​How to Measure a Mortise Lock Cylinder？

When it comes to door security, the mortise lock is a hallmark of strength and reliability. Found in both commercial and high-quality residential properties, its robust mechanism offers superior performance. However, the heart of this lock's operation and security is the mortise lock cylinder. Whether you're looking to replace a damaged one, rekey for security, or simply upgrade, knowing how to measure it correctly is crucial.

A wrongly sized cylinder will not function, potentially leaving your door insecure or inoperable. This comprehensive guide will walk you through the entire process of measuring your mortise lock cylinder accurately, ensuring you purchase the correct replacement every time.

Understanding the Mortise Lock Cylinder

Before we take out the tape measure, it's important to understand what we're dealing with. A mortise lock cylinder (often called the "key-in-knob" or "key-in-lever" mechanism in this context) is the part of the lock where you insert the key. It houses the pin tumblers that interact with the key's cuts to operate the lock.

In a mortise lock system, this cylinder is separate from the main lock body and slides into a pre-drilled hole in the face of the door, connecting directly to the lock mechanism inside. The two critical measurements are the Backset and the Overall Length (OAL).

Why Accurate Measurement is Non-Negotiable

Measuring a mortise lock cylinder isn't just about getting a part that fits; it's about security and functionality.

• Security: A cylinder that is too short will not engage properly with the lock mechanism, making it easy to force open.

• Functionality: A cylinder that is too long will protrude excessively from the door, looking unsightly and being vulnerable to wrenching attacks.

• Aesthetics: The correct size ensures that the trim plates (roses) sit flush against the door, providing a clean, professional finish.

Tools You Will Need

The process is simple and requires minimal tools:

• A tape measure or a ruler (a digital caliper is ideal for precision).

• A screwdriver (usually a Phillips or flat-head).

• A notepad to jot down your measurements.

Step-by-Step Guide to Measuring Your Mortise Lock Cylinder

Step 1: Remove the Cylinder from the Door

First, you need to access the cylinder. Locate the fixing screw. This is typically found on the edge of the door, in the faceplate of the mortise lock body. Loosen this screw completely. Once loose, you should be able to grip the knurled (ridged) end of the cylinder from the outside of the door and pull it straight out. If it's stubborn, inserting and gently turning a key can help.

Step 2: Identify the Key Measurements

Once the cylinder is in your hand, you will see it's a barrel-shaped device with a "collar" in the middle. There are three primary dimensions you need to identify:

1. The Backset: This is the distance from the center of the fixing screw hole to the outside of the collar.

2. The Forend Length: This is the distance from the center of the fixing screw hole to the opposite end of the cylinder (the part that goes into the lock body).

3. The Overall Length (OAL): This is the total length of the entire cylinder, from end to end.

Step 3: Take Your Measurements

Lay the cylinder on a flat surface. Using your ruler or caliper, carefully measure the following:

• Measure the Backset: Place the ruler at the center of the screw hole and measure to the outer edge of the collar. Common backsets are 1/2", 5/8", or 3/4".

• Measure the Forend Length: From the center of the screw hole, measure in the opposite direction to the end of the cylinder barrel.

• Calculate the Overall Length: Simply add the Backset and the Forend Length together. Alternatively, measure the entire cylinder from one end to the other to confirm.

Pro Tip: The most critical specification when ordering a replacement is the Backset and the Overall Length. Most suppliers will list cylinders by these two figures (e.g., "1-1/8" OAL, 1/2" Backset").

Mortise Lock Cylinder Measurement Table

This table summarizes the key dimensions and their importance for quick reference.

Special Considerations: Double Cylinder Mortise Locks

Some situations, like doors with glass panels, require a double cylinder mortise lock. This has a keyed cylinder on both the inside and outside. To measure this, you follow the same process but for both sides. You will have two backsets and a total OAL. It's vital to measure each side individually, as they can sometimes be different.

Troubleshooting Common Issues

• My new cylinder is slightly too long. Many cylinders come with interchangeable "thrust washers" or spacers that can be placed over the thread before installation to take up small amounts of excess length.

• The cam doesn't line up with the lock mechanism. This is almost always a backset issue. Double-check your backset measurement against the specifications of your mortise lock body.

• I can't find the fixing screw. On some older locks, the screw may be hidden behind the internal lever or rose plate. You may need to remove this interior trim first.

Conclusion: Measure Twice, Buy Once

Accurately measuring your mortise lock cylinder is a simple yet essential DIY task that can save you time, money, and frustration. By following this guide—removing the cylinder, identifying the backset and forend length, and calculating the overall length—you can confidently find a perfect replacement. Always double-check your figures and, when in doubt, take the old cylinder to a reputable locksmith to ensure you get the right part for your door's security needs. A correctly measured and installed cylinder is the key to maintaining the integrity and safety of your door for years to come.

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