Near number 1 and 6 exhaust outlets are two flat areas a little larger than the rest. There should be a recess the size of a dime, if ground down so they disappear you have gone to the max or beyond.
88 and before is a pencil eraser size "dimple" cast into the head. Quick indicator whether a head is thick enough to use or surface. When it's gone the head is below standard size.
BMW suggests maximum of 16-17 thousandths of material to be cut.
In the pic it is at the bottom just to the left of the exhaust stud
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