2009 Yamaha F40 Outboard Service Manual, Page 220Get this manual
Camshaft and valve If the valve seat contact area is too narrow and situated near the top edge of the valve face, use 30 cutter to cut the valve seat top edge, and then use 45 cutter to center the area and set its width 0 2
b Previous contact width 5Use 45 cutter to adjust the contact width of the valve seat to specification
b Previous contact width If the valve seat contact area is too narrow and situated near the bottom edge of the valve face, use 60 cutter to cut the valve seat bottom edge, and then use 45 cutter to center the area and set its width