Optical engines allow for in-depth analysis and optimization of in-cylinder combustion events to reduce pollutants and to explore combustion of non-traditional fuels. Despite the research utility of these engines, the use of optical materials and methods impose material, geometric, and operational constraints which complicate their design. This paper provides a method for design and validation of piston assemblies for optical engines, with a focus on modularity, maximized experimental times, and reduced window damage during installation. A Bowditch-type optical piston is developed for use in a 2 L, heavy-duty compression-ignition engine, which is convertible for use in both an optical and all-metal thermodynamic configuration. The design objectives required that the new piston: allow for increased operation time between window cleanings, facilitate rapid window cleaning, replicate geometry of the corresponding thermodynamic piston, and enable other piston crown geometries and configurations to be used with minimal overhead. The new design utilizes continuous (i.e., non-split) glass-filled PTFE compression rings. Plain bearing theory is not sufficient to design these rings and the intermittent and variable nature of the loads and speeds experienced by piston rings allow for the use of a lower grade material than this theory would predict. Compared to the previously used greased, split metal rings, these continuous rings are shown to increase maximum cylinder pressure, as evidenced by higher peak pressures during their operation. A new window cartridge arrangement is presented which protects the window during assembly, disassembly, and application of the gasket preload force. Stress analysis of all components and heat transfer through the piston crown were assessed using an FEA simulation. These simulations helped to arrive at a modular piston crown, and at material selection to provide adequate static and fatigue strength; manufacturability was also considered in the final design.