Modern pin tumbler construction was developed in the mid 1800s and the bottom pin, driver and spring design has changed little in the last 150 years. One of the weaknesses of the pin tumbler lock is that it has to be fully disassembled in order to change the combination. Some companies have developed cylinder versions which can be changed without disassembly, but these designs do have limiting factors such as either not allowing masterkeying or having a relatively small amount of possible key changes before disassembly is required.
Frank Best received a patent for an Interchangeable core (IC) lock cylinder in 1921. IC Core locks solved the need for instant rekeying without lock cylinder disassembly at the door. The Best Lock invention included a control key which could quickly remove or install the lock cylinder portion containing the pin tumblers. The shape of the IC core insert has changed over the years, but the general design of using a control key for cylinder removal remains.
By the time that Frank Best invented IC core cylinders, the lock industry had generally settled on approximately 1 1/8” as the diameter for mortise lock cylinders. The eventual Best IC core design contained an outer shell, an inner hollow control lug and the lock plug. The diameter of the lock plug had to be reduced in comparison to a standard lock plug in order to allow additional room for the outer shell and control lug within the 1 1/8” diameter dimensions.
A smaller plug size also restricts the height of the key blanks. Best Lock devised a series of different keyways which are usually identified by one or two letters such as B or TD. The smaller plug size precludes the use of standard keys blanks such as Y1, KW1 or SC1 keys.
Some IC core keyways are known for their thin cross-sections. Normal key machine vises press the sides of the blank together to hold the key in place. Blanks for many IC core keyways have a tendency to be deformed when being tightened into the key vise. The key blank will return to its original shape when removed from the vise, but the resulting key cuts will be out of tolerance and will not properly operate the lock. Manual stamping machines with dedicated vises for holding Best key blanks can be an option if your business does a large amount of IC core lock work.
IC core lock cylinders have helped extend the life cycle of the pin tumbler lock design. Today most new construction sites of even moderate size will include IC core lock cylinders. Any business which has frequent changes in employees such as fast food chains often use IC core locks so they can immediately change the IC core cylinder at any hour as needed. Rekeying old IC core cylinders can form a lucrative part of your locksmith business.
The popularity of IC core locks (and perhaps the end of Best Lock patent rights) was a signal for many other companies to introduce their version of IC core locks. The modern generic term for all of these different brands is called Small Format IC core (SFIC). Many different lock companies now offer SFIC lock cylinders and most can furnish keyways to fit many existing Best lock key systems.
High security is a term used for lock cylinders which have some secondary exclusive patented feature. By adding a secondary pin, sliding bar or sidebar pins, some companies have added to the security of SFIC cylinders by making them harder to pick open and by offering stricter key control for end users. Even without secondary features, some companies have developed restricted keyways for SFIC cylinders. While in many cases this does not restrict the lock cylinders from being picked open, it does add to better key control. At least two companies, Medeco and Videx, offer electronic versions of SFIC cylinders which provide an added benefit of audit trail reports.
The small plug diameter has caused tolerances in SFIC locks to be kept to a minimum. Tight tolerances allow the same masterkeying capabilities as with larger plug diameters without causing key interchanges. Years ago Best Lock also introduced three keying systems known as A2, A3 & A4. which are still in use today.
The most popular system is A2. There are two shearlines, one for all operating key functions which is located at the plug shearline. The control sleeve forms a second shearline which is used by the control key. The dimension between the plug shearline and the control sleeve shearline is .125”. The A2 system uses depth increments of .0125”. Therefore a distance of 10 increments (10 X .0125 = .125) is formed between the two shearlines. There are a few exceptions depending on the cylinder usage, but the total .0125 increments of the bottom, master, control and top pins in an IC core cylinder (pin stack) usually adds up to 23.
Pinning example: Master key cut = 4, Change key cut = 8, Control key cut = 6.
Bottom pin = 4 (highest operating key cut / shortest bottom pin)
Master pin = 4 (4 + 4 = 8 / deepest operating key cut)
Add ‘10’ length to Control key cut (6 + 10 = 16 / added pin length for control sleeve shearline)
Chamber already has ‘8’ (4 + 4 = bottom & master pin total)
16 - 8 = 8 / Control pin length (bottom, master & control pin increment totals = 16)
Total in chamber = 4 + 4 + 8 = 16. 23 - 16 = 7 (top pin length needed to reach 23 total )
The A3 system uses depth increments of .018. Pin stacks equal 16. The A4 system uses increments of .021. Pin stacks equal 14. A3 and A4 systems were originally designed to allow development of large single-step master key systems but maximum adjacent cut rules prevented many key changes from being used. The result was that there was little advantage for using the A3 and A4 systems and the A2 master key system is generally used today in most cases.
Partial list of SFIC manufacturers past and present: Arrow, Best, CX5, Dom, Falcon, Kaba Peaks, KSP, Marks, Master, Medeco Keymark, Sargent, Schlage, Yale.