technique

The following technique is presented as a concept. I do not have the required aparatus to test this method, but the required appartus should be available in many machine shops. If you try this method, successfully or not, please write to me with your results: [email protected]

This method should work on all combination locks using the same fundamental design as the Master Lock described on these pages.

Note that the cams can be independently positioned. By turning the knob right two turns and stopping at a number, the third cam is positioned. By then turning left one turn and continuing to a second number, the second cam is positioned without moving the third cam. By turning right less than a full turn, the first cam is positioned without moving the other two cams. The procedure requires repositioning one cam at a time and making accurate measurements of how far the shackle pulls away from the lock, in an effort to locate the deep notches on each cam.

On the "why it works" pages, we describe how the lock has three cams with deep notches and that the lock will open if all three deep notches line up under the latch. When the deep notches are not lined up, the latch will press against one or more of the cams and the lock will not open. If all three cams were the same size and had only the deep notches, it might be possible to pull up gently on the shackle, turn the knob and "feel" the notches, or hear clicks, as the notches passed under the latch assembly. This is a safecracking technique used on early safes. To thwart this method, Master made the first cam, the cam driven directly by the knob, larger than the other 3 and added 11 "fake" shallow notches on the first cam.

The fake notches should be below the level of the other two cams, but they may not be. If they aren't, the knob will be harder to turn at a fake notch when the shackle is pulled hard, because the latch assembly will be bearing on the first cam. Thus, if the knob is harder to turn, it is a dead giveaway that the notch is fake and another notch should be tried. Select a notch where the knob is just as easy to turn with the shackle pulled hard as it is with a light pull on the shackle. The dial should be at this number for all measurements.

When the deep notch, or a deep enough shallow notch, is aligned under the latch assembly, the latch assembly will bear evenly on the two other cams when the shackle is pulled. If the deep notch of the second or third cam is aligned under the latch assembly, the latch assembly bears on the remaining unaligned cam.

Using accurate methods to apply the "same" force and to measure how far the shackle pulls out of the lock, it may be possible to locate the deep notches on the second and third cams. The third cam would be positioned at each of the 40 numbers in turn, with other two cams restored to the same positions for each measurement. (First cam in the deep notch or a deep enough shallow notch.) The shackle would be pulled with the same force, and the distance that the shackle pulled out would be measured. The shackle should pull out measurably further with deep notch aligned than it would at nearby numbers. Once the first number was determined, the deep notch on the third cam could be aligned and the notch on the second cam located. Both notches would then be aligned and it would then be easy to locate the third number, even if the measurements were being made at a shallow notch.

Note that, within the tolerances of the test setup, the same amount of force must be applied for each measurement. Even if the cams were machined very precisely to the same radius, it would be possible to determine whether the latch was supported by two, or whether a deep notch was aligned and the latch was supported by the remaining cam. The reason is that the force will compress the cams slightly, but they return to normal when the force is removed. If the force is applied to 1 cam, the amount of compression will be more than when the force is spread over 2 cams.

The proposed test setup would have the lock, upside down, clamped in a vise, overhanging the workbench. A weight would be hung from the shackle for each measurement. The weight would be removed before the dials were turned and the weight returned for the next measurement. The hard part is making precise measurements of how far the shackle pulls away from the lock.

In practice, the cams are likely to not be quite the same radius and also to be out of round. Even so, with careful measurements, it should be possible to locate the deep notches and determine the combination.