2000-2005 Yamaha F40B Outboard Service Manual, Page 238Get this manual
IpOWRI 411: CYLINDER HEAD Valve seat cutter set YM-91043-C90890-06803 Cut section as follows Section
C Refacement steps
[AI Valve seat is centered on valve face but
is too wideValve seat cutter set Use lightly 90 cutter 30 cutter Desired result To reduce valve seat width