Is this a frame drain plug?
 

If so, that is such a cool idea.


https://cimg3.ibsrv.net/gimg/bikefor...83202e33a.jpeg

What does the removed piece look like? My first thought would be it is for adding grease but cannot really tell without seeing what was removed (fingers blocking adequate view).

For draining water? How could enough water get in there to necessitate a drain plug? +1 on the grease port idea, just not the standard location.

Looks more like an attachment braze-on of sorts to me. Rear fender? Generator? :foo:

I doubt it's a drain plug. Collecting water in a BB for later removal doesn't make sense, just use the BKM and put a hole on the bottom for drainage.

Rear fender attachment also doesn't make sense, it would attach to the chainstay bridge. Fenders curve away from the BB in that area.

Maybe a generator?

Is there a mount high up on the back of the seat tube for a lever to activate a generator?

I've seen generators somewhere that are placed in that area. They contact the rear wheel in the center rather than the side of the tire. I don't remember how they were activated. Sheldon Brown shows a picture of one in their descriptions of different kinds of generators. They note they are no longer being made and have the difficulty of turning on and off.

Typically there is a lever on the back of the seat tube that looks like a shift lever and it pulls the generator into contact with the tire via a shift cable. I’ve seen them most often on Japanese touring bikes

This is a 1991 Trek 990 by the way.

An oil drain hole was standard equipment on the Cinelli SC from 1951 to about 1965. Not to say I think the OP ting is an oil drain hole, I don't. Bad location.

https://cimg1.ibsrv.net/gimg/bikefor...ebbd6d17d8.jpg

In that case, that is the anchor point for the cable for the front derailleur.

There was a short period of time where some MTBs used a modified bottom pull FD with top tube mounted cabling.

https://cimg3.ibsrv.net/gimg/bikefor...775646e3f.jpeg

Note: I think my pic is for the same year/model frame. I can't recall which year my frameset is, but the colors look the same as yours.

On some MTBs the FD cable coming from the TT/ST went through a pulley to reverse its travel to work with a bottom pull FD. That looks like the mounting place for that pulley.

Haha! Love this forum section. I'll be damned......................

Yup. In the early 90s, mountain bike builders realized that running the cable along the top tube is cleaner but top pull derailers hadn’t been developed yet. There were a number of pulley systems around that era but Trek’s approach was unique.

In general, using the outer cable to push rather than the inner cable to pull is a Bad Idea. It makes the action less precise, and this is worsened unless the cable outer is cut and mounted as best it can be to minimize that loss. Of course, you loose extra points for having a U-shaped outer section with the U at the bottom, and if you don't run stainless inners you get another reverse bonus; and if you DO have all those things and you DON'T use oil then you deserve what happens.

Good idea. I had never seen that before. It is an effective way to get top pull before top pull derailleurs became available. I always thought that a pulley like a Sturmey-Archer 3-speed pully would reverse the direction, but this is more direct. Of course, it does require a braze-on.

Which is why you don't see builds done this way anymore, excellent analysis however.