On rotary valve engines, the process is similar except instead of a reed, a rotary valve is used (incorporated in the crankshaft), which opens and closes as the piston moves up and down. The rotary valve is more efficient and adjustable (at design time) as there is a larger and clearer path to the crankcase than in the reed valve setup, but such engines can run in only one direction; pusher configurations require a special propeller, sometimes difficult to find.