The top sink drain pipe was rusted out at the joint and broke. I am struggling to get the hub cleaned out so I can reuse threads for a threaded pvc connector. I was able to chisel a little of the old threaded pipe out at the front but still can't get to the threads because its caked in so thick. I'm worried the threads won't be usable. Any suggestions to scape/chisel/clean this and save the threads? If threads are unusable is the only solution to cut out mainstack hub?