2005 Yamaha F90TR 90HP Outboards Factory Service Manual, Page 97Get this manual
Power unit 8If the valve seat contact area is too narrow and situated near the bottom edge of the valve face, use 60 cutter to cut the bottom edge of the valve seatIf necessary, use 45 cutter to center the area and set its width
5Use 45 cutter to adjust the contact width of the valve seat to specification
S69J5900
b Previous contact width Specified contact width
6If the valve seat contact area is too wide and situated in the center of the valve face, use 30 cutter to cut the top edge of the valve seat, 60 cutter to cut the bottom edge to center the area and set its width
b Previous contact width