PHOTO (COLOR): This OHV 1200 Sportster engine traces its four-cam valve train to 1929 and its compactness to the unit construction of the postwar side-valve K-model. Along the way, Harley's hugely successful XR-750 dirt-track racing engine was developed from the Sportster in 1970. PHOTO (BLACK & WHITE): This is the original Big Twin, the 61-inch EL, or "Knucklehead", of 1936, Harley's first large OHV production engine. [Extracted from the article]
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