Question: The photo I sent you shows the self-locking nut on my engine that mysteriously keeps getting loose, allowing my alternator drive belt to also get loose and the tin heat shield that it also holds in place starts to rattle. Can you tell by looking at the photo why this nut and bolt keep getting loose?
Answer: The red arrow I added to your photo is pointing to the cause for your problem. This nut and bolt arrangement is using what is known as a “nylock” nut to keep everything tight. The problem is that the bolt you are using here is just a little bit too short. The white nylon material you can barely see on the inside of the hex nut needs to be completely engaged with the threads on the bolt to provide the locking capability you need. The washers that are visible appear to be a simple flat washers, or possibly a flat washer and a heat shield, with no lock washer. So basically you have no nut-locking in play here and normal engine vibration is rattling things loose periodically. To fix this situation, replace the bolt with one a half-inch (12mm) longer, install a new nylock nut, and you should be all set. Incidentally, nylock hex nuts are only designed for one-time use. Once a bolt’s threads have compressed the nylon in the nut, its locking capabilities are set. If you back it off and retighten it, its locking effectiveness will be dramatically diminished.