The other day I heard someone say something along the lines of:
"The Iron Head Sportster is the first time the transmission and engine were together."
The person is referring to a 1957 Harley Davidson Sportster, and the engine configuration is called unit construction, which means the engine and transmission are held within the same engine case. They are correct about the 57 Sportster having a unit construction engine, but they are incorrect about it being the first unit construction engine from the Motor Co. The first unit construction engine design by Harley Davidson appeared in the 1952 Harley Davidson K model. The first K model engine was a 750 cc, flat head style engine and existed in the following variations:
KH - A stroked K model engine that increase displacement from 750 to 883 cubic centimeters
KR - Racing version
KHK - Same stroked K model engine as above, but with a factory installed Speed Kit, which included polished ports, performance cams and roller bearings for the crankshaft.
KL - This was an experimental engine that was discontinued and never saw production
Harley Davidson manufactured the K model from 1952 to 1956, and was marketed toward young adults. The problem with the K model is it was slow and did not compete well against the imported Triumph and BSA motorcycles, so Harley Davidson was working behind the scenes on a new OHV engine, which appeared in 1957 on the Harley Davidson Sportster. British motorcycles at the time were using smaller displacement, OHV, parallel twin engines which could easily be modded to out perform the Harley Davidson K model.
They say a picture is worth a 1000 words so here are a few:
1952 Harley Davidson K model
1957 Harley Davidson Sportster
1953 Triumph Trophy