I would have had no way to know how high up the fillet would need to go to be fully above the part thinned by rust. My guess is "way too high".

Main reason this repair was so much work was I replaced the BB shell with a larger-diameter one, and offset the miter upward so as much as possible of the rusted tube was mitered away.

Mitering:


New shell fitted:


Taking the old shell off obviously made more work, but I wouldn't have done the repair otherwise, because I wouldn't have been able to know if it was all a waste of time. With the old shell off, I was able to see inside, remove the rust, measure how much thickness was lost, and how high up. Only then was I convinced to go ahead with the rest of the repair.

Replacing the shell also allowed me to get away from the abomination of setscrews for locking the rotation of the eccentric insert. They suck. Some may say a Bushnell internally-expanding insert is the best, and they are very clever, but expensive. Plus I still prefer this old way, which has always been reliable and easy to adjust, anywhere including mid-ride, no matter how old or corroded-in-place everything is.

Finally, the eccentric that came with the bike wasn't large enough diameter to have enough eccentricity (I call it "E") to adjust to proper tightness, on any chain. The bike came to me maxed out, eccentric all the way forward, but the chain was still a bit loose. But we couldn't take a link out, because then the chain wasn't long enough, even with the eccentric all the way back. To work with any length of chain, you need at least a half-inch of E, and this Burley had less than that. Yes I know about half-links, a sad kluge to have to use when the correct diameter shell can avoid needing one. So replacing the BB shell had at least three advantages.

Note: by length of the chain, I'm not talking about number of links, I'm talking about chain wear, or "stretch" as some call it. The whole reason why you need an eccentric in the first place. The interplay between keel tube length, chainring size and chain length, chain wear and E, has been well known for well over a hundred years, but somehow Burley didn't get the memo. They're not alone there; some other brands of tandem have been made with less than 1/2" of E as well. That requirement seems obvious to me, but then I've been a tandem specialist since my first FB job in '76 at Santana, so I've spent a lot of time thinking about these things.

Note, I liked the Burley company, mourned it when it died, and I liked almost everything else about this bike — other than the lack of vent holes (causing rust-through) and the too-small BB shell with setscrews to lock down the eccentric. Everything else was amazing, how great a bike they made for the money. Tremendous value.

But like I say, buying a new tandem might well have been cheaper.