If your engine's ever encountered severe overheating or head gasket failure, there's a good chance the cylinder head or block is warped. Unless you're Superman, there is no physical way to visually inspect for flaws or feel a cylinder head for flatness, so the only correct way to measure your cylinder head is to use a straight edge and feeler gauge. Aluminum cylinder heads and blocks require flatness checks prior to reassembly and gasket installation to ensure they are not out of factory specification. Most aluminum heads using a multi-layer steel (MLS) head gasket should have no more than 0.002-inch (0.05 mm) of total distortion (block plus head combined) in any direction.
Before examining the cylinder head, the surfaces must be cleaned (with brake cleaner and clean paper towels) of any gasket debris, carbon deposits, or silicone that would otherwise affect the measurements. We used a Snap-On GA438A 24-inch straight edge to check our '03 Subaru WRX cylinder heads.
To use the straight edge, place it on the face of the cylinder head or block, then use a feeler gauge to check any gaps between the straight edge and the surface. When checking for flatness, avoid running the feeler gauge butt-end against the straight edge. This will curl or warp the feeler gauge, preventing entry under the straight edge, resulting in incorrect readings.
Lay the feeler gauge leaf on various points of the head as highlighted in our photo, and place the straight edge on top of the leaf. A very easy pull or looseness of the leaf under the edge will indicate a depression or a possible sign of warpage of the surface. If the clearance between the straight edge and surface exceeds the engine's maximum limits, the head or block is not flat enough to hold a good seal and should be resurfaced by a machine shop, prior to reassembly.