what size is the nut that holds the downpipe to the O2 housing? mine decided to vibrate loose and fall off

Some are 17mm and some are 19mm.

thread size is 12mm x 1.5

Good call miami...

thanks! should save me some trouble

i lost my nuts last week(everybody quote that and make a joke out of it).
autozone had what i needed, which was surprising as hell.
i threw on some lock washers and loctite.. i'm hoping it will hold
i do have a turbo bolt that keeps backing out on me at the exhaust manifold though... if you're looking at the engine from the front, its the back left bolt.. i tightened it up.. next day it was loose again