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I just bought all new stainless bolts and washers to bolt on my intake manifold. I replace most all the bolts I take off with new automotive grade. Weird fetish.
Question is--- Is there any issues putting stainless bolts into aluminum?
I spent non-trivial time researching SS fasteners into steel and aluminum a couple of years ago. I was never able to find anything that wasn't open to interpretation. However, my take away from that time spent was that nickel-based anti-seize should be used in that application (and also for titanium fasteners.)
Edit: the primary issue I ran across was galling of aluminum due to SS fasteners.
Note that the "silver" anti-seize that is most commonly referenced 'round here and in the WSM is aluminum-based.
All that written, I've got tons of SS fasteners in D.R.'s twin-screw install and until a few days ago when I put it back in I've used either AL-based or no anti-seize.
The corrosion issue is a trade-off: Any two dissimilar metals will create a battery in the presence of moisture, and one of them-- the aluminum in this case-- will be eaten away. A stainless fastener itself is on the favorable side of that equation and won't corrode, but is father from aluminum than mild or alloy steel, and will corrode the aluminum faster.
Here's a basic galvanic scale-- the stuff towards the left side gets eaten to protect the stuff on the right. In this case the aluminum block will be sacrificed to protect some inexpensive fasteners-- faster with stainless than steel.
As long as things stay dry there is no problem-- sealer on the threads for example.
Stainless is also not generally as strong as steel, but that depends on the specific fastener grade.
But leaks (and rain) happen, I wouldn't be comfortable with stainless fasteners in most applications.
Daniel responded with the appropriate articles. I'm an engineer in the medical field, and it's common to find Stainless steel and aluminum components. Stainless steel is antimicrobial, and aluminum is lightweight and easy to machine.
That being said, our cars almost exclusively use steel fasteners which have been galvanized/plated (cadmium), which is really the safest option for aluminum or magnesium base metal. If you use a 304 SS fastener, which has high anti-corrosion properties, you SHOULD be okay not galling up the base metal threads if you use an anti-seize (as others have already said). Regardless, you must be 100% sure of the properties of your fastener, because if it is low grade, passivated, or martensitic stainless steel you WILL have corrosion and failure in time... sometimes alarmingly quickly if there's repeated exposure to moisture.
There is also a possibility of aluminum oxide forming where the head of the SS bolt makes contact with the aluminum, so if you're worried about that you can use a washer between the SS and aluminum. Just make sure you select a washer type that handle the heat cycles of the engine.
Also, steel fastener coatings like anodization or black oxide don't help with preventing corrosion, and shouldn't be treated as such.
Jcorenman, you should also be aware that it's not just where a metal falls on that scale, but the mass of each material has influence as well. You want your anode to be larger than your cathode in galvanic corrosion, to decrease the rate of oxidation. For example, it would be very unwise to have aluminum fasteners in a steel structure. The aluminum would be consumed rapidly. Using a steel fastener in aluminum structure is fine, since the small cathode has little effect on the aluminum anode. Here's a great example: http://corrosion-doctors.org/Corrosi...ea-effects.htm
Last edited by traxtar928; 09-11-2015 at 06:09 PM.
Just use Duralac Zinc Chromate jointing compound. It's what the marine industry uses for joints between dissimilar metals. If it's good enough in a salt water environment it's more than good enough on a 928.
Don't use SS bolts where strength is the primary criteria e.g suspension, brakes etc. Fine to use SS on stuff like an inlet manifold or oil pan where strength is not a primary criteria, these have many bolts to achieve distributed clamping force on the gaskets. Not because the items need lots of strong bolts to hold them on. I suspect that 2 M6 bolts would be sufficient to hold the Oil Pan not the 30 or so that are used. From the perspective of achieving sufficient distributed clamping force M2 bolts would probably be sufficient but would be impractical due to their tiny size.
09-10-2015, 09:36 PM
Stainless bolts and aluminum
