Stress fractures from varying heat expansion rate is a plausible explanation. It never showed up in the factory one thousand hour test engines and I have never heard of any catastrophic failures because of it. There was a company in the mid ’50s that made and promoted a Water ski tow rope attachment that bolted to the narrow section of the Exhaust housing. These quickly developed cracks at that point in the housing which caused OMC to issue a warning that the use of this device would void the factories warrantee.
