An adjustable depth-of-drive mechanism for a fastener driving tool having a housing structure which defines an axis, and a nosepiece which extends generally axially from the housing structure, comprising: a lower work contact element having a sleeve configured for reciprocatingly receiving the nosepiece; an upper work contact element configured to be attached at a first end to the housing structure; a rotable thumbwheel assembly having a first portion engageable with said lower work contact element, a thumbwheel accessible by a user, a second portion engageable with a second end of said upper work contact element, at least one detent assembly and means for retaining said thumbwheel assembly relative to said upper work contact element; wherein when said thumbwheel is rotated relative to said first portion in either direction, the position of said lower work contact element is moved relative to the nose piece, characterized in that said detent assembly is provided on said second portion; said second portion is configured for being received by said second end of said upper work contact element and said retaining means is constituted of a retaining mechanism, and in that said detent assembly includes a detent spring and a detent element, said retaining mechanism is a bushing having an orifice configured for receiving said thumbwheel assembly second portion, and said detent element is urged to protrude slightly out of an opening in said second portion in a locked position and is depressible to retract from said opening in an unlocked position, wherein said detent element moves between said locked and unlocked positions during rotation of said thumbwheel, said bushing orifice includes a chamber in communication with said orifice and configured for receiving said detent element in said locked position, wherein when the user rotates said thumbwheel in either direction, the user action overcomes a detent assembly biasing force, releasably unlocking said detent element from said bushing chamber. a lower work contact element having a sleeve configured for reciprocatingly receiving the nosepiece; an upper work contact element configured to be attached at a first end to the housing structure; a rotable thumbwheel assembly having a first portion engageable with said lower work contact element, a thumbwheel accessible by a user, a second portion engageable with a second end of said upper work contact element, at least one detent assembly and means for retaining said thumbwheel assembly relative to said upper work contact element; wherein when said thumbwheel is rotated relative to said first portion in either direction, the position of said lower work contact element is moved relative to the nose piece, characterized in that said detent assembly is provided on said second portion; said second portion is configured for being received by said second end of said upper work contact element and said retaining means is constituted of a retaining mechanism, and in that said detent assembly includes a detent spring and a detent element, said retaining mechanism is a bushing having an orifice configured for receiving said thumbwheel assembly second portion, and said detent element is urged to protrude slightly out of an opening in said second portion in a locked position and is depressible to retract from said opening in an unlocked position, wherein said detent element moves between said locked and unlocked positions during rotation of said thumbwheel, said bushing orifice includes a chamber in communication with said orifice and configured for receiving said detent element in said locked position, wherein when the user rotates said thumbwheel in either direction, the user action overcomes a detent assembly biasing force, releasably unlocking said detent element from said bushing chamber.