Pick-proof lock

A key-operated locking mechanism for use in conventional latch-operated doors having a conventional cylinder plug housing said mechanism comprising a cylinder plug comprising said latch and having a keyway, a shell aligned with and adapted to receive and hold said cylinder plug, said cylinder plug and shell assembly adapted to fit within said conventional plug housing so as to be a replacement for a standard cylinder assembly, said cylinder plug being axially rotatable within said shell, a plurality of bores located in said shell, a plurality of bores located in said cylinder plug axially aligned with said bores in said shell, a plurality of sets of movable pin tumblers carried in said shell and cylinder plug bores connecting said shell to said cylinder plug and intersecting the clearance between said shell and cylinder plug, said pin tumblers blocking said axial rotation of said cylinder plug in said shell, said pin tumblers being movable within said bores by respective cuts in a key to clear said clearance between said shell and said cylinder plug and provide a shear line clearance permitting axial rotation of said cylinder plug within said shell, a latching means connected to said cylinder plug, coupling means coupling said axial rotational movement of said cylinder plug to operate said latching means, a plate, blocking pin means carried on said plate, a recess at the rear of said shell for receiving said blocking pin means, a pair of guide pins having one end respectively attached to the rear of said plug, spring members bearing against said plate and attached to said guide pins to force said blocking pin means into said recess, said plate being directly moved by the front end of the key to move said blocking pin out of engagement with said recess.