I'm posting this here because, after a bit of searching, it seems as though this forum *may* have the community that actually knows the answer! Well, hoping at least.
I've got a steel frame that is fairly new. It has a pair of threaded holes on the seat stays for the installation of a rear rack (one on each seat stay, of course). Since, for the time being, I don't plan on using a rear rack, I'd like to put bolts in place - thereby preventing water & gunk from getting inside the tubing of the frame. However, without entirely buying a rear rack, I've got *no* idea what size bolts I should be looking for...
I want to say they're M5x0.8. Or they might be M8x1.25. I'd have to actually go look at my bike to tell, and I'm too lazy to do that. See if a bolt from a water bottle cage will fit. I think those are typically M5s
Edit: You might have better luck with this in the mechanics forum.
The frame is constructed in Taiwan (sorry guys) - but hey, it is a very quality frame and not a cheapo-piece of junk! I'm just not able to afford a custom frame just yet. I tried taking the water-cage bolts and applying them there, but no luck: they seemed a little bit too big.
Just go to you LBS they usually hand me some for little or free, and it isn't because I am a good customer. One might as well get some chromed decent bolts now since just inserting some junk could mean it gets rusted in there or even breaks off. If the holes have beefy surrounds some people prefer expanding them to 6mm, which at the very least means you will have options if they rust out. Or just try oiling the holes.