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Twin Cam

Development of the Twin Cam started in the early 1990s, as Harley sought to address problems affecting the previous Evolution engine, particularly structural weaknesses within the crankcase, oil circulation and leakages. While aftermarket firms such as S&S Cycles previously responded with stronger crankcase components for high-performance Evolutions, Harley went for a completely new design, while keeping the engine fundamentally and aesthetically similar to the 45-degree, air-cooled overhead valve concept Harley has used since it introduced the V-twin to its lineup.

The Twin Cam only shared 18 parts with its predecessor, as almost all of the components were unique to the engine. As the name implies, the engine uses two chain-driven cams, to comply with EPA noise regulations. The drivetrain was reinforced through a mounting scheme called the "Revised Rear Interface", allowing the transmission case to be mounted directly to the rear of the engine, with the primary drive and clutch covers playing less of a structural role; this was also partly the reason for Softails not initially receiving a Twin Cam for the 1999 model year.