In short:
- A replacement lock has to match the existing one on several measurements: backset, case size, centres, faceplate size and, for cylinder locks, cylinder length and profile.
- Measure the old lock itself, not the hole in the door. Old mortices are often oversized or altered.
- Door thickness, door material and which way the door opens (handing) narrow the choice further.
- Decide on keys before you buy: whether several doors should share one key, and whether you want a restricted-key system where extra keys can only be ordered through authorised channels.
Why an exact fit matters
A lock that "nearly fits" causes problems that aren't obvious on the day it's fitted. A case that's too big means cutting a bigger pocket in the door, which removes timber that holds the lock and frame together. A different backset puts the keyhole or handle in a new position, leaving old holes exposed and sometimes weakening the door. A cylinder that's too long can stick out beyond the handle; one that's too short can be hard to use. A bolt that doesn't line up with the existing strike plate rubs, sticks, or doesn't fully engage.
The aim is usually a like-for-like fit, or a deliberate, planned change (for example, a higher-security lock in the same size). If you haven't yet decided whether the old lock really needs replacing, start with our guide to lock repair versus replacement.
Step one: identify the lock you already have
Before measuring, establish what type of lock it is. A lever mortice lock, a mortice case driven by a euro-profile cylinder, a rim nightlatch and a multipoint lock are all chosen and measured differently. Our guide to mortice, cylinder and multipoint locks shows how to tell them apart by looking at the door edge, the keyhole, the key and the handle.
Then look for markings. Lock makers usually stamp a name, logo or product code on the faceplate, and on multipoint locks sometimes on the central gearbox as well. A known make and model is often the quickest route to an exact replacement. Photograph every marking you can see with the door open.
The measurements that decide fit
| Measurement | What it is | Why it matters |
|---|---|---|
| Backset | From the face of the faceplate to the centre line of the spindle, keyhole or cylinder | Sets where the handle and keyhole sit. A different backset means new holes through the door. |
| Case depth | The lock body behind the faceplate, not counting the faceplate | Must fit the existing pocket without cutting it deeper. |
| Case height (and thickness) | The full height of the lock body | A taller or thicker case means enlarging the pocket. |
| Centres | From the centre of the handle spindle to the centre of the keyhole or cylinder | Must match the handles or handle plates you'll use, or new handles are needed. |
| Faceplate (forend) | Length and width; square or rounded ends; on multipoint locks, flat or U-shaped section | Must fill the existing recess in the door edge neatly. |
| Locking points (multipoint) | The type and position of each hook, roller or bolt, measured from the spindle centre | Each one has to meet an existing keep on the frame. |
| Spindle | Square size, single or split spindle | Affects which handles work with the lock. |
Lock Shop Direct's measuring guides make a point worth repeating: measure the metal lock case, not the hole in the door, because old mortices can be oversized or altered. There's no single standard backset or centres in South Africa: common sizes vary by maker and lock type, so the existing lock is the reference. On euro-profile multipoint locks, 92 mm centres is a common size, but others exist, which is why it has to be measured rather than assumed.
Cylinder length and profile
If your lock uses a cylinder, it has its own two measurements: profile and length.
Profile is the shape of the cylinder's cross-section: euro-profile (a round body with a narrower tail), oval, or a round rim cylinder. A replacement must match the profile the lock case was made for.
Length on a euro-profile cylinder is measured from the centre of the fixing-screw hole to each end, giving two numbers, such as 35/35 or 40/50. The two sides don't have to be equal; offset sizes are common where the door or handles are thicker on one side. A thumbturn cylinder has a key on one side and a turn on the other.
The right length leaves the cylinder sitting flush, or nearly flush, with the handle or escutcheon on the outside. UK lock suppliers advise against a cylinder that sticks out more than a few millimetres, because a protruding cylinder is more exposed to snapping, a forced attack on the cylinder body. Too short is a problem too: the key may be hard to insert fully, or the cam may not line up properly. If you're changing handles at the same time, the cylinder length has to suit the new handle thickness, not the old one.
Door thickness, material and handing
Door thickness
Thickness affects almost everything: whether a mortice lock fits with enough timber either side of the case, how long the cylinder and handle spindle must be, and which handle sets suit. Manufacturers state the door thicknesses each lockset or cylinder is made for, so measure the door itself and include it when you ask for a quote. Hollow or very thin doors may not take a mortice lock at all.
Door material
- Solid timber doors usually take standard mortice locks. The condition of the timber around the existing pocket matters: a split or patched door edge may need repair before a new lock goes in.
- Aluminium and steel-framed doors often have narrow frame sections that need narrow-stile locks or locks designed for that door system.
- uPVC and composite doors often use multipoint locks with euro-profile cylinders. These are measured as a set: backset, centres, faceplate length and section, and each locking point.
Handing
Handing describes which side the hinges are on and which way the door opens. A sashlock's latch must face the closing direction. Some locks have a reversible latch; others are made in left- and right-handed versions. Multipoint locks may be handed, and on doors in pairs a rebated lock or rebate kit may be needed. Note which way your door opens and where the hinges are, and check whether the latch is reversible or the lock is handed before ordering.
Security level and standards
A replacement is a chance to improve on the old lock, but only if the higher-security option still fits the door. Things to compare:
- Lever locks: the number of levers. Lever locksets sold in South Africa commonly have two, three or four levers, and ASSA ABLOY lists Union locksets in all three with the SABS mark. The UK standard BS 3621 calls for five levers along with a minimum 20 mm bolt throw, according to Union.
- Cylinders: the EN 1303 classification, especially the key-security and attack-resistance digits. In the UK, TS 007 and SS312 are additional attack standards for cylinders.
- Multipoint locks: the cylinder grade, the strength of the keeps and the number of locking points.
- Strike plates and frames: a strong lock in a weak frame is still a weak door. Secured by Design notes that the door, frame and other hardware can fail before the lock does.
If you have insurance, check your own policy wording for anything it says about locks, and ask your insurer if it isn't clear. If you live in a complex or estate, check whether your body corporate or managing agent has rules about front-door locks.
Keys: keyed alike and restricted-key systems
New locks and cylinders come with the keys their manufacturer supplies. We don't offer key cutting, so it's worth deciding how keys will work before choosing the product.
Keyed alike
Some cylinders and locks can be ordered "keyed alike", so one key operates several doors. That's convenient, but it also means one lost key affects every door on that key. Whether keyed-alike sets are possible depends on the product and supplier, and it usually has to be arranged when the locks are ordered rather than added later.
Restricted-key systems
Restricted or patented key systems control who can obtain extra keys. Mul-T-Lock, for example, describes a key-control card with a magnetic strip holding an encrypted key code: extra keys can only be ordered when the original card is presented to an authorised dealer. ASSA ABLOY explains that key patents make copying unlawful and blanks harder to produce, so extra keys go through authorised partners. These systems suit homes and offices where you need to know exactly how many keys exist. Keep the card or code certificate somewhere safe and separate from the keys, and plan for how many keys you'll need, since extra keys are ordered through the system's own process.
Mistakes to avoid, and what to send a technician
- Don't buy by eye or from a photo alone. Locks that look the same can differ by a few millimetres in backset or case size.
- Don't measure the hole in the door instead of the lock. Remove nothing yourself if you're unsure; a technician can measure on site.
- Don't enlarge a mortice or drill new holes to make a lock fit unless that change has been planned. It weakens the door.
- Don't assume a cylinder from another door will fit. Length and profile must both match.
To get an accurate quote, send photos of the faceplate (readable markings), both faces of the door around the lock, the key, and the strike plate or keeps. Add the door thickness, the material, which way the door opens, and whether you want keyed-alike or restricted keys. If you've already bought a lock, give the make and model so it can be checked against your door before the visit.
When you're ready, our lock replacement service in Sandton fits the new lock on site and checks it with the door and frame. As a mobile locksmith in Sandton, we come to the door rather than asking you to bring the lock in. If you're replacing locks because you've moved, see our moving-house lock security checklist.
Questions
Can I just measure the old lock and buy a new one myself?
You can, if you're confident about the type and every measurement. Many fitting problems come from a single wrong measurement or the wrong handing. If you've already bought a lock, give us the make and model and we'll tell you whether it suits the door before the visit.
Is a new cylinder enough, or do I need a whole new lock?
On a euro-profile cylinder lock that's in good condition, a new cylinder of the right length and profile is often enough, including after lost keys. If the lock case is worn, damaged or low-security, the whole lock may need replacing. See lock repair versus replacement.
How many keys come with a new lock?
It depends on the manufacturer and product. We'll tell you when we quote, so you can choose a product that comes with enough keys or that has a suitable process for ordering more.
Need a locksmith to look at it?
If this hasn't solved the problem, our lock replacement service covers it. We're a mobile locksmith in Sandton, so we come to your door.
Sources and further reading
- Lock Shop Direct: Mortice locks measuring guide — Definitions of backset, case depth, case height, centres and forend; measure the case, not the hole; handing and rebated doors
- Lock Shop Direct: Multipoint lock measurement guide — Multipoint backset, centres, faceplate, locking-point positions, spindles, handing and maker markings
- Lockshop Warehouse: Installing euro cylinders — Measuring from the centre of the fixing-screw hole to each end; offset sizes; flush fitting and protrusion
- Mul-T-Lock: MTL800 key copy control — Key-control card; extra keys only through authorised dealers on presentation of the original card
- ASSA ABLOY: Standards and patent protection for mechanical cylinders — Patents restrict key copying to authorised partners; EN 1303 and TS 007
- ASSA ABLOY DACH: Technical requirements, DIN EN 1303 (PDF) — EN 1303 key security and attack resistance classes
- Union: What are BS3621 locks? — Five levers and 20 mm minimum bolt throw under BS 3621
- ASSA ABLOY South Africa: Union locksets — Two-, three- and four-lever locksets with the SABS mark
- Secured by Design: Locks and associated hardware standards — Door, frame and hardware can fail before the lock; TS 007 and SS312 for replacement cylinders