Portable gas-powered fastener driving tool

A portable fastener driving tool comprising a housing, a main cylinder in said housing, a piston in said main cylinder and movable from a driving to a driven position, a driver attached to said piston, a magazine for supplying fasteners into position to be driven by said driver, a combustion chamber within said housing having a wall portion defining a plurality of openings and said piston as a wall portion thereof, a fan in said combustion chamber and controls therefor to operate same to cause turbulence in said chamber, means for controlling the flow of gases into and out of said combustion chamber, a work sensitive probe assembly cooperating with said means for controlling the flow of gases whereby the combustion chamber is not closed off until the tool is in position to drive a fastener into a work-piece, means for providing fuel into said combustion chamber and igniting same for driving said piston to drive a fastener and trigger operated means responsive to movement of said means for controlling the flow of gases into and out of said combustion chamber whereby the tool cannot be fired until the combustion chamber is closed and the chamber cannot be opened until the trigger is released, and the improvement wherein the wall portion of the combustion chamber is fixed relative to the housing and said means for controlling the flow of gases includes a slidable member disposed adjacent said openings, movable relative to said combustion chamber, and secured to said probe assembly.