Replacement Furniture Locks: Comparing Cylinders and Backset Dimensions

The backset defines the exact distance from the centre of the keyhole to the outer edge of the lock, whilst the cylinder represents the mechanical core that operates the locking mechanism.

A broken lock on a cupboard or drawer requires a swift replacement. Often, original components from manufacturers are no longer available in identical specifications years later, or a compatible, functional alternative is required. To ensure a seamless replacement, joiners and fitters must carefully analyse the technical specifications of the old part.

This article examines the technical differences between lock types and explains how to determine cylinder dimensions and backsets correctly to integrate accurate furniture locks as replacement hardware.

What Are the Cylinder and the Backset?

To replace a lock professionally, understanding the two most critical installation dimensions is essential. If these do not match, the replacement lock cannot be fitted without drilling new holes into the furniture front.

The Backset

The backset is the most critical measurement during replacement. It describes the distance between the centre of the cylinder (the centre point of the hole drilled in the front) and the outer edge of the lock casing. Typical dimensions in furniture construction range between 15 mm and 40 mm. The backset is like the DNA of a lock; if it deviates by even a few millimetres, the bolt will no longer engage with the striking plate or the cylinder will not fit through the existing front hole.

The Cylinder

The cylinder (or cylinder core) is the component into which the key is inserted. When selecting a replacement part from the furniture lock cylinder category, two dimensions are decisive:

• Cylinder diameter: The size of the hole in the wooden front. Common standard diameters in furniture making are 16.5 mm, 18 mm, or 19 mm.
• Cylinder length: This must match the material thickness of the furniture front. For a standard front made of chipboard or MDF with a thickness of 19 mm, the cylinder requires an appropriate length (usually 20 mm or 22 mm) so that it sits flush and the key can be inserted completely.

Differences Between Lock Types: Surface-Mounted Locks vs Espagnolette Locks

Depending on the piece of furniture and security requirements, different lock mechanisms are used. Identifying the existing type is the first step before measuring.

Surface-Mounted Lock (Rim Lock)

The surface-mounted lock is the standard choice for single doors and drawers.

• Installation: The lock casing is screwed onto the inside of the front. The cylinder protrudes outwards through a drilled hole.
• Function: Turning the key extends a solid bolt, which engages behind a striking plate or the cabinet carcass.
• Area of application: Pedestals, desks, chests of drawers. When planning drawers that run on full extension drawer runners, the surface-mounted lock is usually positioned centrally at the top of the front.

Espagnolette Lock (Multi-Point Locking Bar Lock)

For tall doors that need to be secured at multiple points, a single bolt is not sufficient. In this case, a cupboard lock with rods is used.

• Installation: The main lock sits centrally on the inside of the door. From there, two metal rods (the linkage) run upwards and downwards.
• Function: When the key is turned, the rods rotate or slide vertically. At the ends, locking hooks or pins engage with corresponding brackets in the top and bottom panels of the carcass.
• Area of application: Large wardrobes, office cupboards, filing cabinets.

Finding Compatible Alternatives

When replacing a faulty branded lock, you are not strictly dependent on the original manufacturer part. Compatible alternatives can be used without issue, provided the technical parameters are identical. Follow a systematic approach when determining the dimensions:

1. Determine the backset: On the old lock casing, measure from the edge facing the door edge directly to the exact centre of the cylinder.
2. Measure cylinder diameter: Measure the outer diameter of the old cylinder using callipers. Alternatively, measure the diameter of the drilled hole in the furniture front.
3. Check locking direction: Note whether the lock is designed for a right-hand door, a left-hand door, or a drawer. The direction in which the bolt extends changes accordingly.
4. Check the drilling pattern: Check the spacing between the fixing screws on the lock casing. Many manufacturers align with System 32, which greatly simplifies the compatibility of replacement fittings.

Tip for fitters: When a new lock is fitted into an existing door, the weight distribution of the front may alter slightly. It is advisable to inspect the alignment of the door on the hinges after installation. If necessary, adjust the installed concealed cup hinges / cupboard hinges via the adjustment screws so that the bolt of the new lock slides cleanly into the striking plate. For heavy doors, using concealed cup hinges / cupboard hinges with soft close is recommended to minimise shock to the locking mechanism when closing.

Which Lock Is Suitable for Which Application?

The choice of the right replacement part depends on the specific construction of the furniture unit.

• For individual drawers: The classic surface-mounted lock (often referred to as a drawer lock) is the correct choice here. The bolt extends vertically upwards into the carcass. Important for modern drawer systems: ensure that the bolt travel is long enough to bridge the gap between the front and the frame rails.
• For medium-height filing cabinets (single door): A surface-mounted lock with horizontal bolt extension. Pay attention to the handing (left-hand or right-hand).
• For full-height wardrobes: An espagnolette lock is mandatory. A single bolt in the middle of a 2000 mm high door would not prevent the door from being levered open at the top or bottom.
• For glass doors: Specialised glass door locks that do not require drilling into the glass and are secured using a clamp plate instead.

Frequently Asked Questions (FAQ)

How do I measure the backset correctly if the old lock is missing?

If only the drilled hole in the front remains, measure from the outer edge of the furniture front to the exact centre of the hole. This measurement in millimetres is the required backset for the replacement lock.

Can I replace a branded lock with an unbranded model?

Yes. Mechanical furniture locks are highly standardised. If the backset, cylinder diameter, and cylinder length match, a compatible alternative will fit directly into the existing millings and drill holes.

What do keyed alike and keyed to differ mean?

With keyed alike cylinders, multiple locks (such as across an entire wall unit) can be opened with the same key. In keyed to differ systems, each lock has an individual profile and its own unique key.

Why cannot the key be removed from the new lock when it is unlocked?

Some cylinders feature a key-retaining function. This technical feature ensures that the key can only be withdrawn when the lock is in the secured position. This prevents cupboards from accidentally being left unlocked.