The original thickness is 3”. The factory originally said they could be skimmed by 10 thou, and it has been suggested that 20 thou is quite ok. Indeed I have seen 100 thou mentioned but that would be a brave man.
The other point to consider is that if too much is taken off, the inlet manifold bolt holes will not line up, although of course they can be enlarged. I also believe that thicker head gaskets have been used.
Best wishes
Stewart
