when you said the head checked out ok..what was done to check the head...was the vales taken out and bowls,etc checked.. ? Mine also checked good on the initial rounds of pressure checks but after much closer with the inspection ( with dish washing liquid) we saw the tiny bubbles coming from the crack in the bowl itself.
it was seeping so slowly that it would pass the Pressure checks.... bit would eventully seep enough water into the cylinder once the head got hot........if your heads have had any port work at all i would start where the work was done..
One thing for sure its soming from the heads or the block ..lol
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i would really look close at my heads before i got into the block...
Terry my hairline crack was so small it was almost undectectable.
just telling you how i would handle if it were my motor.
hope you find it.
also what brought you to conclusion the problem was in the exhaust seat valve #4 ?
thanks brian