To address one of the main environmental concerns, the engine out emissions, an enhanced understanding of the combustion process itself is fundamental. Recent optical and laser optical measurement techniques provide a promising approach to investigate and optimize the combustion process regarding emissions. These measurement techniques are already quite common for passenger car and truck size engines and significantly contribute to their improvement. Transferring these measurement techniques to large bore engines from low to high speed is still rather more uncommon especially due to the bigger challenges caused by the engine size and thus much higher stability requirements and design effort for optical accessibility. To cover this new field of research a new approach for a medium speed large bore engine was developed using a fisheye optic mounted centrally in the cylinder head to design a fully optically accessible engine test bench. This new approach is detailed with a test setup layout and a stability concept consisting of cooling systems and the development of a suitable operation strategy based on simulation and experimental verification. The design of this single cylinder engine with 350mm bore and 440mm stroke providing 530kW nominal load at 750 rpm was tested up to 85% nominal load in skipped fire engine operation mode. The measurements of the flame chemiluminescence of a dual fuel combustion of the diesel gas type present proof of the feasibility of the new design as a starting point for future systematic studies on the combustion process of large bore engines.