Have a coolant leak on a to30 cylinder head. Engine is a newly rebuilt Z129 continental. Coolant initially shows up externally around #2 and #3 spark plugs, eventually puddles beneath the plugs a runs down the side of the block. Leak is a slow one, takes several days before it runs over the edge and down the block. Goes away when the engine runs and warms up, but always returns after a few days of not running. No signs of any coolant leaking internally. Head was checked for cracks and surfaced during the rebuild process which showed all good. Wondering if the checker missed something, etc. Has anyone else had this problem? All I can figure is a hairline crack around these 2 spark plug ports.