Stainless Bolts on Exhaust Manifolds
 

Is there any reason NOT to use stainless bolts on my Exhaust manifold? I thought it would make it easier if I have to remove them in the future and cause less of a problem with rust. However I got to thinking (ussually a dangerous thing) and thought that there may be a reason not to do this like dissimiliar metals or something.

Jon

Ive used socket head (allen) cap screws for years against gil manifolds with no problems. I think the socket heads are easier to install and tighten than hexhead bolts. By them from McMaster Carr. Oh, I used flat washers too.

My Imco's came with Grade 8 bolts and no washers. I think the SS bolts may not be strong enough. If they are in there a few years and start to sieze you could break them trying to remove them. Also due to the SS being softer metal it will make the head easier to strip.

I replaced all my original bolts with SS on intake and exhaust manifolds. Haven't had any problems. In fact on my other boat, the volvo penta outdrive, which in my opinion is engineered better than mercruiser, uses all stainless socket head cap screws. What a joy to work on, even with just under 1000 hrs run time and generally kept in water from April to November.
Grade 8 bolts are typically misapplied because people think they are stronger. They are in fact harder and therefore you can get more clamping force, but the drawback is they are brittle. That makes them unsatisfactory in applications where the clamping surfaces are not perfectly parallel and a side load is applied to the underside of the bolt head. When you add vibration to this scenario you get breaking bolts. Most surfaces on engines are not perfectly machined and therefore a bolt tough enough to bend is the best choice, usually a grade 5.

My .02

BT

Use an anti seize lube and they will be fine!!

Oh yea, Neversieze for sure. Messy but it works. Especially any stainless to aluminum threads.

Since there's differing opinions here I'll toss another factor in. SS is an OK choice for header bolts but you need to check them more regularly than CS (carbon steel). Since SS has a fairly high chromium and nickel content it also has what is refered to as a high "carbon equivalent". Carbon is the element that determines an alloy's hardenability. Other alloying elements equate to a "carbon equivalent" which is used to compare hardenability.

So, with lot's of alloying elements like SS has, it is very hardenable and therefore will become brittle when hardened. What causes harndness in a hardenable material? One) heat treating, or two) work hardening. What does this have to do with header bolts since (even though they get pretty hot) it doesn't get hot enough to change the molecular structure (heat treatment)? Tightening a bolt causes it to stretch and bend; in essence to "work". A material that work hardens readily will get harder and therefore more brittle each time it is "worked". So after tighteing a SS bolt a few times it may get hard and not provide the required stretch to ensure that it will stay tight over time. Alls you gots to do is check them, and if they don't stay tight after given a chance for the gasket to set, etc, you may have to replace a few. A small price for nice clean header flanges in my mind.

OK - here's a quiz (since I've so effectively transported my mind back to my metalurgy classes and I'm in a fairly geeky mood):

Why is SS so difficult to cut gently but can be cut rather easily using aggressive tools?

:confused:

OOOO, ooo, me, me, pick me, I know this one!

Its because SS work hardens if you take a small cuts. You need to takee heavy cuts to continue getting into fresh, un-work hardened material.

I've never had a SS bolt break on me due to repeated tightening. That is an intriguing thought though. I can see how that could happen if a SS bolt is torqued to the upper end of it's elastic limit. I thought the work hardening process didn't start until the material passed it's yeid point? Therefore if torque properly the hardening shouldn't ocurr.

Great topic!

BT

Use a grade 5 Carbon Steel Bolt. They are much more forgiving than Stainless Steel and always apply a graphite anti seize to the threads. I am very carefull when it comes to connecting dissimiliar metals and I don't do it unless I have to. You can experience galling with SS threads if conditions are not right, and that can get real ugly.

I have always replaced about every bolt with SS also always in aluminum for corosion reasons. never has there been a problem.
One thing carbon grade 5 bolts usually have cut threads SS are usually rolled and dont chew up aluminum as easy.