I called up Hardin yesterday and they say the use grade 5 bolts for their Gil/Hardin aluminum exhaust manifolds. He said that stainless fatigues after a while. I could of swore that stainless marine sent me stainless bolts for my exhaust I had years ago. What should I use? grade 5 or stainless?

Grade 5. Although Stainless is much more corrosion resistant, it is inherently weaker and more brittle then Grade 5 or higher.

I have used both with no problems. If you are in salt use a dab of never seize, and always re tighten after the first few hours of use.

Anyone ever use stainless without problems? I already have the stainless bolts, just wondering if I'm going to have problems.

I don't know, unless leaving the motor open like in a Flat bottom V drive boat or something I think I will be using what Merc uses (reg steel).

There should not be any problem using the SS bolts. Factory HP500's and 525SC's with Gil exhaust used SS hex cap bolts. I used ARP SS studs and nuts on CMI's. No issues with either one and they don't rust.

yep i do,first get all the legnths you need then call arp and order what you need..i did mine 12 yrs ago with arp which are 316 hardened stainless steel bolts..they look nice also..

I will second that and add: You can custom order them to be drilled for safety wire. If you can't get to the header bolts easily once the motor/motors are in place, the best way to insure that they stay put-------safety wire them. IMHO

i use stainless,i get them from MCMASTER CARR.

I've done both. Stainless doesn't seem to do well over time at high temperatures. It stretches out, and you end up re-tightening them until you snap one off. The ARP stuff is probably much better than you're run-of-the-mill 18-8 bolts.

What I'm using these days is Grade8 Ultra-Coated, available from McMaster. They are rated for up to 1000 hrs continuous exposure to salt spray. They have sort of a galvanized color, but it's more of an anodizing. Great luck with them so far. I went to using them for engine mounts, and other things too where bling is not needed, but strength, ability to hold pre-load and reliability is.

I have stuff with them that's been taken apart and reassembled multiple times, and the coating is still good. These fasteners give you a much better feeling, as you know positively when they are down tight, and they don't keep stretching like stainless.