As Harley-Davidson engines evolved, the distinctive shape of the rocker covers led Harley enthusiasts to recognise an engine simply by identifying the head, with names such as "Flathead", "Knucklehead" "Panhead", and "Shovelhead".
Currently,[when?] a number of third-party engine manufacturers produce custom Panhead-style engines in a variety of bores, many much larger than the original-design displacements. Each manufacturer includes upgrades to the original design to improve performance and reliability while still providing the original styling and overall engine structure.[citation needed]