Yamaha F100B F100C Outboards Factory Service Manual, Page 122Get this manual
Power unit
Refacing the valve seat
1Reface the valve seat with the valve seat cutter
4Measure the valve seat contact width where the blueing dye is adhered to the valve faceReface the valve seat if the valve is not seated properly or if the valve seat contact width is out of specificationReplace the valve guide if the valve seat contact is uneven
Valve seat cutter holder: 90890-06553 Valve seat cutter: 30 (intake): 90890-06326 30 (exhaust): 90890-06328 45 (intake and exhaust): 90890-06555 60 (intake): 90890-06324 60 (exhaust): 90890-06315 2Cut the surface of the valve seat with 45 cutter by turning the cutter clockwise until the valve seat face has become smooth