Abstract: A self adjusting ball valve assembly having a ball valve body, a shaft assembly or shaft mounted to the ball valve body, a spherical segment carried by the shaft, and elements for adjusting the position of the shaft. The ball valve body has a port for fluid transit therethrough and a first through passage, located concentrically about a body axis. The shaft rotatably mounts through the first through passage along a shaft axis, which is maintained parallel but offset from the body axis. The spherical segment mounts to the shaft and radially extends from the shaft axis within the ball valve body, and includes a portion for sealing engagement with the port, upon rotation of the shaft through a predefined angle. The adjustment elements automatically adjust the offset between the body axis and the shaft axis to enhance the sealing engagement between the spherical segment and the port.