I have an aluminum time trial frame (Teschner) that I've owned for several years (bought used) and the bottom bolt for the seat tube water bottle mount has always spun freely. Never really used any brain cells on it as I've never needed to ride this bike for longer than one water bottle would last.

I have now however been bamboozled into riding the bike leg of a 1/2 Ironman relay, so I'll need that second bottle. It turns out the boss spins freely and I managed to remove the bolt by clamping down on the edges of the boss, but odds are if I put a cage on, it won't tighten down adequately and I won't be able to use the same method to remove it.

So, question, is this a rivnut? If so, is there a way to secure it, or otherwise remove/replace it without huge expense?

Most likely the boss is some form of a threaded press fit thing, a RivNut of sorts. First thought is to do nothing and just use a hose clamp to secure the lower cage mount. Of course this means that the cage needs to have a tab like portion that can be trapped under the hose clamp. The other end of the range of options is to remove the old and fit a new one to the frame. This is usually done by some one who has the tool to press/set the boss in place as well as the ability to R&R the BB in case any old boss bits drop down in the tube during removal. I would consider using epoxy when setting in the new boss. In between these two is to try to just retighten the boss. I have found this to be far less reliable then complete replacement. A simple bolt and nut properly used can pull up the boss, lube the bolt threads first.

My vote is the hose clamp. Andy.

Oops meant to post this picture. The seat tube is kind of a wing shape so I don't think a hose clamp or zip tie will work well.

If this is just a loose Rivnut type bottle mount, you can use a quick release lever to snug it up:

Park Tool Co. » ParkTool Blog » Water bottle fittings

That might be the answer! I'll give it a shot tonight.

Easier solution, imagine possibility of one bolt being enough for this one race. Hmmm?