I'm aware that you can mill 2 thousandths off the head and still be in factory serviceable limits for a head. My question is .002 off of what?

 

Most machine shops will indicate the head and redeck it taking off .002 to .005 to get a true surface.

It’s taking off anything that will keep the head from sealing to the new head gasket. A lot of videos on YouTube if you want a step by step and see how it’s done.

 

What if I don't know the history of a head and I need to make sure too much hasn't already been milled off? It's already been milled once, don't know how much. Can't find any specs on how to check it, or even what it should be.