The 5/16-18 screws that hold the exhaust plates together

**pcrussell50** · 12-09-2022, 05:42 PM

Are these supposed to be mild steel or stainless?

So I’m educated as an engineer not a mechanic although I’ve had to learn to be a mechanic on my own and it’s been a blast with help from S&F.

Anyway, “the college book” says galvanic corrosion when you use stainless steel fasteners for aluminum. Particular in water, heat, and salt water. (I don’t boat in salt water any more, so that’s moot). That’s what the “book” says but practical reality may dictate otherwise.

So the exhaust plate below the block and the other plate atop the mid, that both clamp together on the upper mounts… Stainless or carbon steel for the 5/16-18 bolts that hold the plates together? in a heat and (fresh) water environment.

-Peter

**David - WI** · 12-09-2022, 06:21 PM

This? https://www.ultratef-gel.com/tef-gel/

**25two.stroke** · 12-09-2022, 06:45 PM

The ones I've seen original from the 80s and early 90s were mild steel. Late 90s and 2000s I see stainless. The both lead to galvanic corrosion and almost always break when they have been installed dry and been put in service in salt water for a few years (regardless of bolt material). I always use stainless on new builds where mercury originally used steel unless its in a high stress (particularly shear) condition. They look better and don't surface rust as I run mostly fresh but some brackish.

At the shop where I work we put the aviation permatex on EVERYTHING. Water pump bolts, exhaust divider bolts, lower unit mounting studs and bolts, motor mount bolts, orings, carrier retaining nuts, midsection bolts...and it never hurts us. Years later when customers have neglected to flush their stuff in pure salt all the bolts we put in come back out. I always though in high-performance applications it may lead to bolts backing out but I have yet to actually experience that. So in the case of your bolts (I assume you mean the two that are under the powerhead in a merc v6 and not accessible from the outside) I use stainless with the aviation permatex.

**pcrussell50** · 12-09-2022, 07:59 PM

Fantastic ^^^

And I actually have some of that to boot. I will put it to use here.

My 2.0/2.4 v6’s are used fresh only, although the 2.4/200 was a salt motor when I bought it. Currently “de-salting” before I put it in service, which is now mostly done.

None of these motors are any more than stock. 150/200 is plenty for our light little boats. Not searching for more power.

-Peter