check the head for warpage with a straight edge and a feeler gauge.
If you cant do this take it to a machine shop and they can check it for you.
The FSM will say what the max warpage allowed is, normally about 0.004 inches
It also will tell you how much can be removed from the head to give a true surface.
2003 Pathfinder from 2004 SE 4WD
1997 SER 5sp Manual from new Rear Ended 2009 !!
Gone to the Knacker yard...
New ride 3.0 manual RWD not a Nissan !!!
1997 Sentra GXE from new .... Sold 2008
2001 Sunny N16 RHD from new (N16 Nissan Bluebird Sylphy) Sold