Tohatsu 4 Stroke MFS 2/2.5/3.5A Outboards Service Manual, Page 100Get this manual
34) Inspection of Valve Seat 123Remove carbon built up on the valveApply thin coat of red lead on the valve seatUse valve lapper (commercially available item) as shown to push valve onto valve seat lightly4Measure width of area where valve face contacted with valve seat that can be identified with red lead adhered to valve faceCorrect valve seat if contact area is above or below the center or contact area of valve seat is over specified limitValve Seat Contact WidthStandard value Intake and Exhaust Sides1.0 mm (0.04 in) Functional LimitIntake and Exhaust Sides2.0 mm (0.08 in)