Check warpage: Poor man's surface plate. A piece of glass or the cast iron machined portion of a table saw/. Absolutely clean! Place suspect warp surface down and attempt to slide feeler gauges between the flange and surface plate. Should not be over about .010" air gap. Attempting to crush several gaskets into a warpage mostly does not work IMO. If the thing is warped bad you can feel the problem by rocking it or looking for a "light band."