Interchangeable Cores: Removable Cylinders and the Control Key

Interchangeable vs. Removable Cores

The trade vocabulary draws a careful line. The Professional Locksmith Dictionary from the Lock Industry Standards and Training (LIST) Council defines a core as "a complete unit, often with a 'figure eight' shape," that usually holds the plug, shell, tumblers, springs, and retainer. It then separates two kinds:

• An interchangeable core is "a key removable core which can be used in all or most of the core manufacturer's product line. No tools (other than the control key) are required for removal of the core."
• A removable core is "a key removable core which can only be installed in one type of cylinder housing," such as a rim cylinder, a mortise cylinder, or a key-in-knob lock.

The part that holds the core is the housing. A156.5 recognizes IC mortise cylinders and IC rim cylinders, each a housing combined with a matching core. ANSI/BHMA A156.2, the bored lock standard, also defines an interchangeable core cylinder: one "removable from the lock with a designated key without disassembly of the lock." See rim and mortise cylinders and cylindrical locks.

How the Control Key Works

An ordinary pin tumbler cylinder has one shear line, where the plug meets the shell. An interchangeable core adds a second one. The LIST dictionary names the parts:

• the control sleeve, which surrounds the plug
• the control lug, the part of the retaining device "which locks the core into its housing"
• the control shear line, which "allows operation of the control lug"

As Locksmith Ledger explains in its overview of small format cores, the operating keys work at the plug shear line, while the control sleeve forms a second shear line used only by the control key. A user key turns the plug and works the lock. The control key turns the sleeve instead, which moves the lug and lets the whole core slide out of its housing.

Frank E. Best's 1921 patent described the same division of labor in its own words: the core contains "a primary and a secondary lock. The primary lock performs the usual functions of a lock while the secondary lock controls the insertion and removal of the core from the case."

The LIST dictionary is explicit that a control key in this sense is not a door key: "a key whose only purpose is to remove and/or install an interchangeable or removable core."

Origins: Frank E. Best

U.S. Patent 1,384,022, "Lock," was filed by Frank Ellison Best on August 28, 1919 and granted July 5, 1921. The patent text says it improves on an earlier application Best filed in April 1919.

BEST, the company that carries his name, gives this timeline on its own history page:

• 1919: a patent application is filed for the first interchangeable core lock
• 1920: BEST Lock Company is founded in Seattle, Washington
• 1921: the first patent is issued
• 1923: BEST Universal Lock Co. is formed as a separate manufacturing company
• 1938: the company relocates to Indianapolis

A 1957 U.S. Tax Court decision involving Best's companies independently describes the July 5, 1921 patent as "a basic patent upon a lock core mechanism which can be removed from the lock and interchanged."

Formats: SFIC, LFIC, and FSIC

Small Format Interchangeable Core (SFIC). The LIST dictionary defines SFIC as "an IC that replicates the functionality and design popularized by Best." Locksmith Ledger notes that once other companies introduced their own versions, SFIC became the generic term, and that many makers now offer SFIC cylinders with keyways to fit existing Best key systems. Schlage's L Series catalog, for example, offers mortise housings that accept SFIC cores "compatible with manufacturers such as Falcon, Arrow and Best."

Fitting a core, a control sleeve, and a lug into an ordinary cylinder diameter has consequences. Ledger explains that the plug had to be made smaller than a standard plug, which also limits key height. As a result, standard blanks such as Y1, KW1, or SC1 cannot be used, and Best keyways are usually identified by one or two letters.

Large Format Interchangeable Core (LFIC). The LIST dictionary defines it simply: "an interchangeable core which is too large to fit into a small format interchangeable core housing." LFIC designs vary by manufacturer.

Full Size Interchangeable Core (FSIC). Schlage describes its full size IC cylinders as allowing "immediate rekeying at the door" with a special control key, and says they can be integrated into existing 5- or 6-pin Schlage key systems.

Grades and Security

A core is part of the lock's security rating, not separate from it. Allegion's support article on mortise locks lists Schlage's L9000 mortise lock as certified to ANSI/BHMA A156.13 (2005 edition) at Grade 1 Security, then notes that the security grade becomes Grade 2 with FSIC cylinders and Grade 3 with SFIC cylinders. That is one manufacturer's listing for one product line, not a rule for all IC hardware, but it shows that the core format can change the certified result.

Locksmith Ledger also reports that some companies have added secondary pins, sliding bars, or sidebars to SFIC cylinders, and that others offer restricted SFIC keyways for tighter key control. Where pick and drill resistance is the goal, A156.5 points readers to UL 437 and ANSI/BHMA A156.30 for high security cylinders. See high-security locks and ANSI/BHMA lock grades.

Interchangeable Cores and Master Keying

Interchangeable cores are closely tied to master keying. Ledger notes that SFIC locks are built to tight tolerances, which allows master keying without unintended key interchange. Best also introduced keying systems known as A2, A3, and A4, and Ledger reports that A2 is the one generally used today.

The practical point is about change. Ledger describes the core as solving "the need for instant rekeying without lock cylinder disassembly at the door." Ledger's example is a business with frequent staff changes, such as a fast food chain, that can change a core "at any hour as needed." The pinning still has to fit the master key system. Swapping a core changes that one opening; it does not change the rest of the system. See rekeying locks.

Two related ideas build on the same hardware:

• Construction cores. The LIST dictionary defines a construction core as an interchangeable or removable core "designed for use during the construction phase of a building." Construction cores are normally keyed alike and are replaced by the permanent system's cores when construction is finished. See construction keying.
• Key control. Who can get a control key, and who can get blanks for the keyway, are management questions. Interchangeable cores make rekeying fast, but they do not control key distribution on their own. See key control and restricted keyways.

Related Topics

• Rim and mortise cylinders: the housings cores fit into
• Mortise locks and cylindrical locks
• Master keying and construction keying
• Rekeying locks
• Key control and restricted keyways
• Key blanks and keyways
• High-security locks and ANSI/BHMA lock grades
• Pin tumbler locks
• Cam locks

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