I ordered from China off the net a 1/4 20 butterfly nut, but it does not fit the 1/4 20 bolt on the tripod plate. Do I need a bigger nut size? I was under the impression the same size nut should fit. Any tips? Or is the ordered nut faulty?
From China, it may be metric then, or at least not 1/4-20 if it does not fit. 1/4-20 is the standard tripod screw thread, which should fit by definition.
Full name of thread is UNC http://en.wikipedia.org/wiki/Unified_Thread_Standard
If in North America, try the local hardware store.
Most tripods are 1/4 and 20 UNC threads, but some bigger cameras have a 3/8ths and 16 UNC thread so tripods exist that have on their head. If what you're looking at though is the base of a tripod, i.e. just the legs and the top plate, but no ball mount or anything (what is often called the "head") then those are USUALLY 3/8. When you buy a tripod head, it has a 3/8 female at the bottom to connect to the actual tripod, then the 1/4 on the top to connect to the camera.