2001 Mercury Mariner 50-60HP Factory Service Manual, Page 419Get this manual
NON-BIGFOOT GEAR HOUSING
6Inspect impellerReplace impeller if any of the following conditions exist: Impeller blade(s) are cracked, torn, or wornImpeller is glazed or melted (caused by operation without sufficient water supply)Rubber portion of impeller is not bonded to impeller hub
7Remove plate and gasket8Replace plate if groove(s) (other than impeller sealing bead groove) in plate are more than 0.030 in(0.762 mm) deep