You can try some Loctite thread locker ( the blue is medium hold) on the bolt threads to prevent the bolt from loosening. You can also place a lock washer under the bolt head to keep it from turning. But, if you're having ongoing problems with this bolt loosening, it may be that the female threads are stripped. I'm not familiar with the engine, but whether the bolt bolts into a bracket or into the alternator itself, you may be able to use a longer bolt and put a nut and lock washer on the end. I've done that , and it works fine.