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Lock Bumping Explained

Lock Bumping Explained

13 Dec 2007

Many locks contain a cylinder with a pin tumbler mechanism. These cylinders have a set of pins, often five or six in a row. Each set of pins actually contains three parts: a lower pin, an upper pin, and a little spring. The lower pins have different lengths and sit in the barrel part of the cylinder (the plug) and make contact with the key. The upper pins sit above these and run from the plug into the cylinder body and prevent the plug from turning. When the correct key is inserted into the lock each pin is pushed up exactly the correct distance so that the join between each lower pin and upper pin lines up along the sheer line between the cylinder plug and body. This enables the plug to turn and activates the lock.

Lock bumping exploits a fundamental weakness in the design of the pin tumbler locking mechanism. Physics dictate that if you apply enough kinetic energy to the lower pin this will be transferred to the upper pin which will push up on the spring and create a gap between the two pins for a split second. If this gap falls across the sheer line between the plug and cylinder body in each pin chamber, the plug will turn. This is the theory behind lock bumping, but how do you achieve this pin separation effect?

The technique is startlingly simple. All you need is a special key called a bump key and a hitting tool like the handle of a screwdriver. A bump key is a key which has been cut down to the deepest cut at each pin position. Any key that will fit into the lock can be filed down to create a bump key:

The bump key is inserted into the lock and withdrawn a bit. Turning pressure is placed on the bump key with one hand and hit into the lock with the back of a screwdriver (or similar object) with the other. Sometimes a few hits are required until the pins are correctly "bumped", the pins separate for a split second, the plug turns, and the lock opens.