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A first retrofit conversion of a Cummins NTC335 engine to spark ignition was carried out in New Zealand in 1984. The conversion used widely available technology for stoichiometric control of natural gas fuel - air mixtures. Experience from the on-road application in a 40,000 kg GVW truck contributed much to the later development of a 400 hp gas-fuelled variant of the same engine family, using lean mixture carburettion control. A second engine entered service in a logging application in February 1989. The following paper summarises results arising from laboratory testing of the second engine, and from in-field monitoring. Also presented are preliminary results from testing of a third generation engine, using timed multi-point injection of gas fuel.