have the heads had any work done to them ?..i have seen over agreesive port work done where the exhaust valve bowl developed a hair line crack..you could not see by looking at it...i did a backyard pressure test..DAWN dishwashing liquid and a air pressure hose...showed right up.
it did not run hot..when the motor got warm the crack in the heads would expand and leak fluid into the #8 cylinder..after it sat for a while the cylinder would fill up...
if there was any epoxy work done to the heads it could have come loose...SO DO NOT PUT ALL THE EGGS IN THE BASKET ON THE BLOCK BEING BAD AT THIS POINT..
brian
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Bjuice..
"I'M YOUR HUCKLEBERRY ! "