A casing drilling system includes combination conduit of a casing and a pipe inside the casing. A first adapter has a flow diverter to redirect at least flow of drilling fluid returning from a bottom of a wellbore to either an interior of the pipe or an annular space between the casing and the pipe. A second adapter has a flow diverter to redirect flow of drilling fluid into the conduit through the other one of the interior of the pipe and the annular space. The second adapter has a fluid connection between either the interior of the pipe or the annular space and a rotationally fixed fluid outlet. The system includes a casing chuck having means to support the casing and a slidable conduit operable to expose an uppermost end of the casing and a connection between the second adapter and an uppermost end of the pipe.