A downhole tool including a ball valve having a pivotal ball and providing full drift clearance through a fluid pathway of the valve over a portion of a pivotal range of the ball is described. An internal bore of the ball includes a first opening larger than a second opening, and the internal bore transitions between the first and second openings. The first opening overlays or encompasses an opening formed by a first bore portion over a range of pivot of the ball, and the second opening may be fully encompassed by an opening formed by a second bore portion over a pivotal range when the internal bore is not centrally aligned with the first or second bore portions.