Oh boy, I know that one !! Especially if the engine is still in the boat, and you have to lay across the engine and work almost upside down to drill out the bolts, and its 95 degrees out, and it takes 3 days and $50 worth of cobalt drill bits. Thats why I never want to repeat that problem again !!

I'm with Chris on using the hardened steel bolts on anything like mounts or manifolds. I'd like a metal guy to explain why, but on the commercial jet boats and even the marathon race boats I've worked on almost any stainless fastner under load will snap. These boats are subject to continuous high frequency vibration from their jets that breaks all kinds of stuff so it might not apply to most guys, but it seems like the harder stainless is more susceptible to fatigue. I have seen everything from exhaust maniflods to alternator mounts and idler pulleys bust right off with the best quality stainless available..

HaxbySpeed- I am speaking to the ARP guys, and they claim that the stainless can be used in this application (at least so far, but I am pressing my case).

302, 304 and 316 stainless are the common materials in regularly available marine hardware. They have a strength just slightly less than Gr2. Not good for things like motor mounts, pulleys, etc. You can buy "hardened stainless" from people like ARP and Totallystainless.com. They're made from materials like 17-4. The downside is they're very expensive. I looked on McMaster-Carr and a common motor mount plate bolt- the 1/2" x 1.5" length is over $9. I can conversion coat then apply dry-film to them for pennies each.

But the coatings can wear off or flake off, correct ? no such problem with stainless hardware.

That'd be cool if I could get some stronger stainless bolts. I talked to the local stainless fastener dude and he said the strongest stuff they can get is Gr2. I think a lot of people would pay the extra $ for the shiny stuff.
Chris could you elaborate on your coating technique, and what products you use? thanks, Alex

No, they do not. Conversion coatings are a part of the metal. It's a chemical oxide. The dry film coatings that we use don't come off. The can be removed but with serous effort. For a motor mount or header bolt, the resin in your hull will be dust before the coating comes off.

We use a basic oxide technique called zinc or manganese phosphating. The military calls it parkerizing. It's a good, old fashioned steel treatment process that's worked for centuries. Over top we'll use a dry-film lube- typically a moly or ceramic. We get them from lots of different vendors but Techline has a wide variety and good pricing. They'll only sell certain ones over the counter- others are for professional application only.

Start adding up all the bolts on your boat that you might want to replace, then figure $5 to $10 each for 17-4's, plus nuts & washers. Then yoiu'll have a better understanding of why airplanes are do expensive.

Which bolt coach?
 

Proper grade stainless bolts of proper strength are great for external use on marine engines, just always remember to use a small amount of premium anti-seize when first installing or re-installing any bolt externally on a marine engine, especially if you are bolting to aluminum threads with a stainless fasteners or bolts. It also helps to remember to use that good old torque wrench!

Best Regards,
Ray @ Raylar

When I change or add new exhaust I have stainless studs made. It helps that my brother is the shop superintendant of a large sheet metal shop that also owns a machine shop and I get them for free.