Even though the unscavenged prechamber has been extensively applied in lean premixed natural gas engines, the limited understanding of the fundamentals and the lack of predictive modeling tools (3D RANS CFD) place obstacles in the way of prechamber design and optimization. The present study investigates unscavenged prechamber combustion of lean methane/air mixtures in a Rapid Compression Expansion Machine (RCEM) by combining optical diagnostics (high-speed OH*-chemiluminescence and Schlieren imaging) and 3D Computational Fluid Dynamic (CFD) simulations. Data from the former is used to develop and validate the modeling approach for the specific application and the latter aims to provide indispensable interpretation of the experimental observations. Initially, the comparison of the Schlieren and the OH* images confirm the hypothesis that inherent reacting flame jets exit the prechamber, which justifies the applicability of a level set combustion modeling framework for the investigated operating conditions. The employed G-equation combustion model has been extended to account for the specifics of spark ignition and flame wall interaction present in the prechamber configuration studied. Validation of the developed model by means of the experimental data shows good agreements in terms of (i) jet exit timing, (ii) main chamber heat release rate (HRR) and (iii) projected reactive flame area, evidencing encouraging predictive capability of the proposed modeling approach. The combined insights from experiments and CFD simulations suggest two phases of the main chamber heat release rate, dominated by the jet penetration and the turbulent flame propagation respectively. The subsequent analysis on a single flame jet, using OH*-chemiluminescence and CFD images, indicates that the jet head tends to be more reactive due to a higher turbulence levels and larger eddy size. Moreover, the entire dataset reveals an inverse correlation between the initial reactive jet speed and the early phase combustion duration (5% of total cumulative heat release). Overall, this research provides useful guidelines for the future unscavenged prechamber design. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.