I've always wondered just how much similarity there actually is between Yamaha's engine and BSA's final twin.
Yes, obviously there are very major differences in construction, the OHC, and more importantly the horrizontal split cases and all built up crank with forged steel rods.
But both engines share exactly the same bore and stroke.
I wonder how different the valve angles, valve head sizes, and port shapes are ?
Remember that just before the XS1 was designed, BSA's A65 Spitfire was the fastest 650 twin you could buy (at least until it blew up) not the legendary Triumph Bonneville