Fully restored red 1949 Harley. Flat head engine.
Advantages of a flathead (side-valve) engine over an OHV (overhead valve) engine were cost of manufacture and simplicity. With valves positioned in the engine block beside the piston, instead of in the cylinder head as in an OHV engine, push rods and rocker arms were not needed. Cylinder heads need only be a simple casting with threaded holes for the spark plugs.(classic-motorcycle-build.com/harley-45-flathead.ht)