1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 173Get this manual
CYLINDER HEAD
VALVE SEAT POSITIONS AND CORRECTIONS Condition: The valve seat is centered on valve face but it is too wide
Valve Seat Cutter Set Use Lightly 15 Cutter 60 Cutter
Desired Results To reduce valve seat width
Condition: The valve seat is in the middle of the valve face but it is too narrow
Valve Seat Cutter Set Use 45 Cutter