Re:
02gf74,
have you tried gripping the external part of rivnut with mole grips? There's not much you can grip as the external part is virtually flush with frame. If it's sticking up enough to grip it the chances are that you'll distort it if you grip it tight enough to hold it, then it will be tighter on bolt.
Just checked my Cannondale F800 to find that one of the bottle cage bolts/rivnut on the downtube was spinning when I tried to remove it. Gave it a skoosh with GT85 then left it for a bit. Got a flat blade screw-driver and pushed it very firmly against edge of rivnut then carefully slackened bolt. I've had to do something similar on another aluminium frame but luckily it was in the seat-tube, the bottom bolt. Again I used GT85, then I got a long flat blade screw-driver and pushed down firmly, inside the seat-tube, on the internal part of rivnut not bolt threads. This worked OK and bolt slackened. What happens is the rivnut is pushed against side of hole or even rocked over in hole so that it tightens it enough to slacken bolt. Obviously this won't work all the time but it's one way to do it that works.
If you don't have a rivnut tool, luckily I do :mrgreen: , there is a way to tighten rivnut again. Find an Allen bolt, and a nut, the same size but a decent bit longer than the bottle cage bolt. Screw the nut onto the bolt far enough that the remaining threads will reach through the threads in rivnut. Now screw it in to rivnut bolt with a washer between the nut on bolt and face of rivnut, making sure the bolt is far enough in to grip rivnut threads properly, you don't want it chewing them up. Now whilst holding the bolt with Allen key, tighten the nut on bolt AGAINST the rivnut so that it squeezes the internal part of rivnut till it's tight in frame again. A drop of lube between washer and nut on bolt will help it turn without the washer turning rivnut, keep the lube away from between washer and rivnut. If the rivnut tries to turn while turning nut push the bolt sideways to rock rivnut in hole to help stop it turning. Again, it won't always work as the hole in frame may be distorted or the rivnut won't compress any further.