Abstract: A mechanical closure assist is provided for a valve having a valve casing with a wall and an internal surface defining a valve seat. A disc is pivotable within the casing from an open position to a closed position. The disc defines front and back faces. The front face defines a sealing surface for sealingly contacting the valve seat when the disc is in the closed position. A reciprocating rod is movable at least partly laterally relative to the flow path, from a retracted position clear of the flow path to an extended position where the rod bears on the back face of the disc to urge the sealing surface of the disc into engagement with the valve seat. The rod advancing structure can be mounted directly on the valve casing or an adjacent section of conduit. The rod can be inclined or the rod can be provided with a wedging shape such that lateral advance of the rod urges the disc sealing surface closed in a direction parallel to axial flow through the valve.