How to Replace a Mortise Lock Key Cylinder with a Thumb Turn

Hey guys, this is Gary with Door Closers USA. In today’s video, I’m going to show you how toreplace a key cylinder mortise lock with a thumb turn on a commercial storefront door.

In this example, the door has a standard key cylinder on the exterior and a key cylinder on theinterior. We’re going to remove the interior key cylinder and replace it with athumb turn.

All commercial door lock parts shown in this video are available at DoorClosersUSA.com,including cylinders, thumb turns, trim rings, and mortise lock components.

What Is a Thumb Turn?

A thumb turn replaces the interior key cylinder and allows the lock to be operated by handinstead of a key. This is commonly used on:

• Commercial storefront doors
• Doors requiring interior free operation
• Doors where keyed access is only needed from the exterior

The thumb turn uses the same cam mechanism as a keyed cylinder—it simply removes the key function.

Removing the Mortise Lock Faceplate

The first step is to remove the mortise lock faceplate.

• Remove the three screws securing the faceplate
• Carefully pull the faceplate off and set it aside

This exposes the mortise lock body and the interior lock cylinder.

Understanding the Set Screw and Cylinder Grooves

The interior key cylinder is threaded into the mortise lock body.A set screw prevents it from backing out.

If you look closely at the replacement thumb turn cylinder, you’ll notice small grooves.The set screw sits inside one of these grooves to lock the cylinder in place.

Removing the Interior Key Cylinder

To remove the existing key cylinder:

1. Loosen the set screw (do not fully remove it)
2. Once the set screw clears the groove, unthread the key cylinder

If the cylinder feels tight, you may need to slightly loosen the mortise lock body screwsto relieve pressure from the door prep.

Once removed, you’ll see the cam attached to the back of the cylinder.This cam rotates and engages the hook bolt when the key is turned.

Installing the Thumb Turn Cylinder

The thumb turn installs the same way as the keyed cylinder.

• Thread the thumb turn cylinder into the mortise lock body
• Turn slowly to avoid cross-threading
• Align the cylinder so one of the grooves lines up with the set screw

Once aligned, tighten the set screw into the groove.This locks the thumb turn securely in place and prevents rotation.

Reinstalling the Faceplate

After the thumb turn is secured:

• Reinstall the mortise lock faceplate
• Tighten all three screws

At this point, the thumb turn should be firmly installed and aligned.

Testing the Lock Operation

Always test the lock before finishing the job.

• Rotate the thumb turn to confirm smooth operation
• Verify the hook bolt fully locks and unlocks
• Test exterior operation with the key

Both sides should operate cleanly with no binding.

Final Notes

Replacing a key cylinder with a thumb turn is a simple upgrade that can improve convenienceand functionality on commercial doors.

We carry thumb turns, mortise lock cylinders, trim rings, and replacement lock parts forall commercial door makes and models.

If you’re unsure which part you need, send us photos of your lock and door,and we’ll help you identify the correct replacement.

Thanks for watching, and please like and subscribe to the Door Closers USA YouTube channelfor more commercial door repair guides.

Watch the Video

Prefer to follow along visually? Watch the full video below to see how to replace a mortiselock key cylinder with a thumb turn step-by-step.