DubDub,

Knucklehead is also the name given to the 1936 Harley Davidson engine. If interested in info:
http://www.tower.org/museum/harley_davidson/harley_davidson.html

I'm a motorcyclist myself, although not a Harley enthusiast, but I know it is a very common term when describing Harleys. The machines, not the riders, as some would guess!