The knucklehead engine has a distinct shape on the rocker covers. In addition the engine is a two cylinder, with two valves per cylinder, pushrod V-twin. In case your wondering, the knucklehead was the second type of engine used by Harley-Davidson.
When looking at a knucklehead engine, it will have round knobs on the valve cover resembling knuckles. This is how knucklehead engine got its name.


