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Good point TikiHut. Many fasteners come from China or other LCC sources where the reality of what you think you bought and what you actually bought are very far apart, poor material quality, not meeting ratings standards, etc. We often find that where fasteners are sourced can make a difference on whether or not people will buy products.
Quality fasteners are not cheap! Look for marine or aerospace quality fasteners. Stainless fasteners should all be passivated too, this is already done by quality manufacturers but you can do it yourself with a alcohol and nitric acid mix (instructions are available easily). Also, and especially in sea water,you need to keep the water from getting under the bolt head as the lack of oxygen prevents the 'self sealing' stainless does to prevent corrosion. Use sealant, buytl tape, etc under the head when to keep water out, not just at the interface of the joint.
Timsmith--yes that is the part which is only screwed into the transom. Sorry for the confusion.
I also replaced the thru-bolts along the actual swim platform which ended in the engine compartment. I saw the rust stain running down the inside of the engine compartment-once I replaced the thru-bolts with good quality stainless--I figured I had better replace the supports as well figuring they to are wearing and are in the water all the time since they are below the waterline. I to used 3M 4000.I did not have an issue with water intrusion but better safe than sorry.
I read all the nightmares with owners of all boats having rotted out transoms because of poorly sealed screws below the waterline. I was amazed these were only screwed in. I then saw why-the swim platform is low and the supports ended up about 3-4 inches from the hull. There would be no way of putting thru bolts that low on the transom.