I am looking to use high quality hardware to secure my ART pipes to my stock manifold. I'm going to be tightening my bolts because I have a leak somewhere and thought I'd replace my current cheap home depot stuff with good quality hardware. The size of the bolts I have found is (8) -3/8 x 1.5" Bolts
(8) -3/8 x 1.5" Nuts.

From a search, I found a user who used:

1) ARP SS bolt = 3/8-24, 1.5" length, 7/16" hex socket, (725-1500)
2) ARP SS washer = 7/16" ID, 13/16" OD, 1/8" thick (400-8509)
3) ARP SS nut = 3/8-24, 7/16" 12pt socket, (400-8302)

These are quite expensive for hardware. What is everyone else using? I tried my local Fastenal with no luck.

 

You could probably use metric since the originals were metric. Guessing they're either an M8 or most likely, an M10. Stainless have the lowest tensile strength but more than enough for what you're doing. Otherwise I'd stick to an 8.8 as it won't be a major pain in the *** to cut them off if that ever happens.

Bolt grade chart:
https://www.boltdepot.com/fastener-i...ade-chart.aspx

Metric hex bolts:
https://www.boltdepot.com/Metric_hex_bolts.aspx?nv=l

If it's really tight in there, and it is, you may want to consider running one of the metric JIS bolts as the heads are smaller. More room to get a socket on it. But you'll need to use a quality 6pt. for removal.

Make sure to check and see if you need a hex or a fully threaded tap bolt.