Master keying

Updated on January 15, 2014

Master Keying Pin Tumbler Locks

Master keying is the ability to have 2 or more keys work in the same lock. Controlling which keys work which locks allow us to give different people access to different doors.

For example We could have a motel of 6 rooms and an office. We could have 1 key for the manager(master key) that opens all the doors. We could have another key for the cleaners that opened all the room doors. And we could have a key for each guest that only opens an individual room door.

There are many different types of locks on the market these days and the different types require different mathematical techniques to master key them.

The predominant lock on the market is the Pin Tumbler which uses 2 pins in each chamber. There are many more types of locks that use for example one set of pins, rotating discs, disc tumblers, rotating pins and many more.

Pin Tumbler Pins

Pin Tumbler Locks use 2 sets of pins Bottom Pins and Top Pins. The bottom pins are are what gives the lock its combination. The bottom pins match the cuts in the key. A short bottom pin corresponds to a shallow cut in the key. A Long bottom pin corresponds to a deep cut in the key.

To make more then 1 key work in the lock we add a master pin between the Bottom and Top Pins. The master pins correspond to the depths of the key cuts.

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Master Pins

Eg if we had a number 6 cut in a key and a number 3 cut in a key, we would use a number 3 bottom pin and add a number 3 master pin so both key cuts will work. The number 3 cut works on the number 3 bottom pin and the number 6 cuts works on the number 3 bottom pin + the number 3 master pin (3+3=6).

We could also add another number 3 master pin to the chamber to allow a number 9 key cut to work(3+3+3). Now we have 3 different keys working the same lock

Bottom and Master Pin kit

This kit is especially made for Kwikset locks. Kwikset are the most popular lock sold in North America. The kit can also be used for most pin tumbler lock rekeying and master keying.

Lock Tollerances and pin depths

In order to get more keys working the lock a locksmith will progress the key cuts using more pin chambers. This works as long as we do not use any of the master key cuts in our progressions. Using any of the master key cuts will give us ghost keys or keys working locks that are not suppose to.

Also it is important to use cut progressions that cant be forced because they are too close together. Manufacturers might use 9 or 10 different cuts in their keys, but they are too close to be used in a practical way.

So instead of using 1,2,3,4, etc we might use 1,3,5,7,9 or 1,4,7 if the lock tolerances are too sloppy.

If we are to use 2 cuts difference for example and have 10 depths(0-9) our useable depths would be 0,2,4,6,8 for even cut numbers and 1,3,5,7,9 for odd cut numbers.

We also do not want key cuts that put a very deep cut next to a very shallow cut. Cutting the deep cut can cut away too much of the key and allow the shallow pin to sit too low in the lock.

Our master Key should also contain both high and low key cuts. If all the cuts in the master key are deep, Then the change keys would all be shallow. Filing down a change key would allow it to work in other locks, wich is not something you want to happen. If the cuts are all shallow it makes it easier to work out the cuts by using the impressionong method as well as other opening techniques.

Master Keying Matrix

An easy way to create a large master key system is to first choose a master key.

We will use our 0-9 cuts in the key with a number 2 cut difference.

It is a good idea to choose odd and even numbers adjacently so not to end up with possible straight cut progressions that could open the lock.

Our Master Key is going to be 723659.

To make it easy we can make a matrix of our change key cuts. We are only going to use the last 3 pin chambers for this lock.

___898

___476

___234

___012

______

123650

Our possible change keys for this lock are

123012 123032 123072 123092 123212 123232 123272 123292

123014 123034 123074 123094 123214 123234 123274 123294

123016 123036 123076 123096 123216 123236 123276 123296

123018 123038 123078 123098 123218 123238 123278 123298

123412 123432 123472 123492 123812 123832 123872 123892

123414 123434 123474 123494 123814 123834 123874 123894

123416 123436 123476 123496 123816 123836 123876 123896

123418 123438 123478 123498 123818 123838 123878 123898

If we were to use another chamber we could get 4 times more change keys.

You can easily see the progressions in the above keys in the last 3 chambers.

123012 ____32 ____72 ____92 ___212 ____32 ____72 ____92

_____4 _____4 _____4 _____4 _____4 _____4 _____4 _____4

_____6 _____6 _____6 _____6 _____6 _____6 _____6 _____6

_____8 _____8 _____8 _____8 _____8 _____8 _____8 _____8

___412 ____32 ____72 ____92 ___812 ____32 ____72 ____92

_____4 _____4 _____4 _____4 _____4 _____4 _____4 _____4

_____6 _____6 _____6 _____6 _____6 _____6 _____6 _____6

_____8 _____8 _____8 _____8 _____8 _____8 _____8 _____8

Each individual lock would be pinned up to have the smallest bottom pin number in each chamber. You have to look at the individual key and the master key to determine the smallest number. Then the master pin would sit on top of the bottom pin. The master pin is the difference between the highest number key cut and the lowest.

eg if there is a no 2 cut and a number 8 cut. We use a no 2 bottom pin with a number 6 master pin.

At this stage we can make a common door work with all the keys.

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Master Keying more complex

Now this would be fine for a motel for example with x number of rooms and 1 master key to operate all the rooms.

But what if we wanted our master key to operate a motel with 3 different blocks and we wanted each block to have its own mastger key?

We would need to use another chamber in our lock as Master Key cuts.

If we use the same master key as before 123650(it is not a Grand Master Key or GMK)

We can use the 3rd chamber for our master key cuts.

__9___

__7___

__5___

__1___

123650

Our possible master key cuts are

121650

125650

127650

129650

we can use the last 3 chambers still as our change keys.

We could use another chamber to make a Great Grand Master Key System (GGMK)

This is all pretty straight forward, but often a master key system is more complicated then this. Often there will be the need for a cleaner to have access to some doors and the lift technitian to have access to the building, but not individual doors and a manager to have access to some doors and not others that only the ceo has access to, etc.

Adding access for different keys to work in a number or different locks adds to the complexity of the system. Adding a key to open a series of locks will usually cause cross keying, which is the wrong keys opening locks they are not spose to open.

When adding keys to open specific doors you pretty much have to go through each lock's pinning and make sure only the correct keys are going to work.

If there is lots of cross keying, alot of keys have to be deleted out of the system. This can dramaticly reduce the number of usable combinations available in a system.

In we look at the previous change key example and use the motel example. Now we want 6 room keys only to have access to a common bathroom(pretty crumby motel).

123012 ____32

_____4 _____4

_____6 _____6

_____8 _____8

We can use the first 6 keys all to open a bathroom door, but the next 2 keys would have to be deleted from our system as they would also open the bathroom door, which we do not want. No other keys in our system will open the bathroom door. We could repeat this if there were blocks in our motel that used seperate bathrooms.

The more complex the system gets the more chance there is for ghost keys working the wrong locks and the less number of keys we can have in our system. There is a very big limitation of key changes when you get a complicated system.

When designing a master key system you want to try to reduce the need for master pins as much as possible. Adding master pins to a lock makes it less secure and more chance of a random key working the lock. In essence master keying allows multiple keys to work in a particular lock, which is not ideal for security.

Another issue is that you want to design a system that can be expanded on. Often the owner of the system will want to change locks or add a new building, etc. So you have to allow for future expantion.

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72

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