Masterkeying Disc Tumbler Locks

May 1, 2006
Disc tumbler locks require the proper type of plug design to accept a masterkey system.

Every locksmith worth his salary knows how to add master pins to a pin tumbler lock to allow both a change key and master key to operate. A lesser used, but important procedure, is the masterkeying of disc tumbler locks. Disc tumbler locks require not only the proper type of plug design to accept a masterkey system, but also disc tumbler tumblers specially made for masterkeying.

Disc tumbler masterkeying is accomplished by using different key blanks for the change keys and master keys. While the primary millings near the base of the master and change key blanks are identical, the portion of blank which interacts with the tumblers is different. The operating portion of change keys designed for masterkeying will touch the disc tumbler tumbler either on the far left or far right of the keyway. Corresponding master key blanks will touch the disc tumbler tumbler on the side of the keyway opposite of where the change key is touching.

The result is that the master key and change key will never touch a disc tumbler tumbler at the same point. By having steps in the tumbler (photo 1), the master key can, for example, touch a high step on the left of the tumbler while the change key can touch a low step on the right of the same tumbler. This allows two different depths of cut to operate any given tumbler and provides the basis for masterkeying disc tumbler locks.

Photo 2 shows several CCL disc tumbler key sections. JVR and JR are standard key sections for non-masterkeyed systems.

CCL refers to the JS section as a service key, while the JL section is the master key. Primary grooves in the non-masterkeyed section are the opposite of masterkey pairs to eliminate the possibility of inserting random change keys into locks designed for masterkeying. Note that the JS and JL grooving is identical except for the top of each key where cuts are to be made. The JVS section is a service key with the JVL section as the master key. Each pair of service/master key sections can use the same key cuts while the key blanks will not interchange into the wrong system.

One final requirement involves the lock plug. The top part of the keyway must be broached open to allow both “left” key blades and “right” key blades to enter the keyway. Disc tumbler lock plugs for masterkeying are cast with an open keyway during manufacture. Disc tumbler locks which do not originally have an open keyway can sometimes be rebroached with home-made tools in the field, but a better recommendation would be to order new locks ready for masterkeying.

Disc tumblers for disc tumbler locks can vary in size and shape depending on the lock manufacturer. It is important, especially when masterkeying disc tumbler locks, to order a keying kit designed for the brand of lock you are going to be using. One example of a disc tumbler keying kit is the KK125 kit from CCL. Standard and master tumblers are included for depths 0, 1 & 2. Photo 3 shows the CCL KK125 keying kit.

CCL uses a depth increment of .025. This allows use of each number normally used by CCL for their disc tumbler locks. Photo 4 shows the three standard CCL tumblers 0, 1 & 2. The position of the window cutout in each tumbler determines the cut needed. The tumbler on the left requires a high cut on the key to hold it in an unlocked position while the tumbler on the right requires a lower cut to hold it in an unlocked position.

Other companies use increments of .015 or .020. Smaller increments have been found to cause possible interchanges because of the loose tolerances found in some diecast lock housings. As a result, some lock companies will not use every available depth and instead use depths such as 1,3 & 5, which provides a wider variance of two increments between cuts.

The amount of key cuts which can be generated in any single keyway, five disc tumbler masterkey system using three depths and full rotation of master disc tumblers in all five spaces can be mathematically determined. Since there are three possible cuts in any one space, the total possible amount of change key cuts is 3 to the fifth power, or 243 different change key cuts.

A few words of caution. Since all of the possible key cuts are being used, every masterkey system developed in this manner will be using the same 243 key codes. Codes can be used in different orders, but it will always be the same 243 codes. For security or esthetic reasons, approximately 10 percent of the codes should not be used due to repetitive cut numbers such as four or five of the same cuts in a row.

Another system for generating change keys in a master key system is called rotating constant. Chart 1 illustrates a rotating constant system where cuts in two spaces are identical to the master key while three spaces are rotated.

Ten different rotational combinations are available in a five space system when using master discs in three spaces and keeping the remaining spaces identical to the master key cuts. Spaces identical to the master key are 12, 13, 14, 15, 23, 24, 25, 34, 35, 45. Each different rotation combination is shown in the 10 groups of Chart 1.

Your choice of master key cuts dictates what the change key cuts will be. This provides a different variety of change key cuts for each small masterkey system. Note that some change key cuts such as #16 (22222) and #55 (11111) are usable, but not recommended because of their repetition of cuts.


There is one last hurdle to overcome when pinning disc tumbler locks. In order to make disc tumbler locks as compact as possible, most lock companies stagger the spring chambers. Photo 5 shows a disc tumbler lock made by Fort Lock. Notice that the first spring chamber is to the right, the second spring chamber to the left, etc..

Disc tumblers contain a spring tab. If a lock plug uses staggered spring chambers then the tumbler must be rotated 180 degrees from space to space during pinning in order to insert the spring tab into the spring chamber. This means that when a tumbler with stepped depths of 2/left & 1/right is rotated 180 degrees, the cuts suddenly change to 1/left & 2/right. Photo 1 shows the effect of moving the spring perch from right to left.

Pinning kits will contain duplicate pairs of stepped discs, one of which will have the tab to the left, while the other will have the tab to the right. The CCL keying kit contains six master tumbler types (0/1 & 1/0)(0/2 & 2/0)(1/2 & 2/1). In addition there are three non-mastered tumbler numbers 0,1 & 2. These nine different tumblers allow for any combination of master and change key cuts using 0,1 & 2 depths regardless of the spring orientation.

For more information on CCL products, contact your local distributor or: CCL, telephone 800-733-8588.