Are Mortice Locks a Standard Size?

If you're looking to replace a door lock, you might assume it's a simple swap-and-go process. But when it comes to mortice locks, the reality is a bit more complex. These sturdy, reliable locks are recessed into the edge of a door, making them a popular choice for security. However, this design also means that finding an exact replacement isn't always straightforward.

So, are mortice locks a standard size? The short answer is no. While there are common sizes and industry norms, there is no single universal standard that all manufacturers follow. This can make replacing an old or broken mortice lock a challenge if you don't know what to look for.

This guide will walk you through the key measurements you need to understand, explain the common sizes available, and provide a step-by-step process for ensuring you choose the right mortice lock for your door.

Understanding Mortice Lock Measurements

To find a replacement lock, you first need to measure your existing one. Taking a few precise measurements will save you time and frustration, ensuring the new lock fits perfectly into the space cut out of your door. Grab a tape measure and let's get started.

Key Measurements to Take

There are four critical dimensions you need to know when shopping for a new mortice lock:

• Backset: This is the distance from the edge of the door to the center of the keyhole or door handle spindle. It's one of the most important measurements, as it determines where your door handle will be positioned. A common backset size is 2.5 inches (about 64mm), but this can vary.

• Case Depth: This is the total depth of the lock case that is morticed (recessed) into the door. You need to ensure the new lock case is not deeper than the hole in your door, or it simply won't fit.

• Case Height: This refers to the top-to-bottom height of the lock case itself. Like the depth, this needs to match the morticed-out section of your door.

• Centers: This measurement is the distance between the center of the keyhole and the center of the door handle's spindle hole. This is only relevant for sashlocks, which have both a key-operated bolt and a handle-operated latch. Deadlocks only have a keyhole, so they don't have a "centers" measurement. A common center measurement is 2.25 inches (about 57mm).

Taking these four measurements accurately is the first and most crucial step in finding a compatible replacement.

Common Sizes for Mortice Locks

While there's no official standard size, manufacturers have developed common sizes over the years that are widely used. Knowing these can help you narrow down your search.

British Standard (BS 3621)

In the UK, many insurance companies require that external doors be fitted with mortice locks that meet the British Standard BS 3621. This standard is not about a specific size, but rather a level of security. Locks that meet this standard have been tested against common burglary techniques like drilling and picking.

BS 3621-compliant mortice locks often come in a few common sizes, but the key is their security features, which typically include:

• A minimum of five levers (or a similar cylinder mechanism).

• A hardened steel plate to resist drilling.

• A bolt that extends at least 20mm into the door frame.

When replacing a lock on an external door, looking for one that is BS 3621 certified is highly recommended for both security and insurance purposes.

Common Industry Sizes

Across the industry, you'll frequently encounter mortice locks with the following dimensions:

• Backset: 44mm (1.75 inches) and 57mm (2.25 inches) are common for older doors, while 64mm (2.5 inches) and 76mm (3 inches) are more typical for modern locks.

• Case Depth: These usually correspond to the backset. For example, a lock with a 64mm backset will often have a case depth of around 76mm.

• Centers: The most common measurement for centers is 57mm (2.25 inches).

These are just guidelines. You'll find many variations, especially in older properties or with locks from less common manufacturers. Never assume your lock is a "standard" size without measuring it first.

How to Choose the Right Replacement Mortice Lock

With your measurements in hand, you're ready to find a new lock. Follow these steps to ensure a smooth replacement process.

Step 1: Measure Your Existing Lock

As detailed above, carefully measure the backset, case depth, case height, and centers of your current lock. Write these numbers down and take them with you when you go shopping, or have them ready if you're buying online.

Step 2: Identify the Type of Lock

Is it a deadlock or a sashlock? A deadlock only has a keyhole and a bolt, while a sashlock has a keyhole, bolt, and a latch that is operated by a handle. You'll need to replace it with the same type to match the existing cutouts in your door.

Step 3: Check the Number of Levers

Mortice locks are often described by the number of levers they contain. A 2-lever or 3-lever lock is generally used for internal doors where high security isn't necessary. For external doors, a 5-lever lock is the minimum recommended for security. If your lock needs to be insurance-approved, make sure it is a 5-lever BS 3621 certified lock.

Step 4: Compare Your Measurements with New Locks

When you're looking at potential replacements, compare their specifications to your measurements. The closer the match, the easier the installation will be. If you can't find an exact match, it's often possible to find a lock that is slightly larger and chisel the mortice in your door to accommodate it. However, it's much more difficult to fit a lock that is smaller, as it will leave unsightly gaps.

What to Do If You Can't Find a Match

Sometimes, especially with very old doors, you might find that your lock is an unusual size that is no longer manufactured. In this situation, you have a few options:

1. Consult a Specialist Locksmith: A professional locksmith will have access to a wider range of suppliers and may be able to source a vintage or specialist lock. They can also offer expert advice on the best course of action.

2. Modify the Door: A skilled carpenter or locksmith can modify your door to fit a modern, standard-sized mortice lock. This might involve filling the old mortice and cutting a new one.

3. Repair the Existing Lock: If the lock mechanism is simple, a locksmith might be able to repair it instead of replacing it.

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Your Next Steps

Replacing a mortice lock doesn't have to be a daunting task. While there isn't one universal "standard size," a few careful measurements can guide you to the right product. By understanding the key dimensions—backset, case depth, case height, and centers—you can confidently find a lock that fits your door and provides the security you need.

Always prioritize accuracy when measuring, and if you're securing an external door, opt for a BS 3621 certified 5-lever lock. If you run into trouble, don't hesitate to reach out to a professional locksmith. Their expertise can save you time, prevent damage to your door, and ensure your property remains secure.

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