you prob need 1 1/2 pipe thread to 5/8 's NC ( this would have to be made you can't buy it
) if I am picturing on what you want to do as the threaded part on the float is a straight thread with a standered nut to hold it in place and you want to sick it through the 1 1/2 cap right. If you pick up a threaded 1 1/2 plug ( I think they are thicker) you can drill and tap it with a 5/8 tap. Put a nut, washer and a rubber washer on both sides and this should seal it up. Use a nylone (sp) or SS washers and nuts
I will try to find some pics
