Abstract: A key-operated pin tumbler lock has a shell in which is rotatably disposed a plug that defines a keyway and a plurality of bores. The bores communicate with the keyway through the plug and in a locked position of the lock communicate with corresponding bores formed in the shell. A plurality of spring-loaded pin stacks are located within the bores and are moveable within the bores against the spring-loading by a key inserted in the keyway into a position wherein the plug can be rotated relative to the shell. A stop means is provided in one of the bores and is adapted to engage the pin stack which is located therein such that a key pin of the stack is prevented from penetrating into the plug to a level reached by key pins of the other pin stacks.