During the Second World War spare parts for the Stinsons' aging Lycoming R-680 engines were impossible to obtain in Australia and it was decided to re-engine both aircraft with a Pratt & Whitney R-1340 Wasp engine on each wing and eliminate the engine on the nose. The additional power allowed both aircraft to fly faster and carry heavier loads, although fuel dumps had to be fitted to allow them to remain under their maximum landing weight of , in case of an emergency necessitating landing shortly after take-off.