What This Guide Covers and Who It Helps
If you are building a hardware schedule, rekeying an existing facility, or troubleshooting why a cylinder will not operate a lock correctly, the first question is almost always the same: is this a mortise cylinder or a rim cylinder? The two look similar at a glance, but they mount differently, drive the lock mechanism differently, and are not interchangeable. This guide explains what each type is, how to tell them apart on a schedule or in the field, and which applications demand one over the other.
What Is a Mortise Cylinder?
A mortise cylinder is a threaded brass cylinder body that screws directly into a mortise lock case, exit device, auxiliary deadlock, or key switch. A small setscrew in the face of the lock or trim holds it in position once seated. The back end of the cylinder carries a cam -- a rotating tail piece that actuates the lock mechanism when the plug turns.
The critical detail: cams are not universal. A mortise cylinder installed in a lock from a different manufacturer requires the correct cam profile for that lock case. Specifying or ordering the wrong cam is one of the most common field errors on a mortise cylinder job. Most cylinder suppliers publish a cam compatibility chart; always cross-reference before ordering.
- Body: Brass, typically 1-5/32" (roughly 29 mm) diameter
- Mounting: Threaded into lock body; retained by setscrew
- Drive mechanism: Cam (several profiles -- must match the lock)
- Pin count: Six-pin standard; seven-pin available for expanded master keying
- Common applications: Mortise locks, mortise-body exit devices, classroom corridor locks, auxiliary deadlocks, key switches
What Is a Rim Cylinder?
A rim cylinder shares the same body diameter as a mortise cylinder but is not threaded and does not use a cam. Instead, it has a long tailpiece that extends through the door thickness and engages the mechanism of a rim exit device or auxiliary rim lock from the back side of the door. The cylinder is secured by two machine screws that pass through the door from the inside, clamping the backplate against the secure face of the door.
- Body: Brass, same diameter as mortise cylinder but smooth (no threads)
- Mounting: Through-bolted with break-off screws; back plate on secure side
- Drive mechanism: Horizontal tailpiece (not a cam)
- Door thickness range: Varies by pin count; verify min. and max. door thickness with manufacturer data
- Common applications: Rim exit devices, auxiliary rim deadlocks, surface-mounted rim night latches
Side-by-Side: The Differences That Matter on the Job
- Threading: Mortise cylinder is threaded; rim cylinder is smooth.
- Back end: Mortise uses a cam; rim uses a tailpiece.
- Retention: Mortise is retained by setscrew in lock face; rim is through-bolted with machine screws and a back plate.
- Interchangeability: Not interchangeable -- the lock body and the cylinder type must match.
- Cam selection: Required only for mortise; rim cylinders do not use cams.
Which Applications Call for Each Type?
Mortise Cylinders: The Commercial and Institutional Standard
Mortise cylinders are the dominant choice across commercial construction. Anywhere you have a mortise lock case -- a school classroom, a healthcare corridor door, an office tenant entry, or an industrial storeroom -- the cylinder is a mortise type. Exit devices with a mortise lock body (rather than a rim latch) also take mortise cylinders.
High-security platforms from preferred lines like Sargent, Corbin Russwin, and Accentra (formerly Yale) are widely available in mortise format and support expanded master keying, interchangeable core (IC) options, and patented key control. Sargent's 41-series mortise cylinders, for example, support both conventional and IC configurations and carry a range of cam options for compatibility across lock families. Corbin Russwin and Accentra offer comparable depth in mortise cylinder formats for institutional and healthcare projects.
Rim Cylinders: Exit Device and Auxiliary Lock Applications
Rim cylinders belong on rim exit devices -- the most common panic hardware configuration in retail, schools, and light commercial occupancies. When a contractor is keying outside trim on a rim-body exit device, the cylinder specified on the hardware schedule will be a rim type. Sargent's 34-series rim cylinder, for example, is a six-pin unit with a horizontal tailpiece, furnished with break-off screws and a back plate, and available in a range of finishes.
Auxiliary rim night latches and some older surface-applied deadlocks also take rim cylinders. On fire-rated openings, verify that the exit device and its outside trim carry the correct UL listing before finalizing the cylinder choice.
Key Scheduling and Keying Compatibility
Both mortise and rim cylinders can be keyed to the same master key system as long as they share the same keyway. That means a building using a grand master system across dozens of doors can include rim-device cylinders on exit doors and mortise cylinders on corridor locks -- all under one key hierarchy -- provided the keyway is consistent throughout.
Interchangeable core (IC) versions of both types allow facilities like schools and hospitals to rekey openings without tools by swapping core assemblies with a control key. IC mortise and IC rim cylinders both follow the same core-removal procedure; the shell design differs, but the rekeying workflow is identical.
Common Specification and Field Errors
- Wrong cam on a mortise cylinder: The cylinder turns but nothing happens. Always match the cam to the lock manufacturer and function.
- Ordering a mortise cylinder for a rim exit device: It will not engage the tailpiece mechanism. Confirm device type before ordering.
- Mismatched keyway on one opening: A mortise cylinder and a rim cylinder on the same door must share a keyway if they are to be operated by the same key.
- Door thickness out of range: Rim cylinder tailpiece length must accommodate the actual door thickness. Verify before ordering, especially on oversized commercial doors.
- Mixing IC and conventional shells: An IC core will not seat in a conventional cylinder shell. The entire hardware schedule must commit to one format or the other per opening.
Making the Right Call for Your Opening
The lock body determines the cylinder type -- full stop. Read the hardware schedule, confirm whether the exit device or lock case is a rim-latch or mortise-body unit, and order accordingly. If you are retrofitting cylinders into an existing building, pull the face plate and look: threads mean mortise, smooth barrel with through-bolt holes means rim.
DoorwaysPlus.com carries mortise and rim cylinders from Sargent, Corbin Russwin, Accentra, Hager, and PDQ -- lines chosen for stable product platforms and broad cam and keyway availability. If you need help matching a cylinder to a specific lock case or building a keyway-consistent schedule across multiple door types, our team can help you get it right before anything ships.