Improved internal combustion (IC) engine simulations of natural gas (NG) combustion under conventional and advanced combustion strategies have the potential to increase the use of NG in the transportation sector in the United States. This study focused on the physics of turbulent flame propagation. The experiments were performed in a single-cylinder heavy-duty compression-ignition (CI) optical engine with a bowl-in piston that was converted to spark ignition (SI) NG operation. The size and growth rate of the early flame from the start of combustion until the flame filled the camera field-of-view were correlated to combustion parameters determined from in-cylinder pressure data, under low-speed, lean-mixture, and medium-load conditions. Individual cycles showed evidence of turbulent flame wrinkling, but the cycle-averaged flame edge propagated almost circular in the 2D images recorded from below. More, the flame-speed data suggested a different flame propagation inside a bowl-in piston geometry compared to a typical SI engine chamber. For example, while the flame front propagated very fast inside the piston bowl, the corresponding mass fraction burn was small, which suggested a thick flame region. In addition, combustion images showed flame activity after the end of combustion inferred from the pressure trace. All these findings support the need for further investigations of flame propagation under conditions representative of CI engine geometries, such as those in this study.