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When running a roller cam on a block that lacks provision for a thrust plate, a button must be used to limit the cam's lateral movement. If the cam moves too much, ignition timing can be thrown off, hurting performance. The movement is checked by carefully pushing the cam forward in the block against the timing cover. Want a quick way to do it? Install the cover without a gasket, then set the button so it's against the cover. With the gasket in place, the cam will have the proper 0.003-0.005 inch of end play.