Most late-model vehicles use cylinder heads made of aluminum because of that metal's light weight and durability. This comes at a cost, as aluminum heads are more prone to developing imperfections in extreme circumstances. The mating surface of the cylinder head must be clear of cracks and scratches in order to create a reliable seal with the head gasket. Fortunately, aluminum is a soft enough metal that scratches can be removed by sanding, in order to ensure a perfect surface.
Clean the head as thoroughly as possible with an engine solvent. Sand the scratch with a belt sander loaded with 80-grit silicon carbide paper. Do not apply any downward pressure, as this will generate heat and warp the head, in addition to causing deviations in the otherwise level surface. Make gentle passes until the scratches are removed.
Lay a straight metal ruler or bar on its edge against the sanded surface, along the length of the cylinder. Using a series of feeler gauges, determine the flatness of the sanded surface by inserting the feeler gauges between the ruler and the head.
Run the belt sander gently along the entire length of the cylinder head to correct any excesses variation in flatness. Depending on the layout of your engine and the gasket manufacturer's specifications, the maximum allowed gap is between 0.003 and 0.006 inches. Continue to make gentle passes with the belt sander until the maximum gap falls into this range.
Test the flatness along the width of the cylinder head at several points, using the procedure in Step Two. If there is any flatness variation greater than 0.002 inches across the width, correct this by sanding out the high areas.
Inspect for any sudden irregularities in the mating surface. If there is any local surface variation greater than 0.001 inches, sand this out.
