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Made in Japan

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 reports, the production model YD1 and the racing YD's did not share that much in common at all bar, the engine configuration, just a few gearbox internals, fork sliders & wheel hubs were shared, 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.  
 
Above: The Yamaha Teams lined up at Asama, with the YD-A & YD-B 250cc machines in the foreground and the YA-A and YA-B 125cc machines behind.
The rules of the Asama Races stated that each manufacturer could enter 2 teams, the 1st team consisting of 2 machines with the 2nd team having machines with significant different configuration. 
 
Yamaha had chosen to field their team "A" with the 54x54mm bore and stroke machines (hence YD-A), as the Adler had been, whilst the "B" team bikes were set up with short stroke engines of 56x50mm (YD-B).
 
Yamaha Riders at Asama
 
Team "A"   YD-A
 
#60 Fumio Ito
#75 Osamu Masuko
Team "B"   YD-B
#54 Tanehare Noguchi
#65 Yoshi Sunako
#66 Matsuo Shimoseki
 
At the begining of the 131km race, Noguchi was the favourite to win, but alas his YD stalled on the start line and refused to restart. Fumio Ito shot off to set a blistering pace, setting the fastest lap of the race. Unfortunately though his bike seized causing him to retire. This allowed the novice racer Masuko to take the victory on his YD-A, followed by Sunako and Shimoseki both on YD-B's. Yamaha's making a historic 1-2-3 ahead of 4 works Honda's.
 
Above: Osamu Masuko on his way to victory at Asama on his YD-A.
Below: Fumio Ito sets the fastest lap before his YD-A seizes up.
Above : Shimoseki's YD-B #66.  Upper : Yamaha promotional picture. Lower: colourized B&W pictures of the same machine. 
Below : Yamaha's website showing their account of Asama and the early years of racing. It is well worth visiting the website ! Page 3 mentions the Asama Race winning machine being sent to the US along with 4 pre-prepared Catalina prepared YD's. 
Above Noguchi's bike after Asama, fitted with a YA-A/YA-B fuel tank.
Below #65 Yoshi Sunako's YD-B.
A nice colour picture of one of the YA-A / YA-B 125 machines. As can be seen the 125 was virtually the same as the 250cc except being single cylinder.
Yamaha's triumphant and dominant 1-2-3 whitewash at the 1957 Asama Races took everyone, especially Honda by storm. In 1958 they even tested an Adler 250cc engine in one of their privateer teams C70Z racers, which can clearly be seen in the bike in the bottom of the photo.
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