Mercury Mariner 30/40 4-Stroke Outboard Service Manual 1998, Page 210Get this manual
CYUNOER HEAD
Condition: The valve seat is centered on valve face but it is too wide
Valve Seat Cutter Set
Desired Results To reduce valve seat width
Lightly
15 Cutter 60 Cutter
Condition: The valve seat is in the middle of the valve face but it is too narrow
Valve Seat Cutter Set
Desired Results To achieve uniform valve seat width