2001 Mercury Mariner 50-60HP Factory Service Manual, Page 473Get this manual
BIGFOOT GEAR HOUSING
4Lift reverse gear, thrust bearing and thrust washer from bearing carrierReplace thrust bearing and thrust washer if rusted or damaged5Replace reverse gear if gear teeth or clutch teeth on reverse gear are rounded or chippedIf reverse gear must be replaced, pinion gear and sliding clutch should be inspected for damage