How do I know if my mortice lock is a 3-lever or 5-lever?

The easiest way is to look at the key. A 5-lever key has a more complex pattern of cuts on the bit than a 3-lever key, with more steps at varying heights. You can also check the faceplate on the edge of the door — a BS3621 lock will be stamped with the standard number and Kitemark. If there are no markings, it is likely a 3-lever lock that should be upgraded.

Can I fit a BS3621 mortice deadlock to a uPVC door?

No. Mortice deadlocks are designed for wooden doors only. They require a solid timber door body to hold the lock securely. uPVC and composite doors use euro cylinder locks in a multipoint mechanism instead. If you have a uPVC door, see my guide on anti-snap euro cylinders for the appropriate security upgrade.

How much does it cost to fit a five-lever mortice deadlock?

I fit BS3621 five-lever mortice deadlocks from £79 including the lock and fitting. If the door already has a mortice pocket from an existing lock, it is a straightforward swap. If I need to cut a new mortice from scratch, the job takes longer and costs more. Call 07735 336175 for an exact quote based on your situation.

What should I do if my mortice lock is stiff or hard to turn?

First try lubricating the keyhole with a graphite powder or PTFE spray — never WD-40. If that does not help, the levers may be worn or corroded, or the bolt may be misaligned with the strike plate. A locksmith can diagnose the issue on-site. Sometimes a service fixes the problem, but if the lock is old or damaged it is better to replace it with a new BS3621 unit.

The Complete Guide to 5-Lever Mortice Deadlocks

The 5-lever mortice deadlock is the gold standard for wooden front doors in the UK. Here is everything you need to know — how they work, how to identify one, and why your insurance probably requires it.

The Complete Guide to Five-Lever Mortice Deadlocks

If you have a wooden front door, a five-lever mortice deadlock is the single most important lock on it. It's what your insurance company requires, it's what keeps your door secure when you're out or asleep, and it's the benchmark that every other wooden door lock is measured against.

I'm Ross, a Coventry locksmith, and I fit and service five-lever mortice deadlocks every week. This guide covers everything you need to know — how they work, how to identify what you have, and when it's time for an upgrade.

How a Five-Lever Mortice Deadlock Works

The name tells you most of what you need to know:

**Five-lever** — inside the lock body, there are five flat metal plates called levers. Each lever has a slot (called a gate) cut into it at a specific position.

**Mortice** — the lock body sits inside a rectangular pocket (mortice) cut into the edge of the door, so you can't see the lock from either side — only the keyhole and the bolt.

**Deadlock** — the bolt is solid and doesn't spring. It's moved only by turning the key, and it stays locked until you turn the key again.

When you insert the key and turn it, the key's bit (the flat patterned part) lifts each of the five levers to a different height. When all five gates align perfectly, the bolt stump can pass through them and the bolt is thrown (locked) or retracted (unlocked).

This is why mortice keys have that distinctive shape — the stepped pattern on the key bit corresponds to the heights needed for each lever. Get any one of the five heights wrong and the bolt won't move.

Why Five Levers?

Three-lever and two-lever mortice locks exist, but they offer far fewer key combinations and are significantly easier to pick. A five-lever lock provides:

Over 1,000 effective key differs (different key combinations)

Greater resistance to picking — the picker has to manipulate five levers simultaneously

Better resistance to skeleton keys — crude bypass tools are ineffective against quality five-lever locks

Compliance with BS3621 — the standard requires a minimum of five levers

If you have a 2 or 3-lever mortice lock on an external door, it needs upgrading. Not only is it less secure, but it won't meet your insurance requirements.

BS3621 Certification

BS3621 is the British Standard for thief-resistant locks. It's the magic number your insurance company is looking for. A lock certified to BS3621 has been independently tested by the BSI (British Standards Institution) for:

**Drill resistance** — hardened steel plates protect the lever pack and bolt

**Pick resistance** — the levers have false notches (anti-pick features) that make manipulation difficult

**Saw resistance** — the bolt contains hardened steel rollers or inserts that destroy saw blades

**Bolt strength** — the bolt must throw at least 20mm and resist sustained force

**Key security** — minimum number of key differs and controlled key blanks on some models

How to Identify a BS3621 Lock

Here's how to check whether your current lock meets the standard:

**Open your door** and look at the **faceplate** — the metal strip on the edge of the door where the bolt comes out

Look for **"BS3621"** stamped into the metal

Look for the **BSI Kitemark** — a heart-shaped symbol with the BSI letters

If you see both, your lock is certified

If you see a BS number but no Kitemark, the lock may claim compliance but hasn't been independently tested — this is not the same thing

If the faceplate is blank or only shows the manufacturer name, it's likely not BS3621

Important: Some very old locks predate the current BS3621:2007 standard and may carry the older BS3621:1998 marking. These are generally still accepted by insurers, but if you're getting a new lock, insist on the current standard.

The Fitting Process

Fitting a five-lever mortice deadlock involves cutting a rectangular pocket into the edge of the door. Here's what's involved:

If There's an Existing Mortice

If your door already has a mortice lock (even a 3-lever one), replacement is straightforward:

Remove the old lock by unscrewing the faceplate and sliding the lock body out

Check the new lock fits the existing mortice pocket — most standard locks are the same case size

Insert the new lock, secure the faceplate

Mark and drill the keyhole positions on both sides of the door

Fit the keyhole escutcheons (the decorative plates around the keyhole)

Test thoroughly with the door open, then closed

This typically takes me 30-45 minutes.

If Cutting a New Mortice

If the door doesn't have an existing mortice lock, the process is more involved:

Mark the position of the lock on the edge of the door (typically about a third of the way up from the bottom)

Drill a series of holes to the correct depth

Chisel out the mortice pocket to create a clean rectangular hole

Fit the lock body and faceplate

Drill keyholes on both sides

Cut a recess in the door frame for the bolt (the strike pocket)

Fit the strike plate with long screws into the frame timber

Fit escutcheons

Test extensively

This takes 60-90 minutes and is a skilled job. A badly fitted mortice lock compromises both the lock's security and the door's structural integrity. This is not a YouTube DIY project unless you're genuinely experienced with woodworking.

Recommended Brands

Union

Union has been making mortice locks for over 180 years. Their **StrongBOLT 2200** series is my most-fitted BS3621 deadlock — good quality, reliable, and keenly priced. The **2134E** (5-lever deadlock, BS3621) is a trade standard workhorse.

ERA

ERA's **Fortress** range of five-lever mortice deadlocks carries BS3621 certification and offers good value. The build quality is slightly behind Union in my experience but the locks perform well.

Yale

Yale's mortice deadlock range is solid. The **PM562** is BS3621 certified and widely available. Yale's brand recognition also means replacements and spare keys are easy to source.

British Standard Locks

There are several other manufacturers producing BS3621 locks — Legge, Securefast, and others. The key thing is the BS3621 certification and Kitemark, not the brand name.

Why Insurance Requires Them

Home insurance policies in the UK almost universally require BS3621 locks on external wooden doors. The reason is simple: the standard represents a tested, proven level of security that significantly reduces the risk of break-in through the door.

If you make a claim for theft and your wooden external doors don't have BS3621 locks, your insurer may:

**Refuse the claim entirely** — on the grounds that you didn't meet the security requirements

**Reduce the payout** — applying a deduction for non-compliance

**Increase your premium** — at renewal, if they flag the issue

I've seen all three happen to customers in [Coventry](/areas/coventry-city-centre). It's not worth the risk. A BS3621 lock fitted costs from £79 — vastly less than any insurance dispute.

Common Issues I See in Coventry

Working across areas like [Chapelfields](/areas/chapelfields), [Coundon](/areas/coundon), and [Binley](/areas/binley), the most common mortice lock problems I encounter are:

**3-lever locks on front doors** — these need upgrading to 5-lever BS3621

**Stiff or seized locks** — often caused by age, rust, or lack of lubrication. Sometimes a service and lubrication fixes this; sometimes the lock needs replacing

**Misaligned bolts** — the bolt doesn't line up with the strike plate, usually because the door has dropped or the frame has shifted. This needs adjusting before the lock can work properly

**Worn keys** — old keys wear down over time, and the cuts become less precise. If your key feels sloppy in the lock, get a new one cut from the original (not from the worn copy)

**Missing or broken escutcheons** — the keyhole is exposed, allowing dirt and moisture into the mechanism

Maintenance Tips

Five-lever mortice locks are low-maintenance but not zero-maintenance:

**Lubricate annually** — use a graphite-based lubricant or PTFE spray in the keyhole. Never use WD-40 — it attracts dust and gums up the mechanism

**Check the bolt alignment** — close the door and lock it, checking the bolt slides smoothly into the strike plate

**Replace worn keys** — before they snap in the lock

**Check the faceplate screws** — make sure they're tight

**Look for rust** — especially on doors exposed to weather

If your mortice lock needs attention, call me on 07735 336175. Whether it's a replacement, a new fitting, or just a service, I carry all the main brands and sizes in my van. Check the [prices page](/prices) for current rates, or see my [Yale vs mortice deadlock guide](/blog/yale-vs-mortice-deadlock) for advice on which locks your wooden door needs.