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The development phases and test bench results related to the conversion of a bus Diesel engine of 6 cylinder 9.8 L displacement to an aftercooled turbocharged Spark Ignition (SI) compressed natural gas engine are reported. A pre-screening of various piston shapes was performed using the KMB CFD code to determine the most promising intake-chamber geometries according to computed mean flow and turbulence results. Then, the designed pistons were built for engine tests. Ignition characteristics, valve lift laws, bowl shape and swirl level were optimized to find the trade-off between efficiency and emissions. The finally retained solution reaches performances very close to the starting Diesel engine by running ultra lean (λ > 1.8) with a maximum thennal efficiency larger than 40% for a compression ratio of 12.5: 1. The complementary use of an efficient oxidizing catalyst allows nitrogen oxide emissions lower than lg/kWh for the European 13 mode cycle.