2000-2005 Yamaha F40B Outboard Service Manual, Page 346Get this manual
2Adjust: Reverse gear backlash Remove or add shim(s)Reverse gear Shim thickness backlash Less than To be decreased by 0.75 mm (0.030 in) (0.94M)xO.53 More than 1.13 mm (0.044 in) To be increased by (M0.94) xO.53