2007-2009 Yamaha F15/F20 Outboard Service Manual, Page 180Get this manual
Lower unit Reverse gear backlash: 0.951.66 mm (0.03740.0654 in) 15Adjust the current shim thickness if the reverse gear backlash is out of specificationReverse gear backlash
S6AG6540
11Install the propeller, washer, and nut, without installing the spacer 4
Shim thickness
Less than To be decreased by 0.95 mm (0.0374 in) (1.31 M) 0.43 More than To be increased by 1.66 mm (0.0654 in) (M 1.31) 0.43 M: Measurement Available shim thicknesses: 0.1, 0.2, 0.3, 0.4, and 0.5 mm 16Remove the special service tools, and then install the water pump assembly17Fill the gear oil to the correct level