Thanks, everyone. Now I know what I can/can't get, but I'm still unsure as to how you knew. I'm guessing most bikes are the English thread? I have another old road bike I was going to ask about.
Velo-orange, I'm in the 53 quite often. There are some good hills around here with 10% grades. On the way up, it's super hard work. The way down, not so much, but when I do pedal, I'm in the biggest gear ratio.