Many home locks are made with pins and tumblers, and these locks are vulnerable to a technique called lock bumping. This method of breaching a lock is shockingly easy to do, yet most people are unaware of it.
A pin tumbler lock has a rotating cylinder, or plug, which is where the key goes, inside an outer housing. The shear line is the space where the cylinder meets the housing. There are moveable pins in the lock that reach from the inside of the housing through the shear line and into the cylinder. These pins keep the cylinder from turning.
This is similar to how the metal bolt in a deadbolt extends into the doorframe and keeps the door from opening. As long as the bolt reaches from the door into the doorframe, the door cannot move. When the bolt slides out of the doorframe, the door is free to open. The pins in the pin tumbler lock work the same way. As long as the pins extend from the lock housing, past the shear line and down into the cylinder, the cylinder cannot turn. When the pins move outside the cylinder and clear the shear line, the cylinder can turn.
In a pin tumbler lock, each pin is a different length. These pins match a specific cut on the lock’s companion key. So, when the correct key goes into the lock, all the cuts on the key raise all the pins so that they neatly move to the shear line, clearing the cylinder and allowing it to turn.
Burglars use a tool called a bump key. Bump keys are extremely easy to get. So, even though the bump key has to match the brand and model of lock it is going to bump, it is easy for a burglar to have many common bump keys, including one that will work in your lock.
The bump key’s teeth are short and even in height, and the valleys between them are cut as deeply as possible. These low and deep cuts allow bump keys to easily slide into a lock. The burglar inserts the bump key into the lock, stopping just before the key is all the way in. He then taps the key with a mallet or hammer, pushing it in the rest of the way. When that happens, those small teeth on the key push under the pins in the lock cylinder, and “bump” them up with a little force, causing them to jump above the shear line. In that brief moment when the pins are temporarily above the shear line, the cylinder is free and the burglar is able to turn the bump key in the lock.
There are things you can do to make it harder for someone to bump your lock. For example, a qualified locksmith can modify your existing lock by adding security pins. Of course, lock bumping requires the presence of a keyhole in the lock into which the bump key can be inserted. No keyhole means no lock bumping. So, you may want to consider changing your locks to a keypad-operated lock and forgo the key altogether. Aside from other benefits and conveniences these locks provide, they cannot be bumped. If you are not interested in a keyless lock, you may choose one of the newer bump-resistant locks, such as one of Master Lock’s deadbolts with BumpStop™ technology.