some of the later 318 & 360 blocks require sealer on the head bolts. these engines will have a decal on the valve cover with directions. These headbolt holes do not enter the water jacket, however they will stress crack on the deck surface towards the water jacket. this problem usally don't affect the earlier blocks, although i just put together a 1972 340 block that had this problem. i put silicone sealer on the head bolts & have yet to have a problem.