I don't have the factory head thickness. But AW's data seems plausible. We have tried to measure the thickness of just the mating surface of an M54 cylinder head but there is not a smooth, recessed surface on the combustion side of the head from which to take a measurement, which could be compared to several other cylinder heads which have never been milled. All of the recessed surfaces are rough, casting surfaces, which are not good surfaces to measure from.

I have sent 15 M54 cylinder heads to my machinist and he looks and determines how many thousandths of an inch he will need to remove to deck it, and WHETHER that measurement will cut too close to the valve seats.

The two stock M54 head gasket thicknesses are 0.70mm(factory) and 1.00mm(after resurfacing). By inference, BMW thinks you can take, as a maximum, the difference of 0.30mm (approximately 12 thousandths of an inch) off of a cylinder. head.

i have discarded 4+ cylinders heads because they needed more than 12 thousandths removed, or had been previously milled too much, etc. I have also 3 times had a cylinder head "baked" in an oven by a machinist to remove most of the warp before machining. That process was too expensive...about $800 per head

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'03 530i - manual swapped - 425k miles
'06 325Cic auto 115k
'05 X5 3.0 auto 158k
'99 E39 528i 5-speed 130k