Shaped top pin for bump resistant cylinder

A method to retrofit existing tumbler and cylinder locks to enhance their resistance to bumping comprising the steps:

• providing a pin and tumbler cylinder lock comprising:
• a cylinder;
• a plug rotably mounted in the cylinder and having a key slot and forming a shear line between the inner surface of the cylinder and outer surface of the plug, the cylinder, and plug having a plurality of registering radial pin receiving bores which bores have straight non-chamfered sidewalls at the shear line;
• spring pressed top driver pins mounted in the cylinder bores;
• and
• tumbler pins mounted in the registered plug bores;
• removing all of the driver pins in the existing lock;
• and
• replacing all of the top driver pins with a driver pin which has a smaller diameter intermediate section forming at least a lower lip having an inner surface and an outer surface where the outer surface of each top driver pin lip is in contact with the upper surface of the registered tumbler pin and the thickness of the lip of the top driver pin is less than the shear line height minus the tallest tumbler pin stack height of the lock so that the inner surface of each replaced top driver pin lower lip is below the shear line so that if the plug is rotated during bumping the driver pins will be disposed at an angle because of the smaller diameter intermediate section of the top driver pins and the driver pins will be caught at the shear line preventing rotation of the plug and opening of the lock by the bumping.