What Is the Case Size on a Mortice Lock?

If you're upgrading your home security or simply replacing an old door handle, you'll likely encounter the term "mortice lock." These locks are a popular choice for both internal and external doors, offering a reliable and secure way to keep your property safe. However, when it comes to choosing the right one, things can get a bit technical. One of the most important measurements you'll need to understand is the case size.

Getting this measurement right is crucial for a successful installation. A lock with the wrong case size might not fit your door correctly, could weaken the door's structure, or may not align with your existing door furniture. This guide will walk you through everything you need to know about the case size on a mortice lock, helping you choose the perfect lock for your needs with confidence.

What Exactly Is a Mortice Lock?

Before we get into the specifics of case size, let's quickly define what a mortice lock is. Unlike a rim lock that is mounted on the surface of a door, a mortice lock is fitted into a pocket—or mortice—cut into the edge of the door. This means the main body of the lock is hidden from view, leaving a clean and unobtrusive finish.

These locks are known for their strength and durability. They typically consist of a lock body (the case), a latch (the spring-loaded bolt that keeps the door closed), and a deadbolt (the solid bolt that locks the door securely). They are operated by a key and often paired with a set of door handles or knobs. Because they are embedded within the door's structure, they provide a higher level of security compared to surface-mounted locks.

Understanding Mortice Lock Measurements

When you buy a mortice lock, you'll see several measurements listed on the packaging or product description. Two of the most critical are the case size and the backset. While they are related, they are not the same, and it's important not to confuse them.

What Is the Case Size?

The case size refers to the overall depth of the lock body that fits inside the door. It's measured from the forend (the metal plate you see on the edge of the door) to the back of the lock case. This measurement determines how deep the mortice pocket needs to be cut into your door.

Standard case sizes for mortice locks are typically 64mm (approximately 2.5 inches) and 76mm (approximately 3 inches). You might also find other sizes for specialist applications, such as 102mm (4 inches) or 127mm (5 inches), but the 64mm and 76mm options are by far the most common for residential doors.

What Is the Backset?

The backset is the distance from the forend to the center of the keyhole or spindle hole (where the handle's spindle passes through). The backset determines the position of your door handle or knob on the face of the door.

• A 64mm case size typically has a backset of 44mm.

• A 76mm case size typically has a backset of 57mm.

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This relationship is crucial because the backset measurement ensures your hand won't get trapped between the door knob and the door frame when you open or close the door. A larger backset pushes the handle further away from the door's edge, giving you more room for your knuckles.

Here's a simple table to illustrate the relationship between common case sizes and their corresponding backsets:

How to Measure a Mortice Lock Case

Whether you're replacing an existing lock or fitting a new one, knowing how to measure it correctly is essential.

Measuring for a Replacement Lock

If you are replacing an old mortice lock, the easiest way is to measure the one you already have.

1. Remove the Lock: Unscrew the forend plate from the edge of the door and carefully slide the lock case out of the mortice pocket.

2. Measure the Case Depth: Place the lock on a flat surface. Use a tape measure to determine the distance from the front edge of the forend plate to the very back of the lock case. This measurement is your case size.

3. Measure the Backset: While you have the lock out, measure from the front of the forend plate to the center of the keyhole. This will give you the backset.

Matching these measurements will ensure your new lock fits perfectly into the existing pocket and aligns with your current door handles.

Measuring for a New Installation

If you're installing a mortice lock on a brand-new door, you have a bit more flexibility. Your choice will largely depend on the style of your door and the type of door furniture you plan to use.

1. Consider the Door Stile: The "stile" is the vertical rail of the door where the lock is fitted. A narrow stile will require a smaller case size, like 64mm, to avoid weakening the door's structure. For wider stiles, a 76mm case is often preferred, as it allows the handle to be positioned more centrally.

2. Choose Your Door Furniture: If you're using large, decorative door knobs, a larger backset (and therefore a larger case size like 76mm) is recommended. This will provide more space between the knob and the door frame, preventing scraped knuckles. For standard lever handles, a 64mm case size might be sufficient.

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Why Is Choosing the Right Case Size So Important?

Selecting the correct case size for your mortice lock is about more than just getting it to fit. It impacts the door's security, functionality, and even its longevity.

Structural Integrity of the Door

Cutting a mortice into a door inevitably removes some of the wood, which can weaken its structure. If you install a lock with a case size that is too large for the door's stile, you risk compromising the door's integrity. This is especially true for doors with narrow stiles or those made from less robust materials. Always ensure there is enough "meat" left on either side of the mortice pocket to maintain the door's strength.

Functionality and Aesthetics

The backset measurement, which is directly related to the case size, determines where your door handle sits. A well-positioned handle not only looks better but is also more comfortable to use. Centering the handle on the stile creates a balanced, professional look. As mentioned, it also provides the necessary clearance to operate the handle without obstruction.

Security

A correctly fitted mortice lock provides excellent security. If the lock is loose within the mortice, it can be more vulnerable to forced entry. A snug fit ensures the lock is stable and functions as intended, providing maximum protection. For external doors, it's also recommended to choose a mortice lock that meets industry standards, such as the British Standard BS3621, which indicates it has been tested against common burglary techniques.

Ready to Choose Your Lock?

Understanding the case size of a mortice lock is a key step in securing your home. By taking the time to measure correctly and consider the style of your door and handles, you can ensure a seamless installation that is both secure and aesthetically pleasing. Remember to distinguish between the case size (the full depth of the lock) and the backset (the distance to the keyhole), as both are vital for a perfect fit.

If you're ever in doubt, don't hesitate to consult a professional locksmith or a knowledgeable hardware supplier. They can provide expert advice to help you select the ideal mortice lock for your project, giving you peace of mind and a perfectly functioning door.

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