Quick Summary

The word rim causes more confusion in commercial door hardware than almost any other term, because it describes a mounting method rather than a product. Ask ten people what a rim lock is and you will get answers ranging from a Victorian box lock to a modern exit device.

Both are correct, and that is the problem. This guide sets out what rim actually means, where it turns up on commercial doors, and what you need to know when ordering parts for it.

What Is a Rim Lock?

A rim lock is a lock whose body is mounted on the surface of the door rather than buried inside it.

Everything else follows from that. The door needs through holes rather than a routed pocket. The lock can be seen and reached without taking anything apart. Servicing happens with the door hanging, not with the lock case pulled out of the edge. And the lock sits proud of the face, which is why rim hardware is specified where appearance matters less than access.

Is a rim lock an old fashioned thing?

The historic surface mounted box lock is where the name comes from, and it still appears on period buildings. In current commercial specification, though, rim describes a mounting method used on thoroughly modern hardware, most visibly on exit devices.

So a rim device on a fire exit in a new building and a rim lock on a church door from 1880 share a principle and nothing else.

How Is a Rim Lock Different From a Mortise Lock?

A mortise lock lives inside a rectangular pocket cut into the edge of the door. A rim lock lives on the surface. That changes the preparation, the servicing, the security profile and the parts list.

Neither format is better in the abstract. A mortise lock buys concealment and a heavier mechanism inside the door. A rim device buys access, simpler preparation and faster servicing. Our comparison of mortise and cylindrical locks covers the third format in the same terms.

Can a rim device replace a mortise lock on the same door?

Sometimes, but it is a door preparation question rather than a hardware question. The mortise pocket does not disappear when the mortise lock comes out, and a rim device fixes through the face in a completely different pattern. Expect a filler plate and new fixing positions rather than a straight swap.

Where Do You Find Rim Hardware on a Commercial Door?

Overwhelmingly on exit doors, as rim exit devices.

A rim exit device latches into a strike mounted on the frame, which makes it the simplest of the four exit device configurations. There are no rods running to the head and sill, no lock case in the door edge, and nothing concealed inside the door. For a single door with a frame to latch into, it is the default choice.

The second place the word turns up is on the cylinder. A rim cylinder is the format used to key a rim device from the secure side. Our rim exit device guide covers the devices themselves in detail.

When is a rim device not the right choice?

On a pair of doors with no mullion, where there is nothing in the middle for a rim latch to engage. On a door that has to latch top and bottom for fire performance. And where the aesthetic specification rules out surface mounted hardware. In each of those, a vertical rod or mortise configuration takes over, as our concealed vertical rod guide covers.

What Is a Rim Cylinder and How Does It Attach?

A rim cylinder is a cylinder designed to be held against the outside face of the door by a backplate fitted on the inside, rather than threaded into a lock body.

The sequence is the clue to how it works. The cylinder goes through the door from outside. The backplate goes on behind it. Screws through the backplate pull the cylinder tight against the face. A tailpiece or cam on the back of the cylinder then reaches into the device and operates it.

Why does the backplate matter so much?

Because it is the only thing holding the cylinder in the door. A mortise cylinder is held by its own thread in the lock. A rim cylinder is held by a plate and two screws, and if those loosen the cylinder can turn or move in the hole.

A cylinder that spins when a key is turned is a backplate problem far more often than a cylinder problem, and the backplate is a stocked item in its own right.

What Does the Cam or Tailpiece Do?

It is the link between the cylinder and the mechanism. Turning the key turns the plug, the plug turns the cam, and the cam moves whatever the device needs moved.

Cams are not universal. The shape, length and orientation have to suit the device the cylinder is driving, which is why cams are listed as separate parts rather than assumed. Fitting a cylinder with the wrong cam gives you a key that turns smoothly and achieves nothing.

Does the keyway have to match the rest of the site?

If the building runs a key system, yes, and that is a decision made at order time rather than afterwards. A rim cylinder can be supplied to the site keyway so that existing keys work, which is usually preferable to adding another key to a bunch. Our key systems guide covers how that is organised.

What Strike Does a Rim Lock Need?

One positioned for a surface mounted latch, which is not the same as a strike for a mortise or cylindrical lock.

A rim latch approaches the frame from the face of the door rather than from inside its edge, so the strike has to present its opening in a different plane and at a different depth. Rim strikes are listed as their own family for exactly that reason.

On a pair of doors the picture changes again. The inactive leaf has to present something for the active leaf to latch into, which is where double door strikes and mullions come in.

Can the strike be electrified?

Yes, and it is one of the most common ways to add access control to a rim device. The electric strike replaces the fixed strike in the frame, the device stays as it is, and the opening gains remote release without touching the door. Our electric strikes guide covers the selection, and our strike plate guide covers the mechanical version.

Is a Rim Lock Secure Enough for a Commercial Door?

On the right door, yes, and it is used on millions of them.

The honest answer has conditions attached. Surface mounted hardware is visible and reachable, so the fixings matter. Through bolting rather than wood screws, a sound backplate behind the cylinder, and a strike that is properly anchored in the frame are what make the difference between a rim device that resists attack and one that does not.

The other factor is that a rim exit device on an exit door is usually on the secure side of the opening with no trim outside at all. There is very little to attack, because there is nothing out there to grip.

What about the cylinder as a weak point?

It is the component most exposed on the outside face, and that is worth designing around. Cylinder guards, recessed trim and the backplate arrangement behind the door all exist to make the cylinder harder to attack. On an opening with a history of attempts, those are the parts to look at rather than the device.

What Fails on Rim Hardware?

None of those is a device replacement. The point of rim hardware is that the parts are reachable, so the repair is usually a component swap with the door still hanging. Our panic bar parts guide lists the components for the exit devices most of this applies to.

What should you confirm before ordering rim parts?

Five things. The device series, from the stamping. Whether the part is for the device or the cylinder, because they are separate orders. The cam type where a cylinder is involved. The keyway, if the site runs a key system. And the finish, because rim hardware is visible.

If the cylinder format itself is the open question, our mortise cylinder and rim cylinder comparison is the quickest way to settle it, and the exit device terminology guide covers the naming.

About the Author

Security Parts has supplied repair parts for commercial door hardware since 2001, covering Von Duprin, LCN, Schlage, Falcon, Detex and Adams Rite at the component level. Our diagram based lookup exists so a technician can confirm a part before ordering it. This guide was written by the Security Parts team from supplier documentation and the questions our customers bring us.