The round threaded piece is a locknut, just without flats. To adjust properly you need to use that as the locknut. It's ugly but you can just grab it with large pliers to do so if you don't have the right tool. But there should be a keyed washer between the round locknut and the upper race in most cases. Another option is to eliminate the round locknut entirely and replace with spacers. If you do enough work to come across them often then buy a tool for the lockring.