Not common, but it does happen. Mostly on the face of the block deck around the head bolt holes. It might be more the Conquest/Starquest/Starion engines due to boost or overtorqued bolts(?) Another place the blocks have been known to crack is the exhaust side of the block between the freeze plugs (this is on early Astron I 2.6 blocks) Generally the 2.6 block is a tough chunk of metal...