When an idea is brand new, every variation of it is rapidly tried, but eventually a consensus emerges as to which best combines low cost, performance, and ease of manufacture. This is why engine cylinders remain round rather than oval or square; a simple, inexpensive boring and honing process produces usable bores.
Before World War I, motorcycle engines proliferated—parallel twins, fours, flat-twins, V-twins, radials. Valves jutted in all directions. But in Britain through the 1920s, the dominant form became the vertical overhead-valve single, its height shoehorned between the pavement and the flat bottom of a gravity-feed fuel tank. Simplicity. As this occurred, writers of the time commented that, like the bicycle, the motorcycle had assumed its final form.
The tank above required that the engine’s intake port emerge from the…
