This is a new 159 Degree Rotary Valve. A rotary valve refers to the fuel intake valve of earlier Sea-Doo two stroke engines. The rotary valve is driven off of a gear on the rotary shaft in the crankcase between the cylinders. This motion allows the intake port to be opened and closed in time with the engine power pulses. The rotary valve allows the port to have no restriction. Rotary valve engines are a very efficient design with complete control of airflow into an engine. They allow the air/fuel mixture to flow without any airflow restrictions (unlike like reeds). Fits: 420924502290924502