That is a drain for oil that gets past the top main bearing. A slinger ring on the crankshaft throws the oil into a groove above the bearing that is channeled to that pickup tube. An early example of recycling. The center bearing is the slow speed rotary valve. The piston skirt acts as the valve for high speed operation by uncovering the third port behind the carburetor. Dammage or excessive ware to the piston skirt will efect the power output.
