SFIC vs LFIC: interchangeable cores and the locks that take them

This section and those after it were checked on 1 October 2026 against the sources listed on this page.

An interchangeable core carries all of a lock’s keying and comes out from the outside of the door with a control key, so the lock is rekeyed by swapping the core instead of taking the lock apart. BEST, which says it made the first small format interchangeable core in 1921, defines it as a figure-8 shaped device.

Why buildings use them, in the makers’ words:

• Rekeying without taking the lock apart. When keys are lost, BEST’s instruction is to remove the core with the control key, insert a new one, issue new keys and update the records. Sargent says its 7300B cores change the keying with no need to remove the cylinder.
• One key system across lock types. BEST’s padlock catalog says its core is interchangeable throughout most of its line, so locks of any type, size or style can be masterkeyed into one system.
• A separate key for changing cores. A BEST core has two shear lines, one for the operating keys and one for the control key, which is the key that releases the core.

BEST warns that the control key can remove cores and open doors, and asks for adequate security for it.

You tell an SFIC from an LFIC by the cylinder face and the maker’s order code. BEST’s 9K FAQ says an interchangeable core has a figure 8 design where a fixed cylinder has a single hole, and every core family BEST lists is SFIC; Sargent, Corbin Russwin and Accentra make both, so their code decides.

• BEST. Core code 1C7 is a 7-pin Standard core, and a 7 in the core housing position of a BEST lock or cylinder code is a 7-pin housing that accepts all BEST cores.
• Sargent. Options 60-, 63- and 64- are LFIC; 70-, 72-, 73- and 65-73- are SFIC; 7P marks 7 pins. Its cores are the 6300 (LFIC) and the 7300B and 7P-7300B (SFIC).
• Corbin Russwin. C6 and C7 are 6- and 7-pin LFIC, C6S and C7S are 6- and 7-pin SFIC, and CL6 or CLS6 is the same housing less core.
• Accentra. Its 2196 and 2197 mortise cylinders, grouped as LFIC in its collar tables, take the 1210 (6-pin) and 1220 (7-pin) cores; K620, K630, K660, K670, A620 and A630 are its KeyMark and Best keyway SFIC cylinders.
• The key. A BEST key carries a keyway designation near the blade and a series keymark code on the bow, and BEST needs the keymark to make combinated cores.

Look for the code on the building’s hardware schedule or the original order, or photograph the face with the key in it.

Moving a lock to interchangeable cores changes three things in the makers’ terms: the lock’s grade can follow the core’s, since Sargent says the cylinder’s ANSI/BHMA grade then determines the lock’s; the lock can ship less core or with a temporary core; and the permanent cores are keyed to the system, at the factory or by a locksmith.

The grade

Sargent’s mortise catalog says that with interchangeable core cylinders the ANSI/BHMA cylinder grade determines the lock’s grade, even if the lock is certified Grade 1 with a standard cylinder. Cylinders have their own standard, ANSI/BHMA A156.5, whose Grade 1 includes a 40,000 cycle test in BHMA’s summary, and BEST lists its Standard, Premium, CORMAX and Preferred cores as A156.5 Grade 1. Sargent’s and BEST’s Security Grade figures for mortise locks are in the cylinder section of Cylindrical vs mortise lock: how they differ and what has to match.

Converting a lock you already have

BEST’s 9K FAQ says a 9K cannot be switched between interchangeable and fixed core: the two are built differently and the lever will not stay on. Sargent says its 10X chassis supports every cylinder format except Keso. A mortise lock takes an interchangeable core mortise cylinder, a part with its own length and cam: Sargent’s 8200 SFIC cylinders come only in #43 and #46, and its LFIC cams are fixed at the factory and ordered as a suffix, such as 63-44-105. See Mortise cylinders.

Ordering

• Less core. BEST ships the 9K less core as standard and takes L/C instead of a keymark on a cylinder order.
• Temporary cores. Sargent’s 60- and 70- come with a disposable plastic core, turned with a coin or flat screwdriver, for doors that need no key locking during construction, and its 64- and 72- with a keyed construction core; Corbin Russwin offers both kinds in both formats, and BEST an SFIC construction core. BEST’s sample 9K specification keeps construction cores and keys off the permanent system’s keyway and has the cores returned.
• Permanent cores. BEST makes cores uncombinated, for a locksmith to pin, or combinated, for which it needs the keymark and number of keys. Sargent’s 63- and 73- ship masterkeyed, and its 65-73- uncombinated, packed loose for field keying.