How to Identify and Replace a Mortice Lock: Backset, Faceplate and What Furniture Fits

A mortice lock is installed inside a pocket cut into the edge of a door. This concealed design provides a neat appearance, supports a wide range of locking functions and is commonly used on Australian residential, commercial, aluminium and fire-rated doors.

Replacing one, however, is not as simple as finding a lock that looks similar. The new lock must fit the existing pocket in the door, line up with the handle and cylinder holes, work with the existing door furniture and provide the correct locking function.

Measuring correctly before ordering can prevent additional drilling, exposed holes, sagging handles, misaligned cylinders and a door that does not latch properly.

First, Identify the Type of Mortice Lock

Several different products may be described as a mortice lock:

• Mortice latch: Operated by a lever or knob and normally used on passage doors. It has a latch but no keyed deadbolt.
• Euro-profile mortice lock: Common on residential entrance doors and architectural hardware. It normally has a lever spindle above a Euro-profile cylinder.
• Commercial mortice lock: Common in offices, schools, hospitals, apartments and other commercial buildings. These locks may use an Australian oval cylinder, a lever, a turn snib or a combination of components.
• Short-backset or narrow-stile mortice lock: Designed for doors that are aluminium-framed with glass panes and other doors with limited space inside the stile.
• Sliding-door mortice lock: Uses a hook, claw or specialised bolt rather than a conventional swinging-door latch.

Australia does not have one universal mortice-lock footprint. Current Australian product ranges include narrow-stile locks primarily with 23 mm or 30 mm backsets, architectural Euro locks with 30 mm to 70 mm backsets, and commercial mortice locks with 60mm, 70mm or 89mm backsets. These are examples only, and many other configurations are available.

Before Removing the Old Lock

Keep the door open throughout the inspection. Take clear photographs of:

• The faceplate on the edge of the door
• Both sides of the handle or plate furniture
• The cylinder, keyhole or internal turn
• The strike plate on the frame
• Any model number or branding stamped into the faceplate
• The lock case after it has been removed

Also note whether the door is timber, aluminium or steel; whether it swings or slides; and whether it has a rebated edge.

Look for a fire-door tag on the hinge edge of the door or hinge side of the door frame. Hardware on a fire or smoke door should not be replaced solely because another product has similar dimensions. The proposed lock and furniture must be suitable for that particular doorset and comply with the relevant fire certification.

How to Measure a Mortice Lock

Use millimetres and record every measurement. A steel ruler or vernier calliper will usually give a more reliable result than a flexible tape measure.

1. Backset

The backset is the horizontal distance from the front edge of the lock faceplate to the centre of the handle spindle and/or Keyhole.

Remove the handles so that the square spindle hole or key hole in the lock is visible. Measure straight back from the front plane of the faceplate to the exact centre of that hole.

Do not measure:

• To the outside edge of the decorative handle plate
• To the end of the latch
• To the back of the lock case
• From the edge of the door to the edge of the spindle hole

For most locks, the spindle and cylinder sit on the same vertical line. If they do not, record the horizontal position of each opening separately.

Backset is one of the most important measurements, but it does not identify a lock by itself. Two locks can have the same backset while having completely different case sizes, faceplates, fixing holes and cylinder arrangements.

2. Centre-to-Centre Spacing

The centre-to-centre measurement, commonly shortened to CTC, centres or pitch, is the vertical distance between:

1. The centre of the lever spindle
2. The centre of the cylinder, keyhole or turn mechanism

This measurement is especially important when using lever handles on a long plate. The handle spindle and cylinder/key hole cut-out in the plate must line up exactly with the openings in the lock.

For example, a lock described as having 85mm centres or Pitch has 85mm between the centre of the spindle and the centre of the key hole or cylinder opening.

With separate lever roses and cylinder escutcheons, the components are not physically joined by a long plate. However, their openings must still align with the lock, and the fixing screws must not interfere with the lock case.

3. Case Depth

The case depth is the distance from the back of the faceplate to the rear of the lock body.

Do not confuse case depth with backset. A 60mm-backset lock will normally have a case that extends farther than 60mm into the door.

Measure both:

• From the front of the faceplate to the back of the lock body
• The overall depth, including the faceplate, if necessary

Manufacturers may illustrate these dimensions differently, so compare your measurement with the exact reference points shown on the product drawing.

The replacement case must fit inside the existing mortice without breaking through the face of the door or colliding with glazing, framing, reinforcement or other hardware.

4. Case Height and Thickness

Measure the height and thickness of the main lock body.

Also record the positions of the spindle, cylinder, latch and deadbolt relative to the top or bottom of the case. This is useful when comparing an old lock with a manufacturer’s technical drawing.

A replacement case that is slightly larger may require the door to be cut further. A smaller case may fit into the opening but leave insufficient material for secure fixing or allow the lock to move inside the door.

5. Faceplate Size

The faceplate, sometimes called the forend, is the visible metal strip on the edge of the door.

Record:

• Overall height
• Width
• Thickness
• Square or rounded ends
• Distance between the fixing-screw centres
• Position and size of the latch and deadbolt openings
• Finish, such as satin stainless steel, brass or black

The new faceplate should sit flush with the door edge. A narrower or shorter faceplate may expose the old recess, screw holes or unpainted timber. A larger faceplate may require additional routing.

Do not assume that the strike plate supplied with a new lock will line up with the existing frame preparation. Measure the strike separately.

6. Door Thickness

Measure the complete door thickness, including any applied skins or panels.

Door thickness affects:

• Cylinder length
• Spindle length
• Through-fixing screws
• Turn-snib extensions
• Packing plates and mounting components

A cylinder that is too short may not reach the furniture. One that is unnecessarily long may project way beyond the escutcheon. Commercial lock ranges may require extension kits for doors outside their standard thickness range.

7. Strike, Door Gap and Rebate

Measure the strike plate on the frame and photograph its position before removing it.

Check:

• Strike height and width
• Latch and deadbolt opening positions
• Screw centres
• Latch and bolt projection on the lock
• Gap between the door edge and frame
• Whether the door has a rebated edge

A rebated pair of doors may require a lock-specific rebate kit. These kits are not automatically interchangeable between brands or lock series.

Where the lock has a separate auxiliary deadlatch, that small bolt should be pushed in by the strike when the door closes. It should not enter the main latch opening. Incorrect strike alignment can prevent the deadlocking feature from operating.

How to Match Handles and Other Door Furniture

In door-hardware terminology, furniture includes lever handles, knobs, long plates, roses, escutcheons, cylinders, turns, snibs and emergency-release components.

A handle’s appearance does not determine whether it fits a particular mortice lock. Check the following interfaces.

Spindle Size and Position

The square spindle must fit the lock’s spindle hole correctly. Do not assume that all square spindles are identical.

Confirm:

• Spindle size
• Required spindle length
• Whether the spindle is split or solid
• Whether each side of the lock operates independently
• Whether the handle has its own return spring

Large or heavy lever handles may need sprung furniture or a lock with a suitable heavy-duty spring. An incompatible combination can result in a lever that droops or fails to return fully.

Long-Plate Furniture

Lever handles on a long plate normally require an exact match for:

• Backset
• Spindle-to-cylinder centres
• Cylinder or keyhole profile
• Turn-snib location
• Fixing-hole centres
• Door thickness
• Lock function

A plate described only as “mortice lock furniture” will not necessarily fit every mortice lock.

Lever-on-Rose Furniture

With lever-on-rose furniture, the lever rose and cylinder escutcheon are separate. This provides more flexibility and may cover fewer old holes, but the spindle, cylinder and fixings must still suit the lock.

Some furniture is designed for particular lock families rather than being universally compatible. Always check the manufacturer’s compatibility information before ordering.

Cylinder Profile

The most common cylinder arrangements encountered include:

• Euro-profile cylinder
• Australian oval cylinder
• Round or specialised commercial cylinder
• Key cylinder on one side with a turn or snib on the other
• Double cylinder operated by a key from both sides

Euro and oval cylinders are different shapes and cannot be substituted for one another.

The cylinder must also have the correct operating cam. Two cylinders may have the same visible profile but use different cams to provide functions such as latch retraction, deadbolt operation, holdback or classroom locking.

Fixing Pattern

Check whether the furniture is:

• Concealed fixed
• Visible fixed
• Fixed back-to-back through the door
• Attached directly to the lock case

Through-fixing screws must pass through the intended clearances. They must not be forced through the lock body or interfere with its internal mechanism.

Required Function

Before ordering, describe exactly how the door must operate from each side.

For example:

• Should both handles always operate?
• Should the outside handle lock while the inside remains free?
• Should the door lock automatically when it closes?
• Should a key retract the latch?
• Is an internal snib required?
• Must users be able to leave without a key?
• Is latch holdback required?
• Is the door part of an access-control system?

Terms such as entrance, classroom, vestibule, nightlatch and storeroom can vary between manufacturers. Confirm the operating sequence rather than choosing only by the function name.

How to Replace the Old Mortice Lock

Once you have identified a suitable replacement:

1. Keep the door open. Do not test an unfamiliar lock for the first time with the door closed.
2. Remove the furniture carefully. Keep the spindle, screws, cylinder and adaptors together so they can be measured.
3. Remove the cylinder correctly. Depending on the lock, it may be retained by a faceplate screw, pin or internal retainer. Do not force it if the cam is not aligned. A working key may be required to align the cam before the cylinder can be removed
4. Remove the faceplate screws and withdraw the lock. Never open the lock case. Springs and internal components may be under tension.
5. Compare the old and new locks side by side. Check every opening, fixing point and overall dimension—not only the backset.
6. Set Lock Functions. If the lock allows for multiple functions, like one side lockable, one side free or both sides lockable, set the required functions
7. Install the new case and confirm that the faceplate sits flush. Do not overtighten screws or distort the case.
8. Fit the spindle, cylinder and furniture. Confirm that screws do not bind against the lock.
9. Test everything with the door open. Operate each lever, key, cylinder, snib and bolt repeatedly from both sides.
10. Check the strike alignment. The latch and bolt should enter smoothly without the door having to be lifted, pulled or forced.

Only close the door after you are confident that the handle, key and internal release all operate correctly.

What Does AS 4145.2 Mean?

AS 4145.2 covers general design and performance requirements and testing procedures for mechanical locksets and locksets used in Australian residential and commercial buildings.

Manufacturers may state performance levels for characteristics such as physical security, durability and corrosion resistance.

However, an AS 4145.2 rating does not mean that two locks have the same dimensions or that their handles, cylinders and strike plates are interchangeable. It is a performance standard, not a universal mortice-lock footprint.

When replacing a lock in a commercial building, also check for any applicable National Construction Code and state or territory requirements.

Extra care is required for:

• Fire and smoke doors
• Doors on required exits or paths of travel
• Accessible entrances and sanitary facilities
• Apartment entry doors
• Security-controlled doors
• Doors connected to electric strikes or access control

Hardware changes to a fire door should be confirmed against the approved doorset documentation rather than selected by dimensions alone.

Final Check Before Ordering

A suitable replacement should match four things:

• The footprint: Case size, faceplate and strike preparation.
• The geometry: Backset, centres and opening positions.
• The furniture interface: Spindle, cylinder, fixings and door thickness.
• The function: Exactly how the lock must operate from each side.

Matching all four is the safest way to replace an old mortice lock without unnecessary alterations or ordering incompatible handles and cylinders.