The "Mastiff" was one of the few 750cc models Douglas cataloged. Basically a S5/S6 model 500/600cc with larger cylinder bores. The other 750cc side valve was the "Powerflow" of 1935.
Douglas also made 750cc versions of their over head valve engines based on the 500cc TT/I.o.M replicas; not cataloged but available to special order. Again same stroke as the 500/600cc, but a larger bore. And of course their own Works entries in the 750cc sidecar racing class up to their last effort 1932. Earlier they did catalog a 730cc version of the 1922 Sports model, their first public offering of an over head valve model. The S.1 was 494cc and the S.2 the 730cc.
The S.2 was sold also with a Douglas chain reduction drive for light aircraft use. In about 1932 Douglas again made another 750cc engine for aircraft use, called the "Sprite". This was pretty much a 1931-32 F/G model 500/600cc motorcycle engine lightly re-designed to aircraft use, direct drive this time. This had been intended for the BAC glider (the ill-fated "Flying Flea" light plane designed by Henri Mignet came slightly later.)
They then started making purpose designed crankcases to attach to an airframe (single then twin magneto versions) and cylinders with circular fins rather than radial (as used on the motorcycles.) Also the displacement shot up to 1100 and 1500cc, I imagine to make it suitable for other small aircraft and to compete against engines like the Bristol "Cherub". Whether these got past the prototype stage is unknown. But this is rather straying from the topic.
-Doug
