A door lock set including a cylinder lock unit which has a lock shell and a plug is disclosed. The lock shell has an axially extending protrusion which projects outwardly and radially from an outer periphery of the lock shell and which has a plurality of pin holes that receive respective springs and tumbler pins. A cover is disposed over the protrusion to cover the pin holes and is biased outward by the springs. The cover is compressible against the springs to reduce the size of the lock shell so that the lock shell can be inserted easily into a rear shank of a knob of the door lock set for assembly. In a preferred embodiment, the cover has two opposite lateral sides extending over and engaging movably two opposite lateral faces of the protrusion. Methods for assembling and disassembling the door lock set are also disclosed.