To mill the head you don't need to take the pistons out or even touch the block for that matter, thats why it's a cheaper way to raise compression than buying new pistons.
I'd rather spend the money and just buy new pistons than milling the head to it's minimum service limit. What if you need to do it again later?
If you replace pistons, you need to do the necessary prep work ie honing, etc. If you re-ring the motor, same thing. But why put stock pistons back in for?...
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