<br> KnuckleHead was produced from 1936 to 1947 and became a milestone in the history of Harley-Davidson: from it, as they say, all the other upper-valve motors of the company went.<br> The name Knucklehead - “knuckle-headed” or “fist-headed” - comes from the form of a valve cover of this motor, similar to the knuckles of a clenched fist. It is easy to recognize it from the side by a pair of huge "bolts" securing the valve block cover.<br> They tried to put the new Knucklehead engine into series as quickly as possible, despite the fact that it leaked oil quite strongly, mainly from under the valve cover, which was due to the complex shape of the surface, which could not be reliably sealed. However, already then Harley-Davidson put style in priority! In addition, valve springs quite often burst in it, and the rocker arms suffered from a lack of lubrication.