As for the camshaft Yamaha had no experience with a one piece crankshaft for a 4 stroke engine therefore the same press-fit crankshaft type with identical cylinder pitch as for the YR-1 was used. But a 360 degree crank instead of the 180 degree one was used because of vibration. And where the YR-1 had a labyrinth seal between the the left and right crank, the XS carries the sprocket for the SOHC.

The crankcase were designed also horizontal-split with cast-iron inserts for the bearings, 3 roller and a ball-bearing on the right hand side. Lubrication system was wet sump type. Because of the deep crankcases, only the clutch was dipped in the oil. So the function was similar to dry sump.