Master Combination Lock Manipulation Method
by Dennis Heath
 CSM0279
Master Lock Company's 1500 series combination padlocks are probably the most widely used combination padlocks in the world. These locks all have code numbers on the backs of their cases. These code numbers can be used to obtain the combinations for those locks where the combination has been published in a code book, or is listed in code software programs. Combinations for many of these locks are not known and locksmiths can use the procedures outlined in this article to determine unknown combinations by manipulation for locks unknown combinations.
There is no consistent relationship between the code and the combination although locks with code numbers up to 910752 did have combinations in repeating blocks so that when the combination to certain codes became known, additional combinations were also known. For locks with code numbers 910753 and higher, the combinations are no longer in these same blocks. As of the date of writing this article, the new system has not yet been deciphered. As a point of interest, there are 1536 possible combinations that are used.
There are basically two versions of these locks, the older style and the newer style. The changes made to the newer lock were supposed to make it more secure, but it is still fairly quick to open and no increased security was really accomplished. The last number wheel (drive cam) in the lock is attached directly to the dial shaft. The primary differences between the older style and the newer style is the number of false gates on the drive cam and the tolerances for the shackle. The older style had only no false gates, or detents, on the drive cam. The newer style has eleven false gates in addition to the one real gate. The shackle tolerance on newer style locks is much less than on the older style, meaning that it is more difficult to use a shackle shim to open the newer style.
Certain rules apply to all of these locks.
Rule #1: These locks all have a three number combination. The numbers for each padlock will either be all even or all odd. Even and odd numbers are never mixed in a combination.
Rule #2: If the third number of the combination is odd, the last number will be a 5, 15, 25 or 35. No other odd numbers are ever be used as the third number of a combination.
Rule #3: If the third number of a combination is even, any even number between 00 and 38 may be used.
Rule #4: There must be at least a six number difference between the second and third numbers of the combination.
The dialing procedure for these locks is three turns to the right to the first number, two turns to the left to the second number and one turn to the right to the third number. Pull the shackle to open the lock.
Manipulation procedures.
The first step in manipulating these locks is to determine the gate on the drive cam. This is the third number of the combination. On older style padlocks just pull out on the shackle and turn the dial to find the gate. You will recognize this by not being able to turn the dial until pressure on the shackle is released.
On newer style padlocks it is necessary to find the gate and not be misled by the eleven false gates, or detents. Pull out on the shackle and turn the dial to the right (clockwise) feeling each of the detents, one at a time. Of the twelve detents that you will find, five will align with a number on the dial and the other seven will align between numbers. Disregard the seven that align between numbers. One of the five remaining numbers will be the real gate.
Write down the numbers for the five detents that you find. Four of them will end in the same number, such as 1, 11, 21, 31. Discard all four of these. The other one will be the real last number of the combination.
Let's use a sample combination of 081004 as an example. Divide the last number by 4. We are interested in what the remainder, if any, will be. In this case the remainder is zero. The first number of the combination will also be evenly divisible by four. This means there are only ten possibilities for the first number: 00, 04, 08, 12, 16, 20, 24, 28, 32 and 36.
The second number uses the same sequence as the first number except you have to add 2 to each number. This means that the second number has to be 2, 6, 10, 14, 18, 22, 26, 30, 34 or 38. With the last number already known, there is only 10 middle and 10 first numbers possible. This means there are only 100 possible combinations.
We can further reduce the possible amount of second numbers by understanding that the second number has to be at least six numbers higher or lower than the last number. Since the last number is 4, we can eliminate 2 and 6 as possible second numbers. This now leaves 8 possible second numbers: 10, 14, 18, 22, 26, 30, 34 or 38. We have now lowered the possibility to a maximum of 80 numbers.
Due to the way the pins from each of the disks interact with each other, we will only have to dial the lock a maximum of 10 times, once for each of the possible first numbers.
Now to open our make believe lock. Combination is 8104.
Turn the dial right at least 3 times to insure that all three wheels are bound and stop on 0.
Turn the dial left, past the first number, and stop at 10 (last number remainder plus 4). Remember we cannot use 02 or 06, so 10 is our first stop.
Turn the dial to the right and stop on 04. Pull on the shackle. The lock will not open because this is the wrong combination.
The next step is very important because we don't want to have to redial the first number again.
Turn the dial to the left again, this time stopping on 14. Then turn the dial back to the right, stop on 04 and pull out on the shackle again. Remember you always stop on the known third number of the combination to test pull the shackle.
Repeat the above step increasing the second number by 4 each time until you have gone through the 8 second number possibilities. That's left to 18, right to 04; left to 22, right to 04; left to 26, right to 04; left to 30, right to 04; left to 34, right to 04 and, finally, left to 38, right to 04, each time test pulling out on the shackle at 04.
By using this method, you don't have to redial the first number each time. The worst case is 10 redials. You only have to rotate and advance the second number with each possible first number.
After trying the incorrect first number, we do our three turns to the right and start with the second possible 1st number which is 04. Go through the same dialing routine that was just done. Then we start over again with 8 for the 1st number and use the same dialing routine. When the correct combination is found, the lock will open. Write down the combination.
It may take a little longer the first few times, but goes pretty quick later. Also, if you happen to already know the 1st or 2nd number, it only takes a few minutes. If you have a lock with a low first number, practice on it. Or start dialing with a higher first starting number to save time.
With practice you will be able to dial the second number accurately with faster dial turning. With practice, the worst case dialing time is about 10 minutes assuming a very high first number (like 38) and starting at the beginning. The real key to this method is finding the second number without redialing the first number each time.
The rules for the sequencing of the older locks are the same as the newer series. Use the same procedure as used for the newer series. You don't have to go through the procedure to eliminate the false gates because there aren't any.
I first used this method in 1958 when I was in the 7th grade. Most of the gym lockers had the combination locks with the key in the back (V series). I modified a Corbin blank at the time because this was a restricted keyway and I did not have access to the correct blank. The school had a key for these, but had to use bolt cutters on those that did not have the key override. I made a deal with them to let me remove all the locks left on at the end of the school year in return for keeping them. This saved them the time and trouble of cutting them off. I then sold them the next year to make a few dollars. At that time, I used a similar procedure except I used 20 first and 20 second numbers and had to do 20 redials back then.
Special thanks to Pam Anderson from the Locksmith Ledger for the idea of attaching a chart and to Charlie Cole for his patience in educating this writer in the organization of this document.
The following chart list all possible combinations for every last number. Remember, the 2nd number must be a minimum of 6 numbers distance from the last number.
