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For handheld power tools, a four-stroke engine allows compliance with exhaust emissions regulations although four-stroke engines available tend to have unfavorable power to weight. The requirement for a low cost diecast block compromises valve sizes and port flow. While dynamic valve train limitations restrict maximum engine speeds. The use of a rotary valve as opposed to poppet valves avoids these issues and results in an engine with competitive performance. The engine block can be diecast and the engine can operate up to 14,000 rpm without valve related issues. This paper describes the evolution of a rotary valve concept and its application to two 35cc handheld development engines. The HRCV35 is based on a belt driven rotary valve horizontally mounted parallel to the crankshaft axis. The VRCV35 is based on a gear driven rotary valve vertically mounted on the cylinder axis. In both configurations, the rotary valve exposes inlet and exhaust ports providing unrestricted flow.