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MikeK · 06-29-2015, 10:59 AM

Most guys will tell you a block deck (or head) 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!

FWIW, most head gaskets will accommodate very slight distortions, up to 3 or 4 thousanths.

06-29-2015, 10:59 AM	#2
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	Re: Checking Head Surface for Flat? Most guys will tell you a block deck (or head) 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! FWIW, most head gaskets will accommodate very slight distortions, up to 3 or 4 thousanths.