I used to get some special nipples for some people that had jimmed the threads or had very worn or oversized threads and it was the base not the top that was oversized, I had to use a special oversized tap to cut the threads to the right size. I do not know if they are still available as I have been out of the business since 1991 but I can tell you that they would not screw in to a normal 1/4 X28.