If you are just cleaning your threads, buy a oxygen sensor bung for $2.50 from Napa, then also get a small file, and file some groves into it, and use it as a thread chaser. This is only if you can't find one. This is an od ball size for tap and dies. Just went through the same cause I needed to make up some custom nuts for a machine, I was able to find some large size 02 bungs that worked perfect. Walker makes the bungs for Napa, the regular ones are the shape of a nut so you can get a wrench on them. I was able to find some 3/4" tall smooth ones too.