Cracks between the valve seats are considered OK as long as they are less than a certain size. The greenbook gives a spec for maximum acceptable width but generally as long as it is small enough that you can't fit your fingernail in it, the head is considered OK.
Those cracks are very common... I bet that if you pull the head off your D24 there is a good chance you will find it has some cracks too. No big deal if they're within limits -- the motor won't know the difference!
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