Harley-Davidson Knucklehead engine

Harley-Davidson Model EL, 1936 Harley's first overhead-valve big twin is where the American cruiser silhouette was fixed. The rocker covers that gave the engine its nickname, the teardrop tank and the instrument panel set on top of it became styling cues the company carried forward for decades. Look at the diagonal line running from the tank down to the rear axle and you can see every later big twin in outline. The first year was not trouble-free, with oil leaks and valve-spring failures that Harley sent repair kits to dealers to fix, so this is a design landmark rather than a model of early reliability.