Abstract: A portable power tool including a body in which a bore (4) is formed and a piston (3) is slidable within the bore. A combustion chamber (2) is formed in the body and is in communication with the bore. The combustion chamber (2) is arranged to receive a gaseous fuel from a source (5). An inlet port (6) into the chamber (2) and an outlet port (12) therefrom are controlled by a valve means (10). Preferably the arrangement is such that fuel entering the chamber through port (6) scours the chamber (2) of residual gases through port (12). The tool preferably has various additional features such as means for monitoring the valve means in an open condition for a preselected time. That means may include a detent catch (39), and an actuating membrane (32). A lubrication system (94) may be provided for lubricating the valve means. A fuel deflecting vane (80) may be mounted in the combustion chamber to improve turbulence therein.