large cut-outs can weaken a narrow door edge. Fire doors require compatible, approved hardware and correct installation; do not enlarge or alter a rated door casually.
Check compatibility with handles and cylinders
Mortise cases often need trim with the right spindle dimensions, fixing centres and cylinder profile. Confirm whether the set uses a standard square spindle, whether the handle springs are in the lock or the trim, and which cylinder type is required. Some lock cases are supplied without cylinders or handles, so check the package contents before ordering.
Our guide to choosing door handles covers handle form and fit. If you are comparing electronic entry options, see the smart door lock guide; an electronic lock may use a different body and door preparation from a mechanical mortise case.
Replacement checklist
- Photograph the lock, faceplate, strike and all visible markings.
- Record case dimensions, backset, handle centres, cylinder centres and door thickness.
- Identify the current function and note how the latch and deadbolt operate.
- Check the door handing and whether the replacement latch is reversible.
- Compare the manufacturer’s template and installation instructions with the existing cut-outs.
- Confirm cylinder, spindle, strike plate and fixing parts are included or separately specified.
- Have the installer check door condition, frame alignment and any fire or access requirements.
When upgrading, do not cut a new mortise until the lock model and template are confirmed. A mismatch can leave an oversized opening or weaken the door. Keep the removed lock until the new unit has been tested through several complete cycles.
Common problems after installation
A key that turns stiffly, a latch that scrapes, or a door that needs to be pushed shut can indicate misalignment between the lock and strike, a warped door, incorrect case position or unsuitable hardware. Tightening handles harder may hide the symptom without fixing the cause. Check the manufacturer’s troubleshooting guidance or ask the installer to inspect the alignment.
Lubrication requirements vary. Use only the product maker’s recommended lubricant and avoid applying oil or grease indiscriminately inside a cylinder. If the lock is part of an access-control or master-key system, ask the responsible locksmith before changing components.
Frequently asked questions
How do I measure a mortise lock backset?
Use the manufacturer’s reference points, commonly from the lock faceplate or door edge to the centre of the handle spindle or cylinder. Confirm which centre the drawing uses before recording the measurement.
Are all mortise locks the same size?
No. Case depth, height, backset, faceplate, centres, function and compatible door thickness can all vary. Compare the model drawing to the existing preparation.
Can I change a left-hand mortise lock to right-hand?
Some models have reversible latches or field-reversible handing; others are handed. Follow the exact product instructions rather than forcing the latch to turn.
Can I install a mortise lock myself?
A like-for-like replacement may be within an experienced DIYer’s ability, but cutting a new mortise or fitting a lock to an entry or fire-rated door needs careful measurement and may require a qualified locksmith or door installer.
Does a mortise lock include a cylinder and handles?
Not always. Check the package contents and compatibility notes. Some lock cases are sold separately from cylinders, handles, escutcheons and strike plates.
Match the lock to the prepared door
Choose the lock by function and dimensions first, then check the handle, cylinder, strike and door requirements. If the case measurements and door preparation do not match, pause before cutting or ordering. A careful fit check helps the lock operate smoothly and keeps the door hardware working as a complete set. For a brand comparison across related entry hardware, see our Dorset vs Ozone door hardware guide.