I had the same problem only with my power steering pump belt, I ended up getting just a slightly smallter belt, and put a lock nut on it. Never got loose again. I'd recommend having someone push down on the alternator to put tension on the belt while you tighten it, make sure you get both bolts tight, I believe the alternator is the same on all models of 85 and 86. Hope it helps. Just don't go too tight.