A bump key is designed to open pin-tumbler locks without requiring the specific key made for the lock. Bump keys have even valleys filed down low on the key. They are used in combination with a mallet or hammer. To use a bump key, insert it most of the way into a pin-tumbler lock and hit it with a mallet. As you hit the key with the mallet, turn it to open the lock without damaging it. This is a method that both locksmiths and burglars use, therefore it is important for homeowners and renters to understand the technique.
Examine your key. Count the number of grooves, or valleys, on the key. There are usually between four and six, and they are evenly spaced.
Some of the grooves may be at their maximum height--and from a side view, may not be appear to be grooves at all. Look at the top of the key so that you are looking down at the top of the teeth. On some pin-tumbler keys, such as Kwikset keys, the metal will be darker at the maximums. If a key had five "grooves" set to their maximum height, from the side, the key would look like a straight line with no teeth at all; if you looked down at the top, you would see a dashed line consisting of five darker dashes on the metal where the grooves would be cut.
Flip your key over so that the back of the key is facing you. The back is the flat side with one long, horizontal groove.
File the valley closest to the end until it is slightly above the horizontal groove on the back of the key.
Move on to the next valley and continue filing until each valley is filed down to an equal height, with small triangular teeth delineating each valley.
File off a very small amount from the tip of the key, allowing it to have more room to move in the lock.
An alternative way of counting the pins is to insert an unmodified key into a lock slowly, counting the number of clicks until it is completely inserted. Any standard pin-tumbler key can be modified into a bump key, including blanks.
Carrying a bump key with intent to commit a crime is illegal in most states, as with other tools such as master keys and lock-picking sets. Bump keys only work with pin-tumbler locks of the same type as the bump key.