Indian's next major development came in 1916 when Hedstrom's former assistant Charles Gustafson developed the 1 litre "Powerplus".
It had two side valves instead of one (full flathead instead of F head). The middle of WW I (1916) was the first year for the Power Plus, and the first engine not designed by Oscar Hedstrom.
A note on internal combustion engine terminology: Flatheads are also known as L heads, side-valve or for cars and trucks, "valve-in-block".
In 1922, it was enlarged to 1000 cc (1 liter or 61 cubic inches) to become the Chief and to 1200 cc or 74 cubic inches in 1924 to become the Big Chief engine.
Indian was also very advanced with a swinging arm rear suspension (albeit with leaf springs, not coils) from 1913 to at least 1918.
The early "Big Twins" as they were called had F heads.
These early Chiefs had gear driven primary in aluminium casings, in oil bath.
(English and Harley motorcycles were still using leaky pressed steel primary cases decades later.) The 1928 Scout 101 (750 cc.) was and is regarded as Indian's best handling if not best-ever motorcycle. Neither the Ace nor Indian fours should be confused with the Henderson-Excelsior Four, although all three derived from the same design by Mr. Note how the skirted ("valenced") fenders which came out in 1940 completely change the look of the Fours.