For nearly 50 years the Simplex line has represented the most popular combination locks in the North American market. Even if you aren’t actively involved in the lock/security industry, you’ve seen them. Simplex locks are everywhere. Per KABA’s website:

Simplex mechanical pushbutton locks offer a convenient way to control access between public and private areas. There are no keys or cards to manage, no computers to program, no batteries to replace, and combinations can be changed in seconds without removing the lock from the door.

The brain(s) of the Simplex line, to speak, is the combination chamber. It stores the code and validates or denies an entered code. Let’s explore the Kaba Simplex combination chamber.

KABA Simplex Combination Chamber(s) Part Numbers

Combination chamber is really a “catch-all” term as there are more than one type of combination chamber used in the Simplex line. Some combination chambers are specifically suited for a single Simplex model while others can be used for multiple Simplex models. Originally, combination chambers were assigned part numbers that began with “M”. For example, there were/are M56 and M54 and M71 combination chambers. Somewhere along the line the part numbers were changed. I suspect this was due to a design change but whatever the case may be, they are now:

Simplex Model Old Chamber Part No. New Chamber Part No.
900 M55 74080-000-01
1000/L1000 M56/M63 74366-000-01
2000 74459-000-01
2015 742412-000-01
3000 M56/M63 74366-000-01
5000 74832-000-01
6200 M64 74870-000-01
7100 M71 74660-000-01
8100 M56/M63 74366-000-01
9600 M54 74014-000-01


Nearly all models of the Simplex line utilize the same combination chamber, there are really only slight nuances between them. The outlier of the Simplex family is the Simplex 900 which features a design radically different from the rest. The Simplex 900 really deserves its own article and perhaps I will one day write one but, for now, the information in this article relates to every lock in the Simplex line except the Simplex 900.

Inside of the KABA Simplex Combination Chamber and How They Work

Let’s start with the key stems. The key stems are what interact with the buttons of the lock. Each key stem is connected two gears. The first set of gears is directly connected each key stem. Upon the pressing of a key stems, the first set of gear turns a second set of gears.

These second set of gears are what you see in the above picture and what interact with the unlocking slide. These second set of gears are equipped with gates, or recesses, inside of them. When the correct combination is entered, the gates all line up to allow the unlocking slide to move upwards and enter into them. The handle of the lock is turned which rotates the control shaft, via linkages inside of the lock body, and lifts the unlocking slide into the aligned gates. Because the unlocking slide is spring loaded, once the control shaft lifts it, it snaps back down into it’s normal resting position while simultaneously resetting all gears and buttons via cams attached to the control shaft.

Here we can see the correct combination already entered and all of the gates aligned, ready to receive the unlocking slide.

And finally the unlocking slide moves up, allowing the lock to be unlocked.

Going back a moment, we can see the aforementioned first set of gears above. These gears are of no real importance except to the function of the combination chamber itself. It should be noted that these are not the gears that interface with the unlocking slide and should not be confused with them. Also pictured is the unlocking slide’s spring; it is a tension spring so that the unlocking slide is pulled away from the gears at all times unless forced up via the control shaft.

Decoding the KABA Simplex Combination Chamber

Now that we’re familiar with the key parts of a KABA Simplex combination chamber and how it works, let’s go through the process of decoding one. Decoding of the KABA Simplex combination chamber is required when the code is lost or forgotten. There is no possible way of decoding a KABA Simplex combination chamber with the lock on the door; it requires disassembly of the lock and removal of the combination chamber. If you aren’t familiar with the disassembly process for a KABA Simplex lock, KABA has produced videos and published them on their YouTube channel. We also have the instructions for this process for every current Simplex model in our Mechanical Combination page in the Library (under the Manufacturer’s Literature and Manuals tab).

The process for decoding KABA Simplex combination chambers used to be very tedious and frustrating. It wasn’t difficult, it just required many steps and the removal of a few tiny parts that could easily become lost. I’ve decoded dozens and dozens of Simplex combination chambers in the field. Not once have I ever used the method that KABA once taught. Yes, I do have to remove the lock from the door and remove the chamber links and the combination chamber but once the combination chamber is in hand, I’m either done or one more step from being done. Confused? Ok, let me explain.

  • Combination chambers manufactured prior to 12/15/2010 had solid combination covers. You can’t see anything on the inside of these combination chambers; you must remove the combination cover itself to view the gears and thus decode the combination. When KABA redesigned their combination chambers, they decided to utilize covers with viewing holes that allowed your to see the second set of gears.
  • Combination chambers manufactured after 12/15/2010 will have these holes in the cover.

A combination chamber manufactured after 12/15/2010.

The decoding process that I and many others have always used for combination chambers with solid combination covers doesn’t involve disassembly of the combination chamber beyond the combination cover’s removal – we simply take note of the positions of the gears and derive the existing code from it. KABA must have agreed with this method because now all combination chambers are designed and built to facilitate this through the use of the aforementioned viewing holes.

With that said, let’s start the process of decoding a combination chamber. Regardless of when the combination chamber was manufactured, the process is virtually the same. If your combination chamber doesn’t have viewing holes, you simply have to remove the cover by gently prying it away from the rest of the chamber. It’s not hard, don’t force it.

The goal of the decoding process is to align all gates with the unlocking slide. When the gates are aligned with the unlocking slide the shearline is established. This is KABA’s terminology, not mine, but I guess it’s accurate enough to work. When a gear is at the shearline it means that the gear is set, so to speak. This is an important piece of information when decoding a combination chamber: gates already set at the shearline are not used in the combination.

Let’s decode a combination using a brand new, factory default combination chamber. The default code is 2 and 4 pressed together, and then 3. For the purpose of this article, let’s pretend we didn’t know that.

What do you notice?

We can immediately see that gears 1 and 5 are at the shear line. This tells us that they are not used in the combination. Our combination will utilize gears 2, 3, and 4. Furthermore, we can see that gear 3 is very close to being at the shearline. This is our next clue. Each gear utilized in a combination will rotate towards the shearline whenever a button in the combination is pressed. This means that gears closest to the shearline are the last utilized in the combination. After decoding a few combination chambers, you will learn their relationship to the shearline and immediately be able to tell if they are the last or next to last button used in the combination. With this piece of information, therefore, we know that 3 is not the first button utilized in the combination.

By process of elimination, we know that either 2 or 4 or 2 + 4 are the first digit(s) of the combination. But which is it? Here is the next tip: each key stem has enough play in it that you can almost move it’s corresponding gear to it’s next position. In other words, we can see what’s going to happen with out committing to pressing a button and having to start all over if we messed up (more on that shortly). By pressing both 2 and 4 key stems, we see that their behavior is nearly identical. Each gear is in the same position. When two or more gears are in the same position, they are used simultaneously in the combination as long as their original position is not already at the shearline. We can therefore use this information to make an informed decision: the first part of the combination is 2 and 4 pressed together.

Now we’re getting somewhere. We’ve almost got the combination. We can see that the 2nd and 4th gear are almost at the shearline. The 3rd gear has moved slightly but is still the furthest from the shearline. We now know that it is the next part of the combination.

And there we have it. All gears are aligned at the shearline. We have decoded the chamber. Before we start the resetting/code changing process, let’s address a few final points:

  • If you mess up during the decoding process at any point, simply rotate the control shaft counter-clockwise (when viewing from the key stem side). This will reset the combination chamber, so to speak, and allow you to start over. Rotating the control shaft will take some force so you’ll more than likely need to do it with a pair of needle-nose pliers or similar tool.
  • 4 or 5 single digit combinations can be very difficult to decode. That is because certain gears will be so far from the shearline that even slight depression of the key stem won’t allow you to see the gate. A flashlight aimed inside of the combination chamber greatly assists if this is the case.

Changing the Code on the KABA Simplex Combination Chamber

Now that we know the existing combination, we can change the code. This can be done in one of two ways:

  1. Reassemble the entire lock and change the combination on the door.
  2. Change the combination by directly interfacing with the combination chamber.

There is no right or wrong answer. It’s a matter of preference. If you chose to reassemble the lock first and then change the combination, the instructions for your specific Simplex model are available online. Here are the instructions and here is a video showing the process of changing the combination of a Simplex 1000/L1000, for example.

If you choose to change the combination with the combination chamber in hand, that’s no problem either. Let’s walk through that process:

1. Using the key stems, enter the existing code to align the gears at the shearline.

2. Depress the lockout slide.

Yeah, yeah I hear you, “what’s the lockout slide?” The lockout slide is located at the top of the combination chamber. It looks like, in the words of KABA, a spark plug.

Once you depress the lockout slide, you’ll notice that the gears shift down towards the control shaft (or should if you’re doing it right!).

3. When viewing the combination chamber from the side with key stems, rotate the control shaft counterclockwise.

This will “clear the chamber” and prepare the gears to accept their new sequence. After rotating the control shaft, the lockout slide should move back up. The button below, known as the code change button, should stay depressed.

4. Enter the new code.

Each key stem should click once depressed.

5. When viewing the combination chamber from the side with key stems, rotate the control shaft clockwise.

The code change button should pop back up.

6. If correctly done, the gears should be scrambled according to their sequence. Enter the new combination to ensure that all gears line up at the shearline. If not, something was done wrong and you’ll need to either attempt to change the combination again or you’ll need to decode the new, unknown code and try again.