[MikeK]

Most guys will tell you a head or block deck can be checked for flatness with a machinist's straight edge or parallel. Unfortunately, this is WRONG!

This distorted plane will measure perfectly "flat" with a machinist's straightedge along any point perpendicular to any edge AND across opposite corners!! It is extremely deceiving and can only be detected with a surface plate or more advanced electronic measuring that 'feels' or scans the entire surface.

Unfortunately it is also possible to produce this error while machining a block or head to correct an out-of -flat condition! It is very easy to instill a torque on a part when clamped for machining. The result is a distorted planar surface like the one above that may NOT be detectable with just a straightedge! You send a head out to be machined, it checks out with a straightedge, but it is like the above illustration!

It is entirely possible that both the block and a well-used head have distorted into MATCHING profiles over time. A surface plate may reveal head spots several thousandths from flat, but the deck may very well be a perfect geometric match.

Making matters even worse, many guys pull heads that have run for a while and have them planed without checking the block. There seems to be a general belief that heads distort, but not block decks. What was a perfect match of two non-planar surfaces is lost. Then the head gasket gets blamed for short life. Pulling the head reveals it's attempt to assume the block distortion, then the head manufacturer gets blamed for a part that does not hold true.

Truing the block before installing a newly planed head, even in-place by hand, requires pulling the studs. Many guys never want to go that far.

FWIW, most new head gaskets will accommodate slight distortions, up to 3 or 4 thousanths. This accommodation ability is lost if a head gasket is re-used. Once compressed they never recover to their pre-crush condition.