I purchased a Craftsman 5163 vise several years ago and it had exactly the same problem. I completely restored the vise, and in the process of doing that eventually found a replacement bolt for it. I believe that I actually purchased two center bolts for it, and may still have the second one somewhere. I will have to look. If memory serves, I believe that it took a fine pitch 7/8” thread. Does you vise look like it takes a thread of that size?