Not sure on thread size, just remove and match up with longer bolt at hardware store, try to get grade 8, but grade 5 would be fine. Just loosen alternator bolts, as if tightening, and remove bolt. Reassemble with longer lower bolt, tighten belt, tighten bolts and then add nut to exposed portion of longer bolt as added security. And like l say, add flat washer to adjustment bracket side of bolt...it will add surface tension to retainer/bolt.