Hey everyone, I'm having trouble keeping my downpipe connected to my ss 02 housing.
The the bolts keep vibrating loose. I tighten the heck out of it and they still loose. Anyone use certain nuts and bolts?

Lock washers along with larger flat washers, maybe some mid duty loctite.

I think some people use Nord lock washers. I've re-torqued my bolts several times when hot and cold after install and haven't had any issues with them coming loose,

Always had OEM flat and lock washers and the nut. Never had vibration issues causing them to come loose.

Does the nut feel loose while you’re threading them in? If so, maybe try new nuts.

Split lock washers. They keep tension on the nut so it won't back off due to vibrations. When I bought my ARP stainless bolt kit for the FP Race manifold from STM they only included flat washers. I hit up my local Ace hardware and grabbed some stainless lock washers and haven't had any issues with them backing out.

On a side note, I'd also suggest using some Nickel anti-sieze to make sure you don't break anything if/when you need to take it back apart.