I was wondering the nut #12 and turns into the thing to the right of it (not sure term to call it, its called a Bolt Stud in pic below ... I guess its kinda like a bolt without the top hex part)...

http://www.boats.net/parts/search/Yamaha/Motorcycle/1978/XS400E/EXHAUST/parts.html

Think its labeled as #30 here in the pic as well:

[img width="200"]http://www.boats.net/images/diagrams/yamaha/02/0720/0002.png

Part #: 90116-08046-00 BOLT, STUD (823-13558-00)

Anyone know where to buy that? my bolt stud/nut are pretty rusted so i was going to replace them with something new (only saw the nuts on boats.net).

just an exhaust stud. I replaced mine with stainless and aero nuts (reusable lock nut)

if you just want ferrus steel, the're pretty common and don't have to be yam.