1998-2006 Yamaha F20/F25 Outboards Service Manual, Page 157Get this manual
_ SERVICE POINTS
Cylinder head inspection 1Measure: Cylinder head warpage Out of specification ? Resurface or replace the cylinder headWarpage limit: 0.1 mm (0.004 in) Resurfacing steps: Place 400600 grit wet sandpaper on the surface plate Resurface the cylinder head using figure-eight sanding pattern