Harley-Davidson Twin Cam engine

Both engines have two cylinders in a V-twin configuration at 45°, are air-cooled (some touring models use liquid cooling for the heads),[1] and activate valves with push-rods.

As the company determined that a rubber-mounted Softail would affect the line's visual styling, Harley solved the issue by designing an engine variant known as Twin Cam 88B.

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.

[10] While aftermarket firms such as S&S Cycle 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 traditional 45-degree, air-cooled overhead valve V-twin.

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.

[10] Harley sought the help of Paul Troxler, a young engineer from Southwest Research Institute and eventually the problem was traced to a design which drained the cam case into the crankcase, and used a single scavenge pump.

After much testing, the solution was to seal the cam case from the crankcase and use a dual scavenge system, which was incorporated into the engine, rather than as an outboard pump as used on older Big Twins since the original Knucklehead.