[rotorwrench]

There could be a crack in one or more exhaust valve pockets. I've also seen the center head bolt bore crack all the way down into the center siamese exhaust port a time or two. Most times it's just a leaking head gasket if no defects were found in the block by presure test & magnaflux. One fellow on here had a hole drilled down into the water jacket when someone attempted to remove a broken head bolt.