TOPIC: MFG Stringer Bolts - Which metal?

Hey folks,

I'm about halfway through with a transom replacement, and I'm pouring some Arjay 6011 tomorrow into my 1960 Edinboro. The one thing I haven't decided on is the type of bolts to use as replacements for the through-transom bolts that hold the aluminium stringer support plates in place on the inside skin of the transom.

The original bolts appeared to suffer from heavy electrolysis, as were very badly corroded and eaten away within the transom. Several had about a toothpick's width left to them. Unfortunately I didn't set them aside and they were pitched with the rest of the rotten transom core.

So I have a few questions for the experts:

Any suggestions on what type of metal to replace them with?

Would stainless be strong enough, and would that cause issues contacting aluminum?

If not stainless; galvanized or grade 8?

Were the originals galvanized or aluminum?

Use marine grade stainless carriage bolts. Seal them up with 5200, and use polyethylene washers between dissimilar metals.

The most common material for Stainless Steel bolts is type 18-8. That is a general class of moderate corrosion resistant versions like 304. For a higher corrosion resistance use 316 if you can find bolts made of it.

However, if your problem is electrolysis, any type of bolt will be subject to degrading if dissimilar metals are used. Isolating the materials with non-conductive washers is a good suggestion for a preventive measure.

Jerry

there are also some paste products out there that are supposed to limit electrolytic action between dissimilar metals. They help in situations where one metal is threaded into another - typically seen in sailboat mast installations. I've never used them myself but the sailing communities generally give them decent reviews.

McMaster-Carr is my go-to place for fasteners. 18-8 is more resistant to chemicals than 304 or 316 stainless. I use 316 stainless whenever possible. Also +1 on bedding in 3M 5200 or 3M 4000(if you want to get it apart ever) and plastic shims to keep dissimilar metals separated.
best,
Larry

Thanks a bunch for all of the suggestions folks. didn't even consider the washers, great idea. I'll look into the stainless that I can get local, and will head to McMC if needed