The Yamaha XS650 was launched in 1968 and at the time, it had the most advanced parallel twin motorcycle engine in the world. It used unit construction (both the engine and transmission are part of the same casing) and had a horizontally split crankcase for ease of construction (and repair), it also had a SOHC with 2-valves per cylinder.
The XS650 had started life 10 years before as a 500cc parallel twin that had been designed by the engineers at now defunct motorcycle marque Hosk, this design had been acquired by Showa when they acquired Hosk and was then acquired by Yamaha when they acquired Showa. Now you see why Japanese companies sometimes have the corporate structure of a Russian nesting doll.
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