YAMAHISTORY.COM
The early history of Yamaha Racing 

Yamaha had began producing motorcycles in 1955 and had been very successful with their YA1 "Aka-Tombo" 125cc. As well as becoming a commercial success, the machine had won the 1st Asama 125cc races even in a fairly standard state of preparation but competition was hotting up, especially from Honda, so it was decided that dedicated race prepared models should be developed for future race events. By 1957 the all new race ready models were ready, alongside their road going cousins.
Contrary to many publications, the production model YD1 and the racing YD's did not share that much in common at all, bar sharing engine configuration, just a few gearbox internals, fork sliders, top yoke & wheel hubs were used, proving just how ambitious and eager the Yamaha engineers were to develop their racing machines. Even the engine cases themselves were unique castings, allowing for a smaller and lighter engine assembly, with removal of the kick starter assy and provision for one racing carburettor per cylinder. Barrels differed from the road bikes mainly by larger port tracts, head locating spigots and removal of weight by not having any frame hanging brackets, the alloy cylinder heads gained taller heat dissipating fins. The two-piece crankshafts were carefully milled from billet, polished and matched whilst the conrods were knife edged and ground to a mirror finish. Small ends were bushed, whilst the big ends housed two rows of roller bearings.  
The tubular frame and swinging arm were again unique to the race bike and not dissimilar to the Adler RS250. The frame weighed in at 9.3 kg's helping to keep the total weight down to 100kg's.
The 125c YA machines were essentially the same rolling chassis fitted but with smaller fuel tanks. The engines were also very similar but only with single cylinders. They shared similar clutch covers, cylinder head and barrel castings, crank parts, conrods, clutch's and some gearbox components.