Service limit 0.004". But unlike the head, you should be able to remove more surface material. You may then need to use ARP head studs instead. And trim down the dowels. After milling the deck, you will need to make sure that the pistons don't come into a contact with the valves once the timing is set. So, rotation by hand before cranking the engine would be critical. If it does, then you would need to use a thicker head gasket. If it doesn't, you can use oem head gasket, and your compression will be higher.