A mortice lock can look like a simple swap until the new case will not sit in the door pocket, the handles no longer line up, or the key will not turn cleanly. Knowing how to change a mortice lock starts with identifying exactly what is already fitted. Get that right before ordering and the job is usually straightforward; guess the size or type and you may be cutting timber unnecessarily.
Mortice locks are fitted into a pocket, or mortice, cut into the edge of a timber door. They are common on front doors, back doors, internal doors and older commercial premises. A like-for-like replacement is normally the cleanest route, particularly where existing handles, escutcheons and strike plates are in good condition.
Identify the mortice lock before you buy
First, establish whether you have a mortice deadlock or a mortice sashlock. A deadlock is operated by a key only and has a single deadbolt. A sashlock combines a key-operated deadbolt with a spring latch, allowing the door to close and latch shut when used with lever handles.
Do not confuse a mortice lock with a rim nightlatch or a euro cylinder lock. A rim nightlatch sits on the inside face of the door, while a euro cylinder is usually fitted through a multipoint locking strip or a sashlock case. The replacement process and measurements are different.
The number of levers is also relevant. Older locks may be 2, 3 or 5 lever models. For an external timber door, a 5 lever British Standard mortice lock is often the appropriate choice, especially where an insurer requires a lock meeting BS 3621. Check your policy wording rather than assuming any 5 lever lock will meet its requirements. Some properties need a key-operated lock from both sides, while others benefit from an internal thumbturn only where this is suitable for the door and its escape route.
If the lock is on a communal entrance, fire door, rented property or business premises, pause before changing it. The door may need compliant ironmongery, restricted keys, a master-keyed suite or landlord approval. A fire door in particular should not be altered casually, as the lock and keeps form part of a tested door assembly.
Measure your existing mortice lock case
Remove the lock first if you can do so safely, then measure the metal case rather than relying on the faceplate alone. The key dimensions are case height, case depth, backset and centres.
Case depth is measured from the front of the faceplate to the back of the lock case. Common sizes include 64 mm and 76 mm, but older locks vary. The backset is the distance from the front edge of the faceplate to the centre of the keyhole or follower. A 64 mm case commonly has a 44.5 mm backset, while a 76 mm case often has a 57 mm backset. Measure yours rather than treating those figures as a rule.
On a sashlock, centres means the distance between the centre of the handle spindle follower and the centre of the keyhole. This is commonly 57 mm on traditional mortice sashlocks, although other dimensions exist. The case height and the positions of the bolt, latch and keyhole must also suit the existing door cut-outs.
Take a clear photograph of the lock case, faceplate and keep before disposal. Note the handing of the latch: when viewed from the door edge, the bevel must face the direction in which the door closes. Many modern sashlocks have a reversible latch, but it is worth checking before fitting.
Tools and preparation
For a standard replacement, you will usually need a screwdriver set, tape measure, pencil, drill and suitable drill bits, a sharp chisel, hammer and safety glasses. Keep the new lock, keys, handles or escutcheons, and its supplied screws together until the final checks are complete.
Work with the door open and supported. Do not remove a lock from a closed door, particularly if it is the only entrance to the property. If the door is heavy, poorly aligned or difficult to hold open, ask someone to assist. It is also sensible to keep pets and children away from the working area.
How to change a mortice lock step by step
Remove the handles and old lock
Start by removing the handle screws or door furniture. If the handles are fixed with through-bolts, loosen both sides evenly. Remove the spindle, then take off any key escutcheons or cylinder furniture that obstructs the lock.
With the door open, remove the two screws in the faceplate on the door edge. Insert the key, turn it slightly to retract the bolt if needed, and ease the lock case from the mortice. Avoid levering hard against the timber. If paint has sealed the faceplate in place, carefully score around it with a knife before lifting it free.
Check the old lock as it comes out. If the wood around the mortice is split, the screws are stripped, or the case has been forced, fitting a replacement alone may not restore the door's security. Repairing the timber and reviewing the keep may be necessary.
Test-fit the replacement case
Slide the new case into the existing mortice without screws. It should sit flat with the faceplate flush to the door edge. The spindle follower and keyhole should line up with the existing holes in the door faces.
If it fits but is very tight, remove only a small amount of timber at a time with a sharp chisel. Do not enlarge the mortice simply to accommodate a deeper case unless you are confident there is enough timber left around it. A loose-fitting mortice lock can move under use and may leave the door weaker around the lock area.
Where the new faceplate is longer or wider than the old one, mark around it accurately. Recess only enough timber for the plate to sit flush. A proud faceplate can prevent the door closing properly; one set too deep can leave an untidy edge and reduce support around the case.
Fit the furniture and secure the lock
Once satisfied with the fit, screw the lock case to the door edge using the supplied screws. Fit the spindle and handles, ensuring the spindle is cut to length only if the manufacturer instructs it and it genuinely projects too far. A spindle cut too short can cause handles to disengage; one that is too long can make them bind.
Fit the escutcheons or keyhole plates without overtightening. Test the handle action several times with the door open. The latch should retract fully and spring back freely. Turn the key and make sure the deadbolt travels smoothly without the handle or furniture catching.
Adjust the keep, not the lock
Close the door gently and see where the latch and bolt meet the strike plate or keep on the frame. If the door needs lifting, pushing or pulling to throw the bolt, the frame-side keep may need adjustment. This is common on timber doors that move with changes in temperature and moisture.
Mark the contact point with pencil or lipstick on the bolt, then make small adjustments to the keep recess or plate position. Never force the key to operate against a misaligned keep. Continued pressure can damage the key, lock mechanism or frame.
Test the lock repeatedly from both sides with the door closed. Confirm that the latch catches, the deadbolt throws fully, the key removes in the intended position and the handles return correctly. Keep the spare keys somewhere secure, not hidden outside the property.
When a replacement is not a DIY job
Changing a like-for-like mortice lock is within reach for many competent DIYers, but some situations need a locksmith. Call for professional help if the key is snapped inside, the door is locked shut, the mechanism has failed after an attempted break-in, the mortice is badly damaged, or the new lock requires substantial cutting. It is also worth booking a locksmith where insurance compliance, fire-door performance or a master-key system is involved.
For an external door, consider the whole security picture rather than the lock case alone. A high-quality mortice lock is only as effective as the door, frame, hinges and keep holding it. Long, correctly sized fixing screws and sound timber can matter as much as the lock’s security grade.
Secure Choice can help you compare mortice deadlocks and sashlocks by type, brand and size, so you can order a compatible replacement rather than make a costly compromise. If the measurements are uncertain, take them twice and keep the old case until the new lock has been fitted and tested.