Originally Posted by
ts99
Would I have to hunt for a 1977 unit on ebay?
NO!
Bookmark Sheldon Brown's
website. Your trek is about guaranteed to use British I.S.O. threads (1.37" x 24 will be stamped on the fixed cup on the right side of the bike). It was and still is the standard threading on US market bicycles, and it's very common. As long as you match the threading and the BB shell width (I'm going to guess yours is 68mm wide) the replacement will screw in like it's supposed to. While you
can replace your headset, I'd recommend being very careful during removal, since headset standards have evolved more than threaded BBs and a replacement might be a step down in finish and quality.
Bottom brackets
can be complicated, if they're esoteric or were cutting edge BITD. If you're trying to keep your bike 'period correct', sourcing a BB assembly on eBay is probably your best bet, unless you can find a coop and root around in their parts bins for a replacement part. There's nothing wrong with updating the components that are worn out if you want a bike that works and you're not worried about keeping it 'period correct'.