John, Scott Logan is pretty helpful supplying info and parts for his lathes. You can't really expect him to give you specs for a part that he sells. I don't have a Logan right now so I can't help you there. Maybe one of the other members that has that model lathe might be able to help you out. You should be able to use a finish tap with the existing threads in the half nuts. I would think if you cut the threads deeper (obviously if the threads are worn they will have to be cut deeper) you will have to make them close tighter. I'm trying to remember if you can adjust the lever so you can pull them in tighter....seems like it would just be a matter of locking the lever down in a different position on the shaft. Sounds like it's doable.
Chuck
Chuck