A single-cylinder research engine for flow and combustion visualization has been designed, built, and operated. It has a transparent liner and piston-top window that maximize optical access for inspecting in-cylinder flow and combustion. It is an overhead cam engine with a three valve head, nominal 9:1 compression ratio, 92 mm bore and stroke, and maximum speed of 4000 r/min. The engine design is described, and some preliminary results are given. The engine has been motored with a glass liner and fired with a stainless steel liner before a sapphire liner is installed for combustion visualization.