Originally Posted by Stu @ M Developments

The minimum recomended head thickness is 138.68mm and this is because after that you are leaving too little material around the valve seat area, which are, as many of you will know, a totally seperate part pressed into the head. Ask anyone who has a had a valve seat come loose in their engine why you dont really want it to happen to you. LOL

I have used YB heads a LOT thinner than that, but you do so at your own risk of course.