<P> In 1928, the Indian Ace was replaced by the Indian 401, a development of the Ace designed by Arthur O. Lemon, former Chief Engineer at Ace, who was employed by Indian when they bought Ace . The Ace's leading - link forks and central coil spring were replaced by Indian's trailing - link forks and quarter - elliptic leaf spring . </P> <P> In 1929, the Indian 401 was replaced by the Indian 402 which received a stronger twin - downtube frame based on the 101 Scout frame and a sturdier five - bearing crankshaft than the Ace, which only had a three - bearing crankshaft . </P> <P> Despite the low demand for luxury motorcycles during the Great Depression, Indian not only continued production of the Four, but continued to develop the motorcycle . One of the less popular versions of the Four was the "upside down" engine on the 1936 - 37 models . While earlier (and later) Fours had inlet - over-exhaust (IOE) cylinder heads with overhead inlet valves and side exhaust valves, the 1936 - 1937 Indian Four had a unique EOI cylinder head, with the positions reversed . In theory, this would improve fuel vaporization, and the new engine was more powerful . However, the new system made the cylinder head, and the rider's inseam, very hot . This, along with an exhaust valvetrain that required frequent adjustment, caused sales to drop . The addition of dual carburetors in 1937 did not revive interest . The design was returned to the original configuration in 1938 . </P> <P> Like the Chief, the Four was given large, skirted fenders and plunger rear suspension in 1940 . In 1941, the 18 - inch wheels of previous models were replaced with 16 - inch wheels with balloon tires . </P>

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