Michael, Normal threads are right-handed. Reverse threads are left-handed. MSC (http://www.mscdirect.com) has left handed hex nuts - I didn't see castellated. Bob Awgertoo at aol.com wrote: > I am trying to find a right-hand (reverse-threaded) slotted or castellated > nut in 5/8-18". > Anyone have one of these laying about? > > Best--Michael Oritt