It is a 3/4"? National Coarse thread. (or in USA COMMON OR GARDEN VARIETY 3/4" BOLT.It is much cheaper and easier to find a length of 3/4NC stud or threaded rod and saw some slots along the length across the threads. Run this up into the old casting from the underneath. Because it is (corroded?) aluminum, this will clean up and re-cut the threads. The slots give the studs'threads a cutting edge and room for the trash to go. Borrowing a 3/4 NC tap is also good, but you will have to extend it to get it up there and turn it. (Piece of pipe with the end beaten square?)