How to rekey a simple lock cylinder.

Many locksmith, landlords and property management companies re-key locks or "change" locks. The principle of Re-keying locks is very simple. Re-keying a lock is changing the key combination of the cylinder you are working with. For example you are re-keying a Kwikset lock with a combination of 23135 and a new key with combination of 14552.  The cuts aren't similar enough for the old key to open the lock with the new combo.

This picture shows how to properly remove a cylinder from its housing. To insure that the top pins stay in the cylinder housing you must use a follower (the black plastic tool) so the top pins(which are being forced down by little springs) don't fall out and create a problem. Once you have successfully remove the cylinder from its housing you can discard the old pins, remove the old key, insert the new key and match the pins with the new key combination. Once the cylinder has been re-keyed and the pins are flush with the cylinder (making sure the pins don't stick out and sink in to far.) the cylinder can be inserted back into the housing and tested to make sure it rotates smoothly. If the key turns roughly and seems to bind, one or more pins are too long and need to be switched out for slightly smaller pins. If the key works well you can re-assemble the lock and re-install the hardware back on the door.