A tool for installing a retainer ring onto the lower portion of a loosely inserted drain fitting for engaging a mounting flange thereto includes a generally resilient cup structure provided with a plurality of axially aligned and resiliently cantilevered peripherally adjacent strips within which the retaining ring is received in a compressed engagement seated on a helical interior shoulder. A threaded rod extends through a centering plug received in the drain fitting and then through the cup interior to axially advance the axially aligned ring carried by the strips.