Exhaust nut & bolt suggestions

[robinsone]

I have a clogged cat (2004) and I will be replacing the downpipe. I believe the two bolts that secure the downpipe to the catback are 15mm bolts (although I've also seen 14mm posted elsewhere). I want to replace these nuts and bolts along with the 13mm nuts on the downpipe to manifold connection. While i'm at it anyone know the thread pitch to these off the top of their head, I haven't looked yet which i could do...

I wasn't sure if grade 8 zinc coated would work well or perhaps something else. I plan on using copper anti-seize as it looks like a muffler replacement isn't too far away. Let me know any tricks or decent materials to use to avoid the rust on the hardware, thanks.

[Fivefingerdeathpunch]

No idea on thread pitch or anything like that.

Just get bolts that fit is what I do. Stainless bolts/nuts or using a anti-seize will work just fine on grade 8 bolts.

[BillBoost37]

If it's an M8 it should be 1.25 pitch and M10 will be 1.50 pitch. GM was good about keeping it all the same.

15mm nut should be an M10 and 13mm should be M8. If it comes out as a 14mm, then it's a rusted 15mm.