Yamaha Outboard Motors Factory Service Manual F6 and F8, Page 280Get this manual
CAMSHAFT AND VALVES Refacing steps: Valve seat is centered on valve face but it is too wideValve cutter seat 90 cutter Use lightly 30 cutter Desired result To reduce the valve seat width
B Valve seat is in the middle of the valve
face but it is too narrowValve cutter seat Use 45 cutter Desired result To increase the valve seat width