In the twenties and thirties overhead valve engines were well known to be more efficient and consequently more powerful than either side-valve engines or the hybrid “F-head” with its overhead inlet and side exhaust valve. Overhead valve engines were the province of motorsport performance engines where power was at a premium and maintenance could be intensive. But metallurgy that would enable reliability from overhead engines was still developmental and expensive.
Harley-Davidson had already moved away from the hybrid F-head to the Flathead side-valve and so Harley-Davidsons were acquiring a reputation for reliability and deservedly so. By the 1930’s Harley-Davidsons were being supplied to the US Military and other government agencies and those motorcycles were all using the bullet-proof Flathead side-valve engine. But management at Harley-Davidson wanted to get to market with a viable overhead valve engine motorcycle and William Harley created a design and got engineers Hank Syvertson and Joe Petrali to hammer out the details to create a commercially viable overhead valve V-twin engine.