Are All Mortice Locks the Same Size?

When you’re looking to replace a door lock, you might hope for a quick, straightforward swap. It’s a common question: can you just buy any mortice lock and expect it to fit? The simple answer is no. Mortice locks are not all the same size, and understanding their different dimensions is crucial for ensuring your door remains secure and functional.

Choosing the right lock involves more than just picking one that looks good. It requires matching specific measurements to the existing pocket, or “mortice,” cut into your door. A lock that’s too big won’t fit, and one that’s too small will leave unsightly gaps and could compromise your security.

This guide will walk you through the essential measurements you need to know, explain the different types of mortice locks, and provide a step-by-step process for ensuring you choose the perfect replacement. By the end, you'll be able to confidently select a new mortice lock that fits your door correctly.

Understanding Mortice Lock Sizing

Mortice locks come in various shapes and sizes to fit different door types and security requirements. To find a replacement that fits, you need to be familiar with a few key measurements. Getting these right is the difference between a simple installation and a frustrating, time-consuming project.

Key Measurements You Need to Know

Grab a tape measure and let's get specific. There are two primary measurements you must get right when replacing a mortice lock: the backset and the case depth.

Backset Measurement
The backset is arguably the most critical dimension. It is the distance from the edge of the door to the center of the keyhole (or the center of the lever spindle, if you have a sashlock).

• How to measure: Measure from the front edge of the door (where the faceplate sits) horizontally to the center of the keyhole.

• Common sizes: In the US, common backset sizes are 2 ⅜ inches and 2 ¾ inches. In the UK and other regions using metric, you’ll often find 44mm (often referred to as 2.5 inches) and 57mm (often referred to as 3 inches).

• Why it matters: This measurement determines where your doorknob or handle will be positioned. If you get a lock with the wrong backset, your new handle won't align with the existing holes in your door, forcing you to drill new ones and patch the old.

Case Depth
The case depth is the total depth of the lock body that is inserted into the door.

• How to measure: This is the distance from the faceplate on the edge of the door to the back of the lock case.

• Common sizes: Typical case depths are around 64mm (for a 44mm backset) and 76mm (for a 57mm backset). The case is always deeper than the backset measurement.

• Why it matters: The lock case must fit entirely within the mortice pocket in your door. A case that is too deep will require you to chisel out more wood, while one that's too shallow might not be secure enough or may not fit properly.

Secondary Measurements to Consider

While backset and case depth are the most crucial, a few other measurements can impact the fit and finish of your installation.

• Case Height: This is the top-to-bottom measurement of the main lock body. It needs to fit within the height of the existing mortice pocket.

• Faceplate Dimensions: The faceplate is the metal plate you see on the edge of the door. You should measure its length and width. While you can sometimes chisel the opening to fit a slightly different size, matching it as closely as possible makes for a cleaner look. Faceplates can also have squared or rounded corners, so pay attention to that detail as well.

• Screw Centers: This is the distance between the centers of the screws that hold the faceplate to the door. Matching this will allow you to use the existing screw holes.

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Types of Mortice Locks and Their Sizes

The function of the lock also influences its size and shape. The two main types you'll encounter are mortice deadlocks and mortice sashlocks.

Mortice Deadlocks

A mortice deadlock offers a single locking point operated by a key. It does not have a latch or a handle. These are often used as a secondary security measure on front doors or on doors that don't need a latching mechanism.

• Sizing: Deadlocks are generally smaller and more compact than sashlocks because they only contain the bolt mechanism. However, they still come in various backset and case depth sizes, so measuring is just as important.

Mortice Sashlocks

A mortice sashlock combines a key-operated deadbolt with a latch mechanism that is operated by a door handle. This is the most common type of mortice lock found on residential interior and exterior doors.

• Sizing: Sashlocks are taller than deadlocks to accommodate both the bolt and the latch. In addition to the standard measurements, you'll also need to know the lock centers—the vertical distance between the center of the keyhole and the center of the handle spindle. This measurement is crucial for ensuring the handle and keyhole align with the holes already drilled in your door.

How to Choose the Right Replacement Mortice Lock

Now that you understand the different sizes and types, here is a simple, step-by-step process for finding the right replacement.

1. Remove the Existing Lock: To get accurate measurements, you'll need to take the old lock out of the door. Unscrew the faceplate, remove the handles and spindle if it's a sashlock, and then unscrew the lock body itself and slide it out of the mortice pocket.

2. Measure Everything: With the old lock in hand, use a tape measure to find the backset, case depth, case height, faceplate dimensions, and lock centers (for a sashlock). Write these numbers down. Taking a photo of the lock with the tape measure next to it can also be a helpful reference.

3. Take It Shopping: Bring the old lock and your measurements with you to the hardware store. This is the surest way to find a perfect match. If you're shopping online, use your precise measurements to filter the product options.

4. Check for Brand and Model: Look for any brand names or model numbers on the old lock. Sometimes, searching for the same brand can lead you to an identical replacement, which is the easiest solution.

5. Consider Security Standards: If you're replacing an exterior door lock, consider upgrading. Look for locks that meet industry security standards, such as those with a 5-lever mechanism (a common standard for insurance purposes in the UK) or those with ANSI/BHMA ratings in the US.

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What If I Can't Find an Exact Match?

Sometimes, especially with older doors, you may not find an exact replacement. In this situation, you have a few options:

• Choose a Larger Lock: It is almost always easier to modify the door to fit a slightly larger lock than to try and fill gaps left by a smaller one. A larger faceplate can cover old screw holes, and you can carefully chisel the mortice pocket to accommodate a deeper or taller case.

• Use a Repair Kit: If you must use a smaller lock, you can find mortice lock repair kits or plates that help cover old, larger holes to provide a clean finish.

• Consult a Locksmith: If you feel out of your depth, a professional locksmith can source the correct lock or expertly modify your door to fit a new one. This is often the best choice for antique or non-standard doors.

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Secure Your Home with the Right Fit

While not all mortice locks are the same size, finding the right replacement doesn't have to be a daunting task. By taking careful measurements of the backset, case depth, and other key dimensions, you can confidently select a new lock that will fit perfectly and function smoothly.

Paying close attention to these details ensures your door remains secure and maintains its clean, professional appearance. Whether you're undertaking a quick repair or a major security upgrade, the right fit is the first step toward peace of mind.

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