Guide

Woking Homeowner's Guide: How to Replace a Door Lock, From Measuring and Fitting to Knowing When to Stop

Lock failed and the door will not secure tonight? Call 01483 377197. If it can wait, read on and do it yourself.

Decide first whether this is a tonight problem or a weekend job. If it is a weekend job, the answer is encouraging: most door lock replacements take 30 to 60 minutes with a screwdriver, as long as the new lock matches the hole already cut in the door. One rule runs through the whole guide: never close the door on a newly fitted lock until it has passed every test with the door standing open.

This guide covers how to identify what is fitted, how to check the lock is really the fault, the measurements to take before you buy, and how to remove, fit and test the replacement. The measuring step is where most attempts go wrong. It deals in full with timber doors, mortice locks and nightlatches; uPVC and composite doors have their own guide. It is written by Woking Locksmith, and it assumes you are working on a door you have every right to change.

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Which Lock Are You Actually Replacing?

What you are replacing is decided by the door, not by preference, and three questions settle it. Answer them by looking at your own door.

What is the door made of? If it is timber, carry on with this guide. If it is uPVC or composite, the lock is almost certainly a different system.
Where is the keyhole? If it is in the edge of the door, you have a mortice lock. If it is in the face of the door near the handle, it is a euro cylinder. If it is on a box fixed to the inside face, it is a nightlatch with a rim cylinder.
Does lifting the handle throw bolts up and down the frame? If so, that is a multipoint mechanism, and the cylinder is only one part of it.

For a fuller picture, see the main UK door lock types, and for the plastic and composite doors, changing a uPVC door lock.

The type matters because mortice locks and nightlatches are measured and fixed in quite different ways.

Locks Fitted Into the Edge of a Timber Door

If the keyhole is in the edge of the door, you have a mortice lock. It comes in two versions:

Mortice deadlock	Mortice sashlock
A deadbolt only, worked by key from both sides	A latch and a deadbolt, so it takes a handle as well as a key
The type insurers often name for a final exit door	Common on doors used during the day

Five levers is the usual specification on an external door, and the lever count is normally stamped on the faceplate or printed on the key. Some external timber doors carry five-lever mortice locks, including models designed to meet BS 3621; what that standard means for your cover is explained in the guide to BS 3621 door locks. Before you go near a shop, write down whether yours is a deadlock or a sashlock.

Locks Fitted Through the Face of a uPVC or Composite Door

A keyhole in the face of the door, level with the handle, means a euro cylinder sitting inside a multipoint mechanism. The cylinder itself swaps out in minutes once you know the right length, but the measuring and the mechanism behind it are different enough to need their own guide. One distinction is worth taking with you:

The cylinder is the part the key turns.
The mechanism is the strip of bolts and hooks the handle drives.

Replacing the wrong one is the costliest mistake on this kind of door. A handle that no longer lifts points to the mechanism, not the cylinder.

Check the Lock Is Actually the Problem

A new lock fixes a faulty lock and nothing else, so spend two minutes finding out which you have. A stiff or unreliable lock often turns out not to need replacing at all, because worn hinges, a moved frame or a misplaced strike plate produce much the same symptoms. The quickest check: open the door fully and work the lock with the door open. If the key and bolt move freely with the door open but stick once it is shut, misalignment is likely to be part of the problem. That is a strong clue, though it does not rule out wear inside the lock. To weigh up the options, see lock repair or replacement.

Signs the Lock Itself Has Failed

These symptoms point at the lock rather than the door:

the key turns and turns without throwing the bolt
the key will not go in, or will not turn, even with the door open
the turn feels gritty all the way round (repairing a lock that has started to fail)
a lever lock jams now and then
a key has snapped off inside (see removing a broken key)

Do not force a lock that has jammed. Some reasons to replace are not faults at all:

keys lost or stolen
moving into a property
keys unaccounted for after building work or the end of a tenancy

Signs the Door, Not the Lock, Needs the Work

The tell is simple: if the fault only shows up once the door is shut, it is alignment. In practice that looks like:

having to lift, pull or lean on the door to turn the key
the bolt catching the edge of the keep instead of entering it
bright scrape marks on the keep or the strike plate
a gap around the door that is wider at the top than the bottom, or the other way round

The usual causes are hinges that have worn or worked loose, a frame that has shifted, or timber that swells in wet weather. Adjusting a keep or a hinge is often a smaller job than a lock change, and it is the job that actually cures the problem.

Measure the Lock Before You Buy It

The replacement has to match the hole already in the door, so measure the old lock first and shop second. You are not choosing a lock; you are matching one, and a lock that is 12mm out will not go in. For many straightforward changes the tools are just a screwdriver that suits the existing screws and a tape measure or rule, plus a replacement that matches the old lock's type, measurements, fixing points and the security level you need. First establish which part you are replacing—a mortice lock, a nightlatch or just the rim cylinder—as this determines what you need. If you would rather have it done, see lock installation.

The Five Measurements on a Mortice Lock

Five dimensions describe a mortice lock, and three of them decide whether a replacement fits. Take the lock out of the door before you measure. The definitions and common sizes below follow ERA's measuring guide for mortice locks.

Dimension	What it measures	Common UK size
Backset	Centre of the keyhole to the edge of the lock	Commonly 44mm on a 64mm lock, 56mm on a 76mm lock
Case depth	Depth of the lock body set into the door	Typically 64mm (2½ inch) or 76mm (3 inch)
Centre	Keyhole centre to the centre of the follower hole the handle spindle passes through (sashlocks only)	Varies by lock
Case height	Top to bottom of the lock body	Varies by lock
Faceplate	Length and width of the plate visible on the door edge	Varies by lock

Mortice locks sit in a recess cut into the edge of the door, and most conventional ones are held by screws through the faceplate, though the exact fixing varies. Note all five measurements in millimetres, compare them with the new lock's specification, and take the old lock to the shop with you. Choosing the cylinder or lock grade is covered in anti-snap locks.

Measuring a Rim Cylinder or Nightlatch

A nightlatch is measured differently, because its case sits on the inside face of the door rather than inside it. Start by deciding whether the whole nightlatch needs replacing or only the rim cylinder on the outside. Then record:

Backset: from the door edge to the centre of the cylinder. For UK nightlatches, 40mm and 60mm are common backset sizes.
Case size: the height and width of the box on the inside face.
Latch type: a standard nightlatch, or a deadlocking one that can be locked from inside.
Cylinder length: from the outside face of the door through to the inside, where the tailpiece passes through.

The cylinder and the case are often sold separately, so a worn cylinder can usually be swapped without touching the case.

Remove the Old Lock

Before you undo a single screw, make sure you could still secure the house if the job stalls halfway, and work with the door open and properly supported so it cannot swing shut on you. Make sure the deadbolt is fully withdrawn.

For a mortice lock:

On a sashlock, remove the handles and square spindle after undoing the handle fixings.
Take out the faceplate screws on the door edge.
Ease the lock body out of the recess, without levering.
If it will not come, look for a second screw under the faceplate rather than pulling harder.

For a nightlatch:

Undo the case on the inside face.
Remove the connecting screws.
Withdraw the cylinder from the outside.
Set aside the old screws, handles and fittings in one place so you can compare them with the replacement parts if needed.

If a key snaps during removal, stop: a fragment inside makes the rest of the job harder (what to do about a snapped key). Never drill, lever or force any part of the lock. If it will not come out, the answer is to stop.

Fit the Replacement

The new lock should go in without persuasion. Anything that needs forcing is the wrong size or the wrong way round.

Offer the new lock up dry, before any screws go in, and check it sits flush.
Check the bolt and latch face the right way, because a latch fitted backwards will not let the door close.
Fit the faceplate screws finger-tight, work the lock, then tighten them.
On a sashlock, refit the spindle and handles, and check the spindle is long enough for the new case.
Check the keep. If the new bolt projects differently, the keep may need adjusting, and this is where the job most often goes wrong.

If the new faceplate is larger than the old recess, the door needs cutting, and that is the point at which many people should stop and look at having the lock installed for them. A fire door is a different matter altogether: cutting it can change what it is rated to do, so that work belongs with someone qualified.

Test It Before You Shut the Door

This is the most important instruction in the guide: test everything with the door wide open, and do not close it until every check has passed. A latch that has been fitted badly, a bolt out of line or a handle connected wrongly can leave you unable to open the door again.

Use the new key to lock and unlock, turning it gently in each direction. It should move smoothly, without sticking or needing force.
Throw the bolt fully and check that it projects cleanly.
On a sashlock, check that the latch springs back when the handle is released.
Still with the door open, hold it in the closed position against the frame and check the bolt lines up with the keep.
Only then close the door and test from outside with the key you will actually carry. Try every key in the set, because new locks occasionally ship with keys that differ.

Do not proceed if the replacement has changed an escape function the door is required to have. Keep the old lock until you are satisfied with the new one; a spare working lock is a better fallback than a call-out.

Deal With the Old Keys

Most people change a lock because of a key, so the job is only finished when the old keys stop mattering.

Write down how many keys came with the new lock and who holds each one.
If you have just moved in, assume there are keys you do not know about, held by previous tenants, tradespeople or letting agents.
If a key was lost rather than stolen, the lock still needs changing, because a found key works for whoever finds it. If the only key has gone, see locked out.
If you want one key for both front and back doors, ask for keyed-alike locks when you buy. It cannot be arranged afterwards without changing both locks again. [CLIENT TO CONFIRM: whether keyed-alike sets are supplied, and in which ranges]
Keep a spare somewhere other than the property.

A key hidden outside the house is not a spare. It is an unlocked door with an extra step. And only ever change a lock on a property you own or rent.

When to Stop and Call a Locksmith

A straightforward lock change is usually manageable when the new lock matches the old fittings and the door is sound. These are the points to stop, with the reason for each:

The new lock does not fit.

If the case, faceplate or screw holes do not line up, forcing it or cutting the door can make things worse.

The door is a fire door or a certified door.

Cutting it changes what it is rated to do.

It is a flat entrance door onto a shared corridor.

The lock has to open from inside without a key, and the wrong choice creates an escape problem.

There is no key at all, or the old lock will not come out normally.

Seized screws or a jammed mechanism need assessment, not more force.

The lock is part of a multipoint mechanism that has failed.

The door is misaligned. If the new lock works with the door open but binds when it is closed, the hinges, frame or strike plate need attention.

Your insurer specifies a standard

and you are not sure the replacement meets it. Check your policy wording.

Having a lock fitted once usually costs less than a lock change plus a repair to a door that has been cut wrongly. See lock replacement, or check what a lock change typically costs.

Discuss Your Lock Problem

Get a Lock Changed in Woking Instead

Woking Locksmith supplies and fits replacement locks across Woking, including on the doors this guide suggests leaving alone. When you ring, have ready the door material, where the keyhole sits, whether you still have a working key, and whether the door opens normally.

There is no call-out charge, emergency calls are taken around the clock every day of the year, and you get a price on the call from what you describe, with anything found on site explained before work begins. [PROVIDER PACK: DBS check and public liability insurance evidence]

Get a Price for a Lock Change: call 01483 377197

Door Lock Replacement Questions

How long does it take to replace a door lock?

A like-for-like change usually takes 30 to 60 minutes with basic tools, provided the new lock matches the recess and fixings already in the door.

How do I know what size lock to buy?

Measure the old one. Even a small difference in backset, case size or fixing positions can stop a replacement fitting, so record the measurements in millimetres and check them against the new lock's specification before you buy.

What is the difference between a deadlock and a sashlock?

A deadlock has a deadbolt only and is worked by a key. A sashlock has a deadbolt and a latch, so it also takes a handle.

My new lock works with the door open but sticks when it is shut. Why?

That usually points to alignment: the bolt is not meeting the keep cleanly. Check for scrape marks on the keep, and look at the hinges and frame before blaming the lock.

Can I change the lock on a uPVC door the same way?

No. A uPVC or composite door normally has a euro cylinder inside a multipoint mechanism, which is measured and changed differently. See the separate guide to changing a uPVC door lock.