Servicar Rider:
Thanks for validating my "educated guess" that this was a JD series engine. The JD series was an "F" head engine. I saw a 1928 JD model being ridden at a motorcycle event years ago. I remember seeing the rocker arms working, out in the open. Cool to look at, but, if I remember correctly, the rocker arms had to be oiled manually. Those open rocker arms probably left a mess of oil on the rider's thighs, aside from collecting a mess of grunge when road dust mixed with the oil on the open rocker arms. Back 'in the day' when the JD was in production, roads outside of cities were mainly dirt or "improved" with gravel. Road dust was a fact of life.
It took a different breed of rider to own and ride motorcycles back in those days, and any rider had to be a pretty fair mechanic. Rigid rear frames (aka "hard tails") on the motorcycles and spring suspension on the rider's saddle were what was standard. Combined with roads that were bad more often than not, both the motorcycle and the rider took a pounding. The owner's manual for the JD motorcycles must have had a lot more information about mechanical maintenance than the modern H-D manual contain. And, those motorcycles probably came with a place to store a tool-kit, unlike several generations of the newer H-D's.
An interesting process was the building of the motorcycle frames back in that era. Frames were made by bending steel tubing and fitting it into forgings or malleable iron castings for the steering neck and other major connections on the frame. Joining was done by bronze brazing. On the heavier joints, such as the steering neck, heating of the assembled portion of the frame with the heavy steering neck was done using a charcoal fired forge hearth. The tubing and sockets in the steering neck were fluxed beforehand. The frame was clamped in an assembly fixture or jig to hold everything in position, and the section of the frame with the steering neck went into the charcoal fire. When it was hot enough, bronze brazing rod was applied to the joints and some localized heating with an oxyacetylene torch was done. The brazing alloy used "wicked" into the joints and made small and very neat fillets. I remember seeing an old photo of the forge brazing of H-D frames, and always thought it took a good deal of skill to perform that kind of brazing.
The JD model was built at a time when the "founding fathers" of The Motor Company (as Harley Davidson used to be known amongst riders) were alive, well, and involved in the design and production of the JD series of engines and motorcycles. It was a time when Milwaukee, Wisconsin was alive with industry of all types, and firms such as Allis-Chalmers, Nordberg, Filer & Stowell, P & H, Kearney & Trecker, Vilter, and many more were producing heavy industrial machinery of all sorts ranging from milling machines to turbines and large steam and diesel engines. Ole Evinrude was alive and well, and had helped the "boys" (the young Harley and the Davidsons) to perfect a working carburetor for their first motorcycle engines. The JD was built in 1928-29, so Prohibition was likely in force and the brewing industry in Milwaukee was probably producing "near beer" and "pharmaceutical alcohol" if they were working at all. Despite prohibition, Milwaukee at the time the JD motorcycles were built, was probably a very thriving city with all sorts of industries and "spin-offs" such as smaller tool-&-die shops and supply firms all going full bore. A different era, for sure.