Engine produced by the Harley Davidson Motorcycle Company from(1936 to 1947). The engine consists of an overhead-valve v-twin design with covers on the valves that resembled knuckles, hence the name of "knucklehead".
Example: Ah fuck dood... I was cruisin outta school and this big fuckin knucklehead came up to me and was like 'Eh Vato, Gimme your pinche money esse!' So I yelled for la migra, and wouldn'tcha know, the mafacker ran away! Fuckin hate knuckleheads.