Thru-bolting swim platform brackets

1. I used 3/8" SS fender washers. More than adequate lateral surface area.
2. When I read this idea it sounded good, however, if using a standard SS 3/8" diameter hex head bolt with fender washers as I am, the hex head will have to be held with a wrench to adequately torque the fastener. The same holds true for removal of the brackets thus requiring movement of the tank and defeating the purpose. If a backing plate for each bracket were fabricated, the hex heads tack welded to the plates, and the plate itself fastened to the back of the boat from the inside, your idea would work. Your idea would also work if you permanently installed/glued the hex bolts and washers with 5200 so that a wrench on the inside would not be required to tighten or loosen the fasteners. I'm going to stick with the hex heads on the outside ( like the original lags were) and having to move the tank to get at the through bolts from the inside to tighten and loosen if necessary.
3. 4200 because the permanent 5200 doesn't seem correct for this application unless using it to secure the hex heads to the inside of the boat which could have consequences.

I have had T nuts spin during untorquing. Was a PIA at that point. Tapped backing plates would be best, glue them on with some 4200 & never have to mess with it again.

Yeah I definitely want to use backing plates as that will be the strongest. Where did you find your aluminum plates, Chris? I called a metal fab shop- they said they could make me some 1/8" thick stainless plates. Might be an option.

I'd stay away from fender washers since they are typically thin especially in SS. I've seen them crushed and in fact have done it myself when they are on wood. Yes, I also agree with Tim that the condition of the wood backing in the transom needs consideration.