A Guide to Retrofitting New Mortise Locks to Your Existing Doors

You’ll know when it’s finally time to retire your trusty old mortise locks. Whether they’re becoming a security issue, keys are frequently lost or demagnetized, or you're looking to automate key management, the good news is that you can make the switch to a modern smart lock without replacing the entire door.

This guide walks you through the process of retrofitting a new mortise lock onto your existing door, including how to assess compatibility, the steps to take, and how to ensure a smooth installation.

Before You Begin: Assess Your Door

1. Assess the integrity of the doors carefully before committing to retrofitting. In some cases, replacing the door may be the better choice, especially if it’s heavily damaged or has become a “Swiss cheese” door full of patched holes. Replacement may also be required if the door no longer meets fire and safety standards. 

2. Choose a mortise lock if the doors are already equipped with a mortise lock, for the most straightforward replacement. While it doesn’t need to be an exact match, selecting a model with similar characteristics will simplify the installation and minimize adjustments.

3. Compare door preparation specifications carefully. All mortise locks have slightly different door prep requirements, so it’s important to compare the new prep against the old. Minimizing door modifications will help conserve the integrity of your door, continue to conform to fire ratings, and ultimately allow you to get more life out of your door.

4. Choose escutcheon locks for better coverage. Escutcheon locks are better than sectional locks for retrofitting, and have a continuous body that will cover existing holes much better. Sectional locks are made of different components and may not hide some of the old holes.

5. Use cover plates when needed. In the end, sometimes things don’t line up perfectly, and that’s okay. Cover plates can help create a clean, finished appearance while adding extra reinforcement to the door where needed. 

Steps to Retrofitting a Mortise Lock

Step 1: Locate the door templates

To determine how the new lock will fit against the existing door preparation, start by obtaining the door templates for both the new and existing locks.

1. Identify the make and model of your current lock. If you’re unsure of the make and model, send us a picture or ask a local locksmith to help confirm.

2. Once the lock model is identified, you can usually find the door template on the manufacturer's website. If they do not have this, you can contact them directly to request one.

3. Print off (or make digital copies) of both the new and old templates. Make note of the front and back templates, as they are often different. When printing, confirm the scale is accurate before using the templates for comparison.

Step 2: Compare characteristics

Now that you have both door templates, it’s time to compare them. You can do this off-site by comparing the templates side by side, or on-site by removing a lock and overlaying the new template onto the existing door. The on-site method is always preferable, but if you’re looking for a quick assessment, the off-site method can work as well. 

Here’s what to check:

1. General alignment

   Ensure the templates are oriented and scaled correctly. Confirm which template is for the front and which is for the back of the door. 

2. Edge hole alignment

   Also known as the “mortise pocket”, this is where the lock case slides into the edge of the door. The preparation is relatively standard in North America. The dimensions should be somewhere around 8” tall x 1.25” wide x 4.5” deep. If they are slightly off, that’s okay, this can usually be fine-tuned before installing the new locks.

3. Vertical hole alignment

   Commonly known as the “back set”, this is the distance from the center of the handle to the edge of the door. This is typically between 2.5” and 2.75”. If back sets are the same, the holes for the handle, key cylinder and thumb turn will typically line up vertically. Sometimes the mounting holes will be offset, so take note of these.

4. Horizontal hole alignment

   This is where variations between locks are most common. Find the centerline of the lock case (CL Lock Case) on the templates and use that as a reference point to see how the holes line up. 

5. Body alignment

   Now that you’ve compared how all the holes line up, you can determine how well the new lock will cover the existing holes that are not being used. If the footprint of the new lock does not cover some of the holes, a cover plate may be required.

6. Strike alignment

   In many cases, the door strike is offset from the center of the lock case, typically by 0.375” or 0.5”. This can usually be found on the door template, marked as “CL Strike”. Different offsets mean that you may have to adjust the position of the door strike slightly.

Make sure to compare both sides of the door, front and back.

Final thoughts

Retrofitting a new mortise lock onto an existing door may seem daunting, but it’s a common and effective way to save time, money, and the environment. The key is to carefully assess the condition of each door before deciding to reuse it. Taking the time to compare door templates before retrofitting helps ensure proper alignment while minimizing modifications and additional holes, preserving both the integrity and appearance of the door. 

If you require further advice on retrofitting or would like to be put in touch with an expert to help with the comparison process, please reach out to us and we'll be happy to help!

Frequently Asked Questions

• 1. Do I need to replace the door to install a smart lock?

  Not necessarily. If your door is in good condition and already has a mortise lock, you can often retrofit a new smart lock without replacing the door. Careful alignment and template comparison are key.

• 2. Do I need to modify my doors to install 33 Locks?

  In most cases, major modifications are not required. 33 Locks are designed to fit standard commercial and residential door preps (e.g., mortise or cylindrical). Minor adjustments — like drilling new holes for mounting screws or trimming a strike plate — may occasionally be needed, depending on your existing door hardware and frame. 

  We offer decorative plates to help conceal any marks or holes left by previous hardware, ensuring a clean and polished finish after installation. 

  If you’re unsure, we can review your door specs or photos to confirm compatibility. Just reach out to sales@33lock.com for assistance.

• 3. Can my team install 33 Locks themselves?

  Yes. Your in-house team or a general contractor can typically install 33 Locks without the need for a certified installer. Our locks are designed for straightforward installation with standard tools, and we provide clear step-by-step guides to make the process easy. 

  For larger projects or properties with more complex needs, such as advanced access automation or gateway placement, we recommend working with an expert to ensure optimal setup.

• 4. How long does it take to retrofit a smart mortise lock on an existing door?

  Most standard retrofits can be completed in 30 minutes or less per door, assuming the existing door is in good condition and already configured for a mortise lock. 

  Installation time may increase if the door requires repairs, significant alignment adjustments, or modifications to accommodate hardware. Preparing templates in advance and having the proper tools on hand helps streamline the process.

• 5. What if the new lock doesn’t align perfectly?

  Slight differences are common and can usually be managed with minor adjustments. If the new lock doesn’t fully cover existing holes, a cover plate can provide a clean, reinforced finish.