1983 Yamaha 30EN Outboards Service Manual, Page 63Get this manual
INSPECTION AND REPAIR Cylinder head 1Using rounded scraper, remove carbon deposits from combustion chamberTake care to avoid damaging spark plug threadsDo not use sharp instrumentAvoid scratching the aluminumCorrect by re-surfacing as follows: Place 400600 grit wet sandpaper on surface plate and re-surface head using figure-eight sanding patternRotate head several times to avoid removing too much material from one side
2Thoroughly clean the cylinder head water jacket of salt and dirtPatch up any part from which anti-corrosion paint has come offReplace the cylinder head if its inner surface has any corroded areaCheck the cylinder head warpage with straightedge as shownThe warpage shou Id not exceed the specified limit; if necessary, resurface the cylinder headCylinder head warpage: less than 0.1 mm (0.004 in)