Abstract: A retrofit assembly for a chlorinator has a cylindrical disk portion having a lower region for insertion into a cylindrical opening of a chlorinator cylinder, a groove for receipt of a first O-ring and a flange at the top to abut the cylindrical opening, and an upper cylindrical member attached to the lower region having exterior threads thereon, a central bore passing through the cylindrical disk portion for passage of chlorine tablets into the chlorinator cylinder, a cylindrical collar with threads for engagement with threads on the chlorinator cylinder, the collar configured to fit over the lower region of the cylindrical disk portion, and a cap having a cylindrical section with internal threads for engagement with the exterior threads of the upper cylindrical member to cover the central bore of the upper cylindrical member, wherein a recess formed in the cap has a bottom for receipt of a second O-ring.