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The Sentinel AC II was an Australian design for a cruiser tank that could be produced using existing truck engines and components.
In the summer of 1940 Australia faced the increasingly likely prospect of a war with Japan, at a time when the country owned no modern tanks, Britain was rearming after the loss of virtually all of her modern tanks at Dunkirk and the best American tank was the obsolete Medium Tank M2. The Australian Ministry of Munitions decided to investigate the possibility of designed and producing a cruiser tank in Australia. In November 1940 the Australian General Staff set out their requirements, and in December 1940 Britain sent out Colonel Watson, a tank expert. He travelled via the United States, where he saw the plans for the Medium Tank M3, and early in 1941 his team produced the design for the Sentinel AC I (AC standing for Australian Cruiser). This used the M3 final drive and gearbox, commercially available engines and a newly designed cast hull and turret.
The AC I project ran into a serious problem in April 1941 when the detailed drawings for the M3 drive arrived in Australia. The M3 drive was too complex to be produced with the machine tools then available in Australia, and neither Britain nor American could spare any suitable machinery.
In order to bypass this problem the United States suggested that the Australian team should produce a new design that would use commercial truck engines, drive and other mechanical components that could be provided from the United States. Work on this new design, the Sentinel AC II, began in July 1941. The limited factor turned out to be the truck final drive and other mechanical components, which could only cope with a vehicle of 16-18 tons. This restricted the gun to a 2-pounder, already clearly obsolete in 1941, and also limited the thickness of armour that could be carried.
In September 1941 the AC II project was abandoned, and work resumed on a modified version of the AC I. Eventually sixty six Sentinel AC Is were produced in 1942-43, but by the time they appeared American was in the war, her levels of tank production were reaching impressive proportions, and the Australian armoured divisions were able to use American tanks.