The sprag failure isnt necessarily due to the outer race tearing loose, although this is possible. More commonly the sprag roller(s) migrate beyond their pocket, into the next. Once this happens the locking force is gone and the inner race can rotate, allowing the drum to overspeed and explode. A bolt in sprag wont prevent this. A sprag with more rolling elements is better.
Here is a pic from a running 727 that I tore down that was behind a stock 318 poly its whole life. You can see the finger spring retainers are bent and one of the rollers has migrated. This is the start of a failure.
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