A bump key is a device that can be made from a simple house key to help you should you find yourself locked out of the house. Locksmiths have been using them from upwards of 30 years. They can be purchased online as well, however they are much too expensive given the short amount of time and effort involved in constructing one. Such a key will work for any normal multi-pin tumbler lock, such as the kind you have connected to your deadbolt and front door. This is also an effective exercise in home security. Thieves have likewise been using bump keys with relative impunity for just about as long as locksmiths have. It’s unfortunate that people who tout home security have downplayed this information in an attempt to get you to purchase shiny new home security systems that do nothing to keep thieves out. Once you’ve made a bump key and given it a try, maybe you’ll be willing to pony up the extra expense of purchasing new bump-proof locks, which also happen to be proof against normal lock pick attempts as well.
Begin by taking a look at your old house key and determining how many internal pins the door lock has. You can find this out by looking up the model of lock you’ve got, counting the number of individual grooves in the teeth of key, or slowly slotting the key into your lock; there’s a pin for each click you hear as the key is inserted.
Figure out the distance between each pin by measuring the length of the key’s operational surface and dividing it by the number of pins. Find the slot near the back length of the key. Above this slot, near the teeth of the key, is the deepest point at which the lock’s pins can rest. Mark out this point using your ruler down the length of the key. From the points along that line at which the pins will sit, draw triangular teeth, one tooth between each groove where the pin will sit. These teeth should be no more than 5 millimeters farther from the back line you’ve drawn.
Use a file to scrape away all the teeth of the key until the edges are level with the lines you’ve drawn. You might want to do this outdoors as the metal shavings will get everywhere and they can be dangerous if they get in your eyes. You should have a key that’s mostly filed down with teeth of equal height spaced equal distances apart. If done correctly, the first tooth of the key should start at the very end of the key. With a little practice, this key can open any lock that it will fit into.
Use your bump key by slotting it three-quarters of the way into the lock. You might want to keep your door open when you do this for the first time. Any burs in the metal of the key might jam the lock, forcing you to dismantle it in order to remove the key. With your thumb and forefinger, exert very light pressure on the key as if you were turning it normally to lock or unlock it.
Use the back of your file or the heel of your other had to lightly bump the key completely into the lock with a single tap. The kinetic energy exerted on the key as it is slotted into place will in turn be transferred to the lock’s housing, causing the pins to jump momentarily. When the pins reach the correct height in their jump the pressure you’re exerting on the cylinder will turn it at the appropriate moment, opening the lock.