A molten metal pump assembly to fill a mold with molten metal comprised of aluminum, the pump assembly comprising: an elongated shaft connecting a motor to an impeller , the impeller being housed within a chamber of a base member such that rotation of the impeller draws molten metal into the chamber at an inlet and forces molten metal through an outlet of the chamber, the impeller including a first radial edge spaced from a second radial edge such that the first radial edge is proximate the elongated shaft ; and a bearing assembly surrounding the impeller within the chamber , the bearing assembly including: a first bearing ring adapted to support the rotation of the impeller at the first radial edge ; a second bearing ring adapted to support the rotation of the impeller at the second radial edge ; characterized by at least one bypass gap to a surrounding environment interposed between a portion of one of the first and second bearing rings and the associated first and second radial edges , wherein the bypass gap comprises a width between the second bearing ring and the associated second radial edge of the impeller which is at least about 1.25 x greater in width than a lubrication gap between the first bearing ring and the first radial edge of the impeller, the bypass gap being operative to manipulate a flow rate and a head pressure of the molten metal. an elongated shaft connecting a motor to an impeller , the impeller being housed within a chamber of a base member such that rotation of the impeller draws molten metal into the chamber at an inlet and forces molten metal through an outlet of the chamber, the impeller including a first radial edge spaced from a second radial edge such that the first radial edge is proximate the elongated shaft ; and a bearing assembly surrounding the impeller within the chamber , the bearing assembly including: a first bearing ring adapted to support the rotation of the impeller at the first radial edge ; a second bearing ring adapted to support the rotation of the impeller at the second radial edge ; characterized by at least one bypass gap to a surrounding environment interposed between a portion of one of the first and second bearing rings and the associated first and second radial edges , wherein the bypass gap comprises a width between the second bearing ring and the associated second radial edge of the impeller which is at least about 1.25 x greater in width than a lubrication gap between the first bearing ring and the first radial edge of the impeller, the bypass gap being operative to manipulate a flow rate and a head pressure of the molten metal. a first bearing ring adapted to support the rotation of the impeller at the first radial edge ; a second bearing ring adapted to support the rotation of the impeller at the second radial edge ; characterized by at least one bypass gap to a surrounding environment interposed between a portion of one of the first and second bearing rings and the associated first and second radial edges , wherein the bypass gap comprises a width between the second bearing ring and the associated second radial edge of the impeller which is at least about 1.25 x greater in width than a lubrication gap between the first bearing ring and the first radial edge of the impeller, the bypass gap being operative to manipulate a flow rate and a head pressure of the molten metal. <EOS>