To Carl's point, many head gasket manufacturers recommend doing a re-torquing after the engine has warmed up when installing a new head gasket by backing off the head bolts by 1/4 turn one bolt at a time and re-torquing at the correct spec...90 foot pounds for a 235...using the normal head bolt tightening sequence. My guess is that many head gaskets get installed on a cold engine and never get this done.
Matt
