head gasket

The block's head bolt holes have to be tapped prior to the final washing. Especially if the block was decked and the heads milled. I like to make sure the bolts will snug to the heads with no gasket in place. Because if they bottom out in the hole without compressing the gasket you get what you have.. ugliness. If you didn't do this, then yes - you need to pull the other head too... Use a bottoming tap, clean and blow out the holes, and either replace the bolts or verify they are not bottoming out.