Rim and Mortise Cylinders: How Commercial Key Cylinders Mount and Drive a Lock

Rim Cylinders

Schlage's key-systems catalog defines a rim cylinder as "a cylinder with a tailpiece to actuate the lock mechanism typically used with surface applied locks and attached to the lock or door with a mounting plate and machine screws."

The key parts:

• Tailpiece: a flat bar attached to the rear of the cylinder, parallel to the plug. When the key turns the plug, the tailpiece turns with it and operates the lock mounted on the inside face of the door.
• Mounting plate and screws: the cylinder passes through a hole in the door and is drawn tight from the inside with a back plate and machine screws.

Because the lock itself sits on the inside surface of the door, rim cylinders pair with surface-applied hardware. Medeco's catalog lists typical uses as surface-mounted deadbolts, night latches, panic devices, and "jimmy proof" (vertical dropbolt or interlocking deadbolt) locks. See rim locks for the historical surface-mounted lock family and panic hardware for exit devices, whose outside trim is often operated by a rim cylinder.

Mortise Cylinders

A mortise cylinder is a threaded cylinder that screws into the case of a mortise lock (or other hardware prepared for it). BEST's installation instructions for its mortise cylinders describe the arrangement: the cylinder is screwed into the mortise case, and a cylinder clamp screw in the front edge of the mortise lock holds it from turning. A set screw may also secure the cylinder or core.

The key parts:

• Threaded body: the cylinder's outside is threaded to match the lock case. Medeco says its standard mortise cylinders "retrofit into virtually any industry standard hardware." It also lists larger "jumbo/mogul" mortise cylinders, plus a threaded adapter ring that "converts conventional size Mortise Cylinder to Jumbo thread size" for one manufacturer's jumbo hardware. Thread size is therefore one of the things to match.
• Cam: Schlage defines a cam as "a lock or cylinder component that transfers the rotational motion of a key or cylinder plug to the bolt that works the lock." Medeco's catalog lists many cam and tailpiece configurations for different makers' hardware, so the cam must match the lock case.
• Length: mortise cylinders come in many lengths to suit door thickness and trim. Medeco's catalog lists standard mortise cylinders from 1 inch up to 5 inches, and Schlage lists rear mortise housing sizes from 1-1/8 to 1-3/4 inches. BEST's instructions call for noting the length of the old cylinder and using a cylinder ring where needed.

Rim vs. Mortise at a Glance

Manufacturers stress retrofit flexibility. Medeco describes "a large variety of cylinder lengths and cam/tailpiece configurations," and Schlage describes a modular conventional cylinder that can be configured for key-in-lever, rim, or mortise applications by changing housings, tailpieces, and cams.

Key-in-Knob and Key-in-Lever Cylinders

In a cylindrical (bored) lock, the cylinder lives inside the outside knob or lever rather than in the door itself. These key-in-knob or key-in-lever cylinders also use a tailpiece; Schlage's catalog lists tailpieces as "used on rim, key-in-knob or other special cylinders." Medeco sells key-in-knob cylinders as a separate line, "designed to retrofit knob type cylindrical locksets, lever type cylindrical locksets, padlocks and bored type auxiliary deadlatches and deadlocks."

Conventional vs. Interchangeable Cores

Each format above can be conventional or built around an interchangeable core (IC). Schlage's catalog explains the difference:

• Conventional cylinders usually require some disassembly of the lock to remove and replace.
• Small format interchangeable cores (SFIC) are based on an industry-standard plug diameter and fit most manufacturers' SFIC-compatible products.
• Full size interchangeable cores (FSIC) are specific to one manufacturer's design.

With an IC, the rim, mortise, or key-in-lever housing stays on the door and a control key removes the core. See interchangeable cores for how control keys work. Format can also affect how a whole lock is rated: Allegion lists one Grade 1 mortise lock series as Grade 2 security when fitted with FSIC cylinders and Grade 3 security with SFIC cylinders.

Euro Profile Cylinders

The euro cylinder is a separate format, covered on its own page. In Europe, cylinder performance is addressed by BS EN 1303, which BSI describes as "the European standard on cylinders for locks." Check BSI for the current edition.

Standards for Cylinders

• ANSI/BHMA A156.5 covers cylinders and input devices for locks. BHMA's summary says Grade 1 cylinders must pass 40,000 cycles and that inserting or removing the key must take no more than 3 pounds of force. It also gives an example designation for a Grade 1 bored-lock cylinder rated drill- and pick-resistant per UL 437.
• ANSI/BHMA A156.30 (current edition 2025) covers high-security cylinders. BHMA's summary describes three levels, A to C, with C the highest. Key-control attributes include key blank control and cylinder marking restrictions, alongside destructive and surreptitious attack tests.
• UL 437 is a separate listing that manufacturers cite for drill and pick resistance. Medeco, for example, lists its M4 rim and mortise cylinders as UL 437 listed.

See high-security locks and restricted keyways for how cylinder security and key control fit together.

Related Topics

• Mortise locks — the lock case a mortise cylinder threads into
• Rim locks — surface-mounted locks driven by rim cylinders
• Panic hardware — exit devices often keyed with rim cylinders
• Interchangeable cores — SFIC, FSIC, and control keys
• Euro cylinders — the European profile format
• Cylindrical locks — bored locksets with key-in-knob/lever cylinders
• Pin tumbler locks — the mechanism inside most cylinders

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