What is a Rim Cylinder?

A rim cylinder is a lock cylinder designed to mount on the surface of a door, most commonly used with rim locks and surface-mounted deadbolts such as vertical night latches. Unlike a mortise cylinder, which threads into a recessed pocket cut into the door, a rim cylinder attaches through a single round hole drilled through the door face, with the cylinder held in place by a mounting plate and connecting screws on the interior side.

Rim cylinders are widely used on residential and light commercial doors, particularly with auxiliary locks added for extra security alongside a primary lock, such as a rim-mounted night latch installed above a standard entry lockset.

How Does a Rim Cylinder Work?

A rim cylinder works the same way internally as a standard pin tumbler cylinder, using key pins, driver pins, and springs that align at a shear line when the correct key is inserted. What makes it distinct is its mounting method: the cylinder sits on the exterior face of the door, connected through the door to an interior mounting plate and tailpiece by long connecting screws.

When the correct key turns the plug, it rotates a tailpiece that extends through the door and engages the interior lock mechanism — typically a rim lock case or night latch — which throws or retracts the bolt. Because the cylinder and the lock body are separate, connected only by the tailpiece and screws, a locksmith can often replace or rekey the cylinder without removing the entire lock case.

Key Components of Rim Cylinders

A rim cylinder includes several parts specific to its surface-mounted design:

• Cylinder housing – the visible portion mounted on the exterior door face

• Plug – the rotating core containing the pin stack, same as in other cylinder types

• Tailpiece – extends through the door to connect the cylinder to the interior lock mechanism

• Mounting plate/back plate – secures the cylinder to the interior side of the door

• Connecting screws – long screws that hold the cylinder and back plate together through the door

• Key pins, driver pins, and springs – standard pin tumbler components determining which key operates the cylinder

The tailpiece length and mounting screw length must be matched to the door's thickness for a proper, secure installation.

What Are the Common Uses of Rim Cylinders?

Rim cylinders are used in a variety of surface-mounted lock applications, including:

• Vertical night latches and auxiliary deadbolts on entry doors

• Additional security locks installed alongside a primary lockset

• Apartment and residential doors using rim lock hardware

• Storefront and commercial doors with surface-mounted lock cases

• Gates and doors where a mortise pocket isn't practical to cut

Because rim cylinders don't require a recessed mortise pocket, they're often chosen for retrofit security upgrades on doors that weren't originally prepped for mortise hardware.

Main Characteristics of Rim Cylinders

Rim cylinders share several defining traits that distinguish them from mortise cylinders:

• Mount on the surface of the door rather than threading into a recessed pocket

• Connected to the interior lock mechanism via a tailpiece and mounting screws

• Installed through a single round bore hole rather than a rectangular mortise

• Available in various tailpiece lengths and cam styles to match different lock cases

• Compatible with standard pin tumbler rekeying using pinning kits

• Commonly used with auxiliary locks rather than as a building's primary lockset

These characteristics make rim cylinders a practical, easier-to-install option compared to mortise cylinders, particularly for retrofit and auxiliary lock applications.

Advantages of Rim Cylinders

Rim cylinders offer several practical benefits for both new installations and lock upgrades:

• Easier installation – requires only a bored hole rather than a mortised pocket

• Simple replacement – can be swapped without removing the full lock case

• Cost-effective retrofit option – well-suited for adding auxiliary security to existing doors

• Compatible with rekeying – can be rekeyed using standard pinning kits and techniques

• Widely available – common sizing and mounting standards make sourcing replacements straightforward

Because rim cylinders are simple to install and service while still offering solid pin tumbler security, they remain a popular choice for auxiliary locks and retrofit security upgrades.

How to Change a Rim Cylinder Lock

Changing a rim cylinder is a manageable job with basic locksmith tools:

1. Remove the interior mounting screws connecting the back plate to the cylinder through the door.

2. Detach the mounting plate and tailpiece from the interior side of the door.

3. Pull the old cylinder out from the exterior face of the door.

4. Note the tailpiece length and cam orientation before removing it, since the replacement must match the lock case it connects to.

5. Insert the new cylinder through the bore hole from the exterior side.

6. Reattach the mounting plate on the interior side, securing it with the connecting screws through the cylinder.

7. Trim or swap the tailpiece if needed so it properly engages the interior lock mechanism.

8. Test the key to confirm smooth operation before finishing the installation.

If the replacement cylinder needs to match an existing key, rekeying with a pinning kit should be done before or during installation.

Are All Rim Cylinder Locks the Same Size?

No — rim cylinders vary in several dimensions, and sizing must be matched to the specific lock case and door being serviced. Key variables include:

• Cylinder diameter – standard sizing exists, but housing and trim ring diameters can vary slightly by manufacturer

• Tailpiece length and shape – varies significantly depending on the lock case brand and door thickness

• Cam/tailpiece style – some lock cases require a specific tailpiece profile to engage correctly

• Door thickness range – connecting screw length must accommodate the specific door being installed on

Because of these variables, a rim cylinder isn't automatically interchangeable across all lock brands — the tailpiece and mounting hardware in particular need to be matched to the lock case for correct operation.

Can You Convert a Mortise Cylinder to a Rim Cylinder?

Converting from a mortise cylinder setup to a rim cylinder setup is possible, but it typically requires replacing more than just the cylinder itself, since the two use different door preparations and lock bodies. A mortise cylinder threads into a mortise lock body recessed into the door edge, while a rim cylinder mounts through a bored hole in the door face and connects to a separate surface-mounted lock case.

To convert between the two, the door generally needs:

• The existing mortise lock body removed and the mortise pocket filled or left unused

• A new bore hole drilled through the door face for the rim cylinder

• Installation of a compatible rim lock case or night latch on the interior side

• A properly matched strike plate for the new lock mechanism

Because this changes the door preparation rather than simply swapping hardware, this type of conversion is usually best handled by a professional locksmith, especially on commercial doors where fire rating or code compliance may apply.