Large Format Interchangeable Cores

Oct. 15, 2003

Over the years, interchangeable cores have become very popular. The nomadic nature of today's institutions and corporations require flexible keying systems that can be easily moved and re-configured. As large and small facilities change ownership or move to another state, interchangeable cores facilitate the need.

The interchangeable core was originated by Frank Ellison Best in 1912, and patented in 1921. The "figure-eight" format was first produced in 1925 and was slow to be accepted as a hardware standard. The main obstacle relating to product acceptance was that other brands like Corbin, Russwin, Sargent, Schlage and Yale were already established lock companies of choice and offered sufficient keying systems for the times. Additionally, the recombination of cores was perceived as unnecessarily complicated.

Best Lock Company was successful in overcoming these obstacles by working closely with the customer and the customer's administrative designee and evolving the product into a seven-pin configuration that offered four times the key yield as other standard six-pin key systems.

As acceptability grew, existing lock companies succumbed to customer demand by developing their own proprietary interchangeable cores. Customers could then enjoy interchangeability while maintaining their existing keying systems.

Many lock companies offer two types, their own "large-format" proprietary interchangeable cores that work with their established keying systems, and a "small-format" that accommodates "figure-eight" (Best/Falcon) interchangeable cores.


In 1971, the Emhart Company developed removable cores for its Corbin and Russwin lock lines. To distinguish their cores from Best/Falcon cores, the company coined the cores as "recores." Although sales representatives insisted on using the term, it didn't stick, and today they are referred to as the more typical term "interchangeable cores."

The cores share the same shape as Best/Falcon cores, but are significantly larger. Like Best/Falcon cores, they are held into the sleeve by a control lug on the left -side of the core, but are unlike as dual shearlines appear in the middle chambers only.

The Corbin Russwin "recores" have no control shearline in the first and last chambers. Both control and operating shearlines are implemented in the center chambers. These center chambers use plus, zero, and minus build-up pins.

When a control cut is deeper than an operating cut, a "plus" build-up pin results. Consider the following where the control cut is 6, the change cut is 2, and the master cut is 4. The control pin equals the control cut (6). The bottom pin and master pin would equal 4. Subtract the total of the bottom and master pins (4) from the control pin (6) to get the build-up pin (+2).

When a control cut is the same as the plug total, the build-up pin will be zero. Consider the following where the control cut is 4, the change cut is 2, and the master cut is 4. The control pin equals 4 (the control cut). The bottom and master pin would equal 4. Subtract the total of the bottom and the master pins (4) from the control pin (4) to get a build-up pin (0).

The computation works the same when the control cut is shallower than the deepest operating cut. Consider the control cut is 2, the change cut is 2, and the master cut is 4. Subtract the control pin (which is equal to the control cut) 2, from the total of the bottom and master pins (which is 4), you will get the build-up pin -2.

The order of the stack is the bottom pin, master pin, build-up pin, and last the control pin.

In the first and last chambers, calculate the bottom and master pin conventionally, and the last pin will always be a driver equal to .247"

These Corbin Russwin "recores" are offered either in five- or six-pin configurations, in System 70 keyway groups. In Corbin, the keyway groups are X and Z. In Russwin, the keyway groups are K and N.


There are two different large format interchangeable cores from Medeco: Series 31 and 32.

The Series 31 is designed to fit mortise, rim, key-in-knob locks that have been prepped to accept Yale interchangeable cores. Like a Yale, the core is held into the cylinder is held into the housing by a cam at the back of the core. The cam protrudes on the left side of the core and is pulled back when a special control key blank is utilized.

The Series 31 control key blank is longer than the operating key blank. When cut the same as a operating key, the added length and a twist to the right, slides the cam into the core, the core is then removed. Securing the core into the housing in this fashion eliminates the need for "build-up" or "control" pins. This simplifies the keying process.

Series 31 cores are available in six-pin only.

There is a high-level of drill and pick resistance but not to the level of UL437.

The Series 32 interchangeable cores are locked into the sleeve with a control lug positioned in the middle of the core (third and fourth chambers). The control lug protrudes on the right side of the core.

Like the series 31, no "build-up" or "control" pins are needed. To remove the core, a key is cut similar to an operating key with the exception that the third and fourth chambers are cut three increments shallower. The key is inserted and turned slightly to the left, which rotates the control lug into the core. The core is then pulled out.

If the respective operating cuts in the third and fourth positions are 4 and 5, then the control cuts will be 1 and 2.

Care need to be taken when series 32 cores are integrated into a master key system. Because control keys are required to be three increments shallower in the third and fourth positions, this parameter needs to be worked into the system before it is initiated.

Series 32 cores are UL437 listed because of their pick and drill resistance and strength against other forms of forced attack. They are available in both six- and seven-pin configurations.

Both series utilize a double-locking principle providing pick resistance and benefit from utility patents that provide protection from unauthorized duplication of keys.


The "Old-Style" 5100 Series removable core was the original removable core supported by Sargent. The 5100 Series is based on using a seventh position on the control key and in the core. All other operating keys are cut on six-pin length key blanks. When the longer key blank is cut to the bitting of an operating key and is cut to a #3 depth in the seventh position, it will slide back the cam and allow the core to be extracted from the housing.

A five-pin version is available.

It is only available for mortise and rim cylinders and is currently furnished only for additions to existing key systems.

The mainstay of removable cores for Sargent is based on the 6300 Series core. The control lug is on the right side of the core and is pulled into the core when the control key is inserted and rotated to the left.

The control key bittings match the TMK (Top Master Key) in positions 1,2,5 and 6. The control key bittings in the 3 and 4 positions are selected from a key-bitting array.

The stack values in non-control chambers (chambers 1, 2, 5 and 6) equal 15. Three pins are stacked in these chambers the bottom, master, and driver. To compute the driver, subtract out the total of the bottom and master pins from 15.

The stack value in the control chambers (chambers 3 and 4) equal 20. Four pins are stacked in these chambers the bottom, master, control, and driver. To determine the control pin, subtract the total of the bottom and master pins from the total of the control key bit and 8. Determine the driver by subtracting the total of the bottom, master, and control pin from 20.


Schlage's "full-size" interchangeable cores are configured either for conventional (part no. 23-030) or Primus (part no. 20-740) keying. Schlage utilizes a pin on the right side of the core in what would be the seventh pin position.

The pin is visible with a mortise or rim cylinder in hand. An access hole reveals the location of the locking pin (see Schlage Primus graphic, p. 36).

Like Sargent 5100 Series cores, a special control key blank is used that is longer than other operating key blanks. When cut to an operating key bitting in the first six positions and a #3 cut in the seventh position, the control key is rotated to the left forcing the locking pin into the body of the core, then the core is pulled out.

By utilizing this strategy, pinning processes are simplified.

Primus features a patented finger-pin and sidebar design providing a "dual-locking" core that is pick-resistant.

Both 23-030 and 20-740 cores cover all popular Schlage keyways.


Yale 1200 Series interchangeable cores are secured into the cylinder housing by way of a locking cam at the back of the lock in the seventh pin position. The cam secures the core into the housing on the left side of the core.

Like other versions of large-format cores, a longer key blank is used for the control. The control is cut similar to the TMK (Top Master Key) and the longer length of the key slides the cam.

When the control key is inserted and rotated to the right, the locking cam enters the core body and the core can be pulled from the housing.

Like other versions of cores, "build-up" or "control" pins are not necessary.

These cores are furnished in either six- or seven-pin configurations. A new feature with these cores is the addition of a spring cover for top loading.


Something completely different, not necessarily a "large format" but definitely not a Best-type interchangeable core, is the Bilock QCC Series. The "Quick Change Core" is held into the housing by small ball bearings. The control key is has a set of dimples that allow the ball bearings to recess into the core body when the key is turned to between 30-40 degrees to pull the core out.

The QCC features two independent side-bars controlled by two sets of pins. There are two rows of six pins each. There are four usable depths per chamber.

It is possible to generate 16,777,216 theoretical key combinations. The Bilock QCC Series can be master keyed, having over 500,000 operating keys under the Top Master Key.

Features of the QCC are double-locking protection, simple design, and color-coded keys.

BiLock offers a full range of mortise, rim, key-in-knob, cam, padlock, and utility housings to fit its QCC Series.

In Summary

All the mentioned lock companies (except BiLock) furnish small-format cores that are compatible with Best/Falcon type "figure-eight" cores. Many of these interchangeable core types offer proprietary patented protection and unique keyways.

The Best/Falcon type cores are not compatible with large existing key systems. This is the forte of the large format and related core system. Each blends in transparently with each company's existing key systems.

It's nice to know that no matter what existing system you use, there is a proprietary large-format interchangeable core that will work with your system. Compatible and cost-effective, they are the way to go when maintaining existing systems.